Pharmacology

[Endocrinology] The Agonist-Receptor Interaction and Pharmacodynamics of Thyroid Hormone

October 14, 2016 Endocrinology, Pharmacodynamics, Pharmacology, Physiology and Pathophysiology No comments , , , , , , , , , , , , , ,

rs_634x1024-160907091929-634-justin-chambers-greys-anatomy-abcThyroid Hormone Receptors and Cellular Events

Thyroid hormone receptors are expressed in virtually all tissues and affect multiple cellular events. The cellular actions of thyroid hormones are mediated by multiple thyroid hormone receptor isoforms derived from 2 distinct genes (alpha and beta) encoding thyroid hormone receptors. The functional significance of the different isoforms has not yet been elucidated. Thyroid hormone recetpors are nuclear receptors intimately associated wtih chromatin. Thyroid hormone receptors are DNA-binding transcription factors that function as molecular switches in response to hormone binding. The hormone receptor can activate or repress gene transcription, depending on the promoter context and ligand-binding status. Unoccupied thyroid hormone receptors are bound to DNA thyroid hormone response elements and are associated with a complex of proteins containing corepressor proteins. Hormone binding to the receptor promotes corepressor dissociation and binding of a coactivator, leading to modulation of gene transcription. Thyroid hormone receptors bind the hormone with high affinity and specificity. They have low capacity but high affinity for T3. The majority (85%) of nuclear-bound thyroid hormone is T3, and approximately 15% is T4.

Thyroid hormones enter cells by a carrier-mediated energy-, temperature-, and Na+-dependent process. Several transporters have been identified to be involved in their entry into the cell, including those belonging to the sodium taurocholate cotransporting polypeptide (NTCP), the sodium-independent organic anion transporting polypeptide (OATP), L- and T-type amino acid transporters, and members of the monocarboxylate transporter family. Two transporters have been demonstrated to have particular specificity for thyroid hormone transport, the OATP1C1, which shows preference for T4 and the MCT8 which shows preference for T3. Mutations or deletions in the MCT8 gene have been linked to psychomotor retardation and thyroid hormone resistance, indicating their contribution to optimal thyroid hormone function.

Cellular Events of Thyroid Hormone

  • Transcription of cell membrane Na+/K+-ATPase, leading to an increase in oxygen consumption
  • Transcription of uncoupling protein, enhancing fatty acid oxidation and heat generation without production of adenosine triphosphate
  • Protein synthesis and degradation, contributing to growth and differentiation
  • Epinephrine-induced glycogenolysis, and insulin-induced glycogen synthesis and glucose utilization
  • Cholesterol synthesis and low-density lipoprotein receptor regulation

Physiologic Effects of Thyroid Hormone

Thyroid hormones are essential for normal growth and development; they control the rate of metabolism and hence the function of practically every organ in the body (remember that the thyroid hormone receptors are expressed in virtually all tissues). Their specific biologic effects vary from one tissue to another.

The effects of thyroid hormone are mediated primarily by the transcriptional regulation of target genes, and are thus known as genomic effects. Recently, it has become evident that thyroid hormones also exert nongenomic effects, which do not require modification of gene transcription. Some of these effects include stimulation of activity of Ca2+ adenosine triphosphatease (ATPase) at the plasma membrane and sarcoplasmic reticulum, rapid stimulation of the Na+/H+ antiporter, and increases in oxygen consumption. The nature of the receptors that mediate these effects and the signaling pathways involved are not yet completely elucidated. However, T3 exerts rapid effects on ion fluxes and electrophysiologic events, predominantly in the cardiovascular system.

Bone

Thyroid hormone is essential for bone growth and development through activation of osteoclast and osteoblast activities. Deficiency during childhood affects growth. In adults, excess thyroid hormone levels are associated with increased risk of osteoporosis.

Cardiovascular System

Thyroid hormone has cardiac inotropic and chronotropic effects, increases cardiac ouput and blood volume, and decreases systemic vascular resistance. These responses are mediated through thyroid hormone changes in gene transcription of several proteins including Ca2+-ATPase, phospholamban, myosin, beta-adrenergic receptors, adenylyl cyclase, guanine-nucleotide-binding proteins, Na+/Ca2+ exchanger, Na+/K+-ATPase, and voltage-gated potassium channels.

Fat

Thyroid hormone induces white adipose tissue differentiation, lipogenic enzymes, and intracellular lipid accumulation; stimulates adipocyte cell proliferation; stimulates uncoupling proteins; and uncouples oxidative phosphorylation. Hyperthyroidism enhances and hypothyroidism decreases lipolysis through different mechanisms. The induction of catecholamine-mediated lipolysis by thyroid hormones results from an increased beta-adrenoceptor number and a decrease in phosphodiesterase activity resulting in an increase in cAMP level and hormone-sensitive lipase activity.

Liver

Thyroid hormone regulates triglyceride and cholesterol metabolism, as well as lipoprotein homeostasis. Thyroid hormone also modulates cell proliferation and mitochondrial respiration.

Pituitary

Thyroid hormone regulates the synthesis of pituitary hormones, stimulates growth hormone production, and inhibits TSH.

Brain

Thyroid hormone controls expression of genes involved in myelination, cell differentiation, migration, and signaling. Thyroid hormone is necessary for axonal growth and development.

Specific Immunosuppressive Therapy

July 20, 2016 Hematology, Immunology, Infectious Diseases, Oncology, Pharmacology, Transplantation No comments , , , , , , , , , , , , , , , ,

The ideal immunosuppressant would be antigen-specific, inhibiting the immune response to the alloantigens present in the graft (or vice versa alloantigens present in recipient in GVHD) while preserving the recipient's ability to respond to other foreign antigens. Although this goal has not yet been achieved, several more targeted immunosuppressive agents have been developed. Most involve the use of monoclonal antibodies (mAbs) or soluble ligands that bind specific cell-surface molecules. On limitation of most first-generation of mAbs came from their origin in animals. Recipients of these frequently developed an immune response to the nonhuman epitopes, rapidly clearing the mAbs from the body. This limitation has been overcome by the construction of humanized mAbs and mouse-human chimeric antibodies.

Many different mAbs have been tested in transplantation settings, and the majority work by either depleting the recipient of a particular cell population or by blocking a key step in immune signaling. Antithymocyte globulin (ATG), prepared from animals exposed to human lymphocytes, can be used to deplete lymphocytes in patients prior to transplantation, but has significant side effects. A more subset-specific strategy uses a mAb to the CD3 molecule of the TCR, called OKT3, and rapidly depletes mature T cells from the circulation. This depletion appears to be caused by binding of antibody-coated T cells to Fc receptors on phagocytic cells, which then phagocytose and clear the T cells from the circulation. In a further refinement of this strategy, a cytotoxic agent such as diphtheria toxin is coupled with the mAb. Antibody-bound cells then internalize the toxin and die. Another technique uses mAbs specific for the high-affinity IL-2 receptor CD25. Since this receptor is expressed only on activated T cells, this treatment specifically blocks proliferation of T cells activated in response to the alloantigens of the graft. However, since TREG cells also express CD25 and may aid in alloantigen tolerance, this strategy may have drawbacks. More recently, a mAb against CD20 has been used to deplete mature B cells and is aimed at suppressing AMR (antibody-mediated rejection) responses. Finally, in cases of bone marrow transplantation, mAbs against T-cell-specific markers have been used to pretreat the donor's bone marrow to destory immunocompetent T cells that may react with the recipient tissues, causing GVHD.

Because cytokines appear to play an important role in allograft rejection, these compounds can also be specifically targeted. Animal studies have explored the use of mAbs specific for the cytokines implicated in transplant rejection, particularly TNF-alpha, IFN-gamma, and IL-2. In mice, anti-TNF-alpha mAbs prolong bone marrow transplants and reduce the incidence of GVHD. Antibodies to IFN-gamma and to IL-2 have each been reported in some cases to prolong cardiac transplants in rats.

TH-cell activation requires a costimulatory signal in addition to the signal mediated by the TCR. The interaction between CD80/86 on the membrane of APCs and the CD28 or CTLA-4 molecule on T cells provides one such signal. Without this costimulatory signal, antigen-activated T cells become anergic. CD28 is expressed on both resting and activated T cells, while CTLA-4 is expressed only on activated T cells and binds CD80/86 with a 20-fold-higher affinity. In mice, D. J. Lenschow, J. A. Bluestone, and colleagues demonstrated prolonged graft survival by blocking CD80/86 signaling with a soluble fusion protein consisting of the extracellular domain of CTLA-4 fused to human IgG1 heavy chain. This new drug, belatacept, was shown to induce anergy in T cells directed against the graft tissue and has been approved by the FDA for prevention of organ rejection in adult kidney transplant pateints.

Write Again – Receptor Rationale – Pharmacodynamics

April 22, 2016 Pharmacodynamics, Pharmacology No comments , , , , , , , , , , , , , , , , , , , ,

Screen Shot 2016-08-23 at 8.19.15 PMType of Drug Receptors

The effects of must drugs result from their interaction with macromolecular components of the organism. These interactions alter the function of the pertinent component and initiate the biochemical and physiological changes that are characteristic of the response to the drug. The term drug receptor or drug target denotes the cellular macromolecule or macromolecular complex with which the drug interacts to elicit a cellular response, i.e., change in cell function.

From a numerical standpoint, proteins form the most important class of drug receptors. Examples include the receptors for hormones, growth factors, transcription factors, and neurotransmitters; the enzymes of crucial metabolic or regulatory pathways; proteins involved in transport processes; secreted glycoproteins; and structural proteins. Specific binding of drugs to other cellular constituents such as DNA is also exploited for therapeutic purposes.

Drugs commonly alter the rate or magnitude of an intrinsic cellular response rather than create new responses. Drug receptors are often located on the surface of cells, but may also be located in specific intracellular compartments such as the nucleus. Many drugs also interact with acceptors (e.g., serum albumin) within the body. Acceptors are entities that do not directly cause any change in biochemical or physiological response. However, interactions of drugs with acceptors such as serum ablumin can alter the pharmacokinetics of a drug's action.

A major group of drug receptors consists of proteins that normally serve as receptors for endogenous regulatory ligands. These drug targets are termed physiological receptors. Many drugs act on physiological receptors and are particularly selective because physiological receptors have evolved to recognize and respond to individual signaling molecules with great selectivity.

Type of Drugs – From Perspective of Pharmacodynamics

Drugs that bind to physiological receptors and mimic the regulatory effects of the endogenous signaling compounds are termed agonists. If the drug binds to the same recognition site as the endogenous agonist the drug is said to be primary agonist. Allosteric (allotopic) agonists bind to a different region on the receptor referred to as an allosteric or allotopic site.

Drugs that block or reduce the action of an agonist are termed antagonists. Antagonism most commonly results from competition with an agonist for the same or overlapping site on the receptor (a syntopic interaction, or "primary" antagonism, or competitive antagonism), but can also occur by interacting with other sites on the receptor (allosteric antagonism), by combining with the agonist (chemical antagonism), or by functional antagonism by indirectly inhibiting the cellular or physiological effects of the agonist.

Agents that are only partly as effective as agonists regardless of the concentration employed are termed partial agonists.

Many receptors exhibit some constitutive activity in the absence of a regulatory ligand; drugs that stabilize such receptors in an inactive conformation are termed inverse agonists.

Drug Specificity

The chemical structure of a drug contributes to the drug's specificity. A drug that interacts with a single type of receptor that is expressed on only a limited number of differentiated cells will exhibit high specificity. If, however, a receptor is expressed ubiquitously on a variety of cells throughout the body, drugs acting on such a widely expressed receptor will exhibit widespread effects, and could produce serious side effects or toxicities if the receptor serves important functions in multiple tissues.


Quantitative Aspects of Drug Interactions with Receptors

Screen Shot 2016-04-18 at 8.36.23 PMReceptor occupancy theory asumes that response emanates from a receptor occupied by a drug, a concept that has its basis in the law of mass action. The basic currency of receptor pharmacology is the dose-response (or concentration-response) curve, a depiction of the observed effect of a drug as a function of its concentration in the receptor compartment. Figure 3-2 shows a typical dose-response curve; it reaches a maximal asymptotic value when the drug occupies all the receptor sites.

In general, the drug-receptor interaction is characterized by 1.binding of drug to receptor and 2.generation of a response in a biological system, as illustrated in Equation 3-1 where the drug or ligand is denoted as L and the inactive receptor as R. The first reaction, the reversible formation of the lignad-receptor complex LR, is governed by the chemical property of affinity. LR* is produced in produced in proportion to [LR] and leads to a response. This simple relationship illustrates the reliance of the affinity of the ligand (L) with receptor (R) on both the forward or association rate (k+1) and the reverse or dissociation rate (k-1). At any given time, the concentration of ligand-receptor complex [LR] is equal to the product of k+1[L][R], the rate of formation of the bi-molecular complex LR, minus the product k-1[LR], the rate dissociation of LR into L and R. At equilibrium, k+1[L][R] = k-1[LR]. Because the equilibrium dissociation constant (KD) is then described by ratio of the off and on rate constants (KDk-1/k+1), thus at equilibrium KD = k-1/k+1 = [L][R] / [LR] (Equation 3-2).

Screen Shot 2016-04-18 at 9.04.40 PMThe affinity constant or equilibrium association constant (KA) is the reciprocal of the equilibrium dissociation constant (KA = 1/KD); thus a high-affinity drug has a low KD and will bind a greater number of receptor at a low concentration than a low concentration than a low-affinity drug. As a practical matter, the affinity of a drug is influenced most often by changes in its off-rate (k-1) rather than its on-rate (k+1).

Screen Shot 2016-04-18 at 9.20.47 PMEquation 3-2 permits us to write an expression of the fractional occupancy (f) of receptors by agonist, Equation 3-3. This can be expressed in terms of KA (or KD) and [L]: f = [L]/([L] + KD). This relationship illustrate that under the condition of equilibrium and when the concentration of drug equals the KD, f = 0.5, that is, the drug will occupy 50% of the receptors. Note that this relationship describes only receptor occupancy, not the eventual response that is often amplified by the cell.

  • Occupation 

The second reaction shown in Equation 3-1 is the reversible formation of the active ligand-receptor complex, LR*. The ability of a drug to activate a receptor and generate a cellular response is a reflection of its efficacy. A drug with high efficacy may be a full agonist, eliciting, at some concentration, a full response. A drug with a lower efficacy at the same receptor may not elicit a full response at any dose. When it is possible to describe the relative efficacy of drugs at a particular receptor, a drug with a low intrinsic efficacy will be a partial agonist. A drug that binds to a receptor and exhibit zero efficacy is an antagonist. When the response of an agonist is measured in a simple biological system, the apparent dissociation constant, Kapp, is a macroscopic equilibrium constant that reflects both the ligand binding equilibrium and the subsequent equilibrium that results in the formation of the active receptor LR*.

Potency and Efficacy

Agonist

When the relative potency of two agonists of equal efficacy is measured in the same biological system, and downstream signaling events are the same for both drugs, the comparison yields a relative measure of the affinity and efficacy of the two agonists (Figure 3-3). It is convenient to describe agonist response by determining the half-maximally effective concentration (EC50) for producing a given effect. Thus, measuring agonist potency by comparison of EC50 values is one method of measuring the capability of different agonists to induce a response in a test system and for predicting comparable activity in another. Another method of estimating agonist activity is to compare maximal asymptotes in systems where the agonists do not produce maximal response (Figure 3-3B). The advantage of using maxima is that this property depends solely on efficacy, whereas drug potency is a mixed function of both affinity and efficacy.Screen Shot 2016-04-22 at 7.52.20 PM

PS: Potency refers to the concentration (EC50) or dose (ED50) of a drug required to produce 50% of that drug’s maximal effect.

Antagonist

For antagonists, characteristic patterns of antagonism are associated with certain mechanisms of blockade of receptors. One is straightforward competitive antagonism, whereby a drug with affinity for a receptor but lacking instrinsic efficacy competes with the agonist (i.e., endogenous ligands) for the primary binding site on the receptor. The characteristic pattern of such antagonism is the concentration-dependent production of a parallel shift to the right of the agonist dose-response curve with no change in the maximal response. The magnitiude of the rightward shift of the curve depends on the concentration of the antagonist and its affinity for the receptor. A partial agonist similarly can compete with a "full" agonist for binding to the receptor. However, increaseing concentrations of a partial agonist will inhibit response to a finite level characteristic of the drug's intrinsic efficacy; in contrast, a competitive antagonist will reduce the response to zero. Partial agonists thus can be used therapeuticallu to buffer a response by inhibiting excessive receptor stimulation without totally abolishing receptor stimulation.

An antagonist may dissociate so slowly from the receptor that its action is exceedingly prolonged, as with the opiate partial agonist buprenorphine and the Ca2+ channel blocker amlodipine. In the presence of a slowly dissociating antagonist, the maximal response to the agonist (i.e., endogenous ligand) will be depressed at some antagonist concentrations. Operationally, this is referred to as noncompetitive antagonism, although the  molecular mechanism of action really cannot be inferred unequivocally from the effect. An antagonist may also interact irreversibly (covalently) with a receptor to produce relatively irreversible effects. Noncompetitive antagonism antagonism can also be produced by another type of drug, referred to as an allosteric or allotopic antagonist. This type of drug produces its effect by binding to a site on the receptor distinct from that of the primary agonist, thereby changing the affinity of the receptor for the agonist. In the case of an allosteric antagonist, the affinity of the receptor for the agonist is decreased by the antagonist. In contrast, a drug binding at an allosteric site could potentiate the effects of primary agonists; such as drug would be referred to as an allosteric agonist or co-agonist.

The Management of Hypertension (Pathophysiologic Basises)

September 10, 2015 Cardiology, Pharmacology, Physiology and Pathophysiology No comments , , , , , , , , , , ,

Hypertension is a common diseases and is defined as persistently elevated arterial blood pressure of >= 140/90 mm Hg. Most of patients belong to essential hypertension and a small percentage belong to secondary hypertension for which the most common causes include renal dysfunction resulting from severe chronic kidney disease (CKD) or renovascular disease. Besides, certain drugs or other products (Table 3-1), either directly or indirectly, can cause hypertension or exacerbate hypertension by increase BP.

Table 3-1 Secondary Causes for Hypertesnion

Screen Shot 2015-09-08 at 9.42.15 PM

Classification of Hypertension

  • Normal: Systolic lower than 120 mm Hg, diastolic lower than 80 mm Hg
  • Prehypertension: Systolic 120-139 mm Hg, diastolic 80-89 mm Hg
  • Stage 1: Systolic 140-159 mm Hg, diastolic 90-99 mm Hg
  • Stage 2: Systolic 160 mm Hg or greater, diastolic 100 mm Hg or greater

Hypertension Crisis: These are clinical situations where BP values are very elevated, typically >180/120 mm Hg. They are categorized as either hypertensive emergency or hypertensive urgency. The former are extreme elevations in BP that are accompanied by acute or processing target-organ damage. The latter are high elevations in BP without acute or progressing target-organ injury. Prehypertension is not considered a disease category but identifies patients whose BP is likely to increase into the classification of hypertension in the future.

Cardiovascular Risk and Blood Pressure

Hypertension must be treated and the reason why is that hypertension is a major cardiovascular risk factor and there indeed is a causal relationship between hypertension and cardiovascular diseases. Also, epidemiologic data demonstrate a strong correlation between BP and CV morbidity and mortality. (Starting at a BP of 115/75 mm Hg, risk of CV disease doubles with every 20/10 mm Hg increase.) Even patients with prehypertension have an increased risk of CV disease. Because hypertension and CV morbidity/mortality has a casual relationship, treating patients with hypertension with antihypertensive drug therapy provides significant clinical benefits.


Pathophysiology

To further discuss the pathophysiology, we first need to know the mathematic formula to estimate arterial BP. According to the physic law, steady flow (Q) through a closed hydraulic circuit is directly related to the pressure gradient across the circuit (Pin – Pout), and inversely related to the resistance to flow (R) through the circuit. So Q=(Pin – Pout)/R. In the cardiovascular system, Q is cardiac output (CO), Pin is mean arterial pressure (MAP) and Pout is right atrial pressure (RAP), whereas resistance to flow (R) is total peripheral resistance (TPR). So CO=(MAP – RAP)/TPR. Because in normal conditions RAP approaches zero mm Hg, so CO=MAP/TPR and after we make a rearrange we finally get the formula of MAP=CO * TPR. Note that in some pathophysiology status RAP increases significantly and cannot be removed from the formula above.

After the discuss above, the two determinants for MAP is the cardiac output (CO) and the total peripheral resistance (TPR). If we distinguish MAP to systolic BP (SBP) and diastolic BP (DPB), CO is the major determinant of SBP, whereas TPR largely determines DBP. So factors that elevate CO or TPR can elevate BP. We category these factors into 1.humoral; 2.neuronal; 3.peripheral autoregulation; and 4.disturbances in sodium, calcium, and natriuretic hormone.

Humoral Mechanisms

RAAS

RAAS stands for the rennin-angiotensin-aldosterone system, which is a complex endogenous system that play a range of functions including the regulation of arterial pressure. The RAAS regulars sodium, potassium, blood volume, and most important the vascular tone. Because the total periphery resistance (TPR) is primarily generated by arterioles, so elevated TPR could be a result of activation of RAAS – the angiotensin II (angII). For the detail discussion of TPR please refer to the threads of http://www.tomhsiung.com/wordpress/2015/06/flow-resistance-of-vessels-in-series-and-vessels-in-parallel/ and http://www.tomhsiung.com/wordpress/2015/07/vascular-resistances-and-compliance-map-and-pulse-pressure/, respectively, by Tom Hsiung. First, angII increase the vascular tone, including arterioles. Second, angII induced increased aldosterone synthesis and secretion sodium and water retention, which increase the blood volume. Increased blood volume and TPR eventually result in elevation of BP.

Vasopressin

Vasopressin is a polypeptide hormone, also known as antidiuretic hormone/ADH, which plays an important role in extracellular fluid homeostasis (blood volume/plasma volume). Vasopressin acts on collecting ducts in the kidneys to decrease renal excretion of water. This is the most important and wide-known function of vasopressin. However, vasopressin is also a potent arteriolar vasoconstrictor.

Natriuretic Hormone

Natriuretic hormones inhibits sodium and potassium-ATPase and thus interferes with sodium transport across cell membranes. Natriuretic hormone theoretically could increase urinary exertion of sodium and water. However, this hormone might block the active transport of sodium out of arteriolar smooth muscle cells. The increased intracellular sodium concentration concentration ultimately would increase vascular tone and BP.

Insulin Resistance and Hyperinsulinemia

Hypothetically, increased insulin concentrations may lead to hypertension because of increased renal sodium retention and enhanced sympathetic nervous system activity. Moreover, insulin has growth hormone-like actions that can induce hypertrophy of vascular smooth muscle cells. Insulin also may elevated BP by increasing intracellular calcium, which lead to increased vascular resistance. The exact mechanism by which insulin resistance and hyperinsulinemia occur in hypertension is unknown. However, this association is strong because many of the criteria used to define this population (i.e., elevated BP, abdominal obesity, high, triglycerides, low high-density lipoprotein cholesterol, and elevated fasting glucose) are often present in patients with hypertension.

Circulating Catecholamines

It is easy to understand the causal relationship between elevated levels of circulating catecholamines and the hypertension, from the perspective of MAP = CO * TPR.

Neuronal Regulation

Synaptic receptors, baroreceptor reflex system, and CNS are involved in the regulation of vascular resistances, cardiac outputs.

Central and autonomic nervous system are intricately involved in the regulation of arterial BP. Many receptors that either enhance or inhibit norepinephrine release are located on the presynaptic surface of sympathetic terminals. The alpha and beta presynaptic receptors play a role in negative and positive feedback to the norepinephrine-containing vesicles, respectively. Stimulation presynaptic alpha-receptors (α2) exerted a negative inhibition on norepinephrine release. Stimulation of presynaptic beta-receptors facilitates norepinephrine release.

Sympathetic neuronal fibers located on the surface of effector cells innervate the alpha- and beta-receptors. Stimulation of postsynaptic alpha-receptors (α1) on arterioles and venues results in vasoconstriction. There are two types of postsynaptic beta-receptors, β1 and β2. Both are present in all tissues innervated by the sympathetic nervous system. However, in some tissues β1-receptors predominate (e.g., heart), and in other tissues β2-receptors predominate (e.g., bronchioles). Stimulation of β1-receptors in the heart results in an increase in heart rate, and the force of contraction (so cardiac output is increased), whereas stimulation of β2-receptors in the arterioles and venues causes vasodilation.

So after the discussion of the two paragraph above, we know that the disturbance of the function of presynaptic and/or postsynaptic receptors would result the imbalance of autonomic nervous system.

Same with the autonomic nervous system but from a different aspect (above is output of autonomic nervous system and now it’s the input of nervous system),  the baroreceptor reflex system is the major negative feedback mechanism the controls sympathetic activity. Baroreceptors are nerve endings lying in the walls of large arteries, especially in the carotid arteries and aortic arch. Changes in arterial BP rapid activate baroreceptors that then transmit impulses to the brain stem through the ninth cranial nerve and vagus nerve. In this reflex system, a decrease in arterial BP stimulates baroreceptors, causing reflex vasoconstriction and increased heart rate and force of cardiac contraction. Also the periphery vascular tone increase too (TPR).

Stimulation of certain areas within the central nervous system can either increase or decrease BP. I think this mechanism must be rather complex, which involves with neurology. If we have time in future, I will take a look at the neurology.

OK. The purpose of the neuronal mechanisms is to regulate BP and maintain homeostasis. Pathologic disturbances in neuronal systems could chronically elevate BP. These systems are physiologically interrelated. A defect in one component may alter normal function in another. Therefore, cumulative abnormalities may explain the development of essential hypertension.

Peripheral/Local Mechanisms (including autoregulatory, etc.) 

Abnormalities in renal or tissue autoregulatory systems, which is just one of several local vascular regulatory mechanisms of human, could cause hypertension. Recall the formula that MAP = CO * TPR. Similarly, the disorders of local vascular regulatory .For detail information of local vascular regulatory mechanisms please refer to the thread of http://www.tomhsiung.com/wordpress/2015/07/arteriolar-tone-and-its-regulation-local-mechanisms/ by Tom Hsiung.

Electrolytes

Epidemiologic and clinical data have associated excess sodium intake with hypertension. Population-based studies indicate that high-sodium diets are associated with a high prevalence of stroke and hypertension. Conversely, low-sodium diets are associated with a lower prevalence of hypertension. For the perspective of pathophysiology, more sodium, more water. We will discuss this phenomenon in threads that discuss the kidney.

Altered calcium homeostasis also may play an important role in the pathogenesis of hypertension. A lack of dietary calcium hypothetically can disturb the balance between intracellular and extracellular calcium, resulting in an increased intracellular calcium concentration. This imbalance can alter vascular smooth muscle function by increasing PVR (peripheral vascular resistance). Some studies have shown that dietary calcium supplementation results in a modest BP reduction for patients with hypertension.

The role of potassium fluctuations is also inadequately understood. Potassium depletion may increase PVR, but the clinical significance of small serum potassium concentration changes is unclear. Furthermore, data demonstrating reduced CV risk with dietary potassium supplementation are very limited.

Antimicrobials – The Basic Mechanisms

September 8, 2015 Infectious Diseases, Pharmacology No comments , , , , , , , , , ,

The antimicrobials produce their bactericidal or bacteriostatic effects by targeting at certain sites of the pathogens which are susceptible to the antibiotics. Generally, the mechanisms of all antibiotics can be divided into five basic types, or four targets toward which these antibiotics act, including: cell wall synthesis, protein synthesis, nucleic acid synthesis, folate biosynthesis, and cell membrane integrity.


Antibiotics Targeting at Cell Wall Synthesis

Antimicrobials that act on cell wall synthesis can be divided into β-lactams including penicillins, cephalosporins, carbapenems, and monobactams, and non-β-lactams including vancomycin, teicoplanin, telavancin, and bacitracin. The β-lactams antibacterial agents interfere with the transpeptidation reactions that seal the peptide cross links between glycan chains. They do so by interference with the action of the transpeptidase enzymes which carry out this cross-linking. These transpeptidase enzymes we call it in short penicillin-binding proteins (PBPs). Among different species, or even within one strain, the PBPs might have distinctions and vary in their avidity of binding to different β-lactam drugs.Screen Shot 2014-11-07 at 9.44.25 PM

The subtype of β-lactams are based on the chemical structure, the β-lactam ring, which is essential for antibacterial activity. Monobactams have a single β-lactam ring, penicillins and carbapenems have a β-lactam ring fused to a five-member thiazolidine penem ring, and cephalosporins have a β-lactam ring combined with a six-member dihydrothiazine cep hem ring. These differences on the structure affect the pharmacologic properties and spectrum of the specific drug. In general, β-lactam antimicrobials are highly bactericidal, but only to growing bacteria synthesizing new cell walls. One of the key factor that deterring the successful pharmacologic and physical effects of β-lactam is that the drug must penetrate or be transported across the outer membrane of the microbes to get in contact with its receptor, the PBPs.

For instance, penicillin G is active against certain Gram-positive organisms, Gram-negative cocci, and some spirochetes, but lack the activity against Gram-negative bacilli, as the outer membrane of these Gram-negative bacilli prevents passage of penicillin G to the site of action on cell wall synthesis.

For cephalosporins, a agent of a higher generation has a wider spectrum, and in some instances, more quantitative activity (lower MIC) against Gram-negative bacteria. Meanwhile as the Gram-negative spectrum increases, the drug typically loses some of their potency (higher MIC) against Gram-positive bacteria. Note that today there is a fifth-generation cephalosporins, ceftaroline, yes, those guys made it.

β-lactamase inhibitors

These inhibitors are capable of binding irreversibly to β-lactamase enzymes and, in the process, rendering them inactive. There are three β-lactamase inhibitors available, including clavulanic acid, sulbactam, and tazobactam. They also be referred to as suicide inhibitors since they must first be hydrolyzed by a β-lactamase before becoming effective inactivators of the enzyme. Therefore, for infections without β-lactamases, β-lactamase inhibitors are not able to enhance the bactericidal effect.

Glycopeptides

Another type of antimicrobials act against cell wall synthesis is glycopeptides. Each of these antimicrobials inhibits assembly of the linear peptidoglycan molecule by binding directly to the terminal amino acids of the peptide side chains (see the figure above). This effect is the same as with β-lactams: prevention of peptidoglycan cross-linking. Now there are three drug available, including vancomycin, teicoplanin, and telavancin (has additional theoretical advantage of cell membrane activity). Both of vancomycin and teicoplanin are bactericidal, but primarily only against Gram-positive bacteria. Because the both drugs are not absorbed by mouth, they could be used to treat CDI via oral route.


Inhibitors of Protein Synthesis

A variety of antibiotics are within this category of antimicrobials, including aminoglycosides, tetracyclines, chloramphenicol, macrolides, clindamycin, oxazolidinones, and streptogramins.

Aminoglycosides (30S subunit)

Amino glycosides are bactericidal drugs especially useful against many gram-negative rods. Certain ahminoglycosides are used against other organisms (e.g., streptomycin is used in the multi drug therapy of tuberculosis, and gentamicin is used in combination with penicillin G against enterococci). Amino glycosides are named for the amino sugar component of the molecule, which is connected by a glycosidic linkage to other sugar derivatives.

Both inhibition of the initiation complex and misreading of messenger RNA (mRNA) occur, the mechanisms for this class of drug to act, where the former is probably more important for the bactericidal activity of the drug. An initiation complex composed of a streptomycin-treated 30S subunit, a 50S subunit, and mRNA will not function – that is, no peptide bonds are formed, no polysomes are made, and a frozen “streptomycin monosome” results.

Misreading of the triplet codon of mRNA so that the wrong amino acid is inserted into the protein also occurs in streptomycin-treated bacteria. The site of action on the 30S subunit includes both a ribosomal proven and the ribosomal RNA (rRNA).

As a result of inhibition of initiation and misreading, membrane damage occurs and the bacterium dies.

Tetracyclines (30S subunit)

Tetracycline are a family of antibiotics with bacteriostatic activity against a variety of gram-positive and gram-negative bacteria, mycoplasmas, chlamydiae, and rickettsiae. They inhibit protein synthesis by binding to the 30S ribosomal subunit and by blocking the aminoacyl transfer RNA (tRNA) from entering the acceptor site on the ribosome. However, the selective action of tetracycline on bacteria is not at the level of ribosome, because tetracycline in vitro will inhibit protein synthesis equally well in purified ribosomes from both bacterial and human cells. Its selectivity is based on its greatly increased uptake into susceptible bacterial cells compared with human cells.

Chloramphenicol (50S)

Chloramphenicol inhibits protein synthesis by binding to the 50S ribosomal subunit and blocking the action of peptidyltransferase; this prevents the synthesis of new peptide bonds. It inhibits bacterial protein synthesis selectively, because it binds to the catalytic site of the transferase in the 50S bacterial ribosomal subunit but not to the transferase in the 60S human ribosomal subunit.

Chloramphenicol inhibits protein synthesis in the mitochondria of human cells to some extent, since mitochondria have a 50S subunit (mitochondria are thought to have evolved from bacteria). This inhibition may be the cause of the dose-dependent toxicity of chloramphenicol to bone marrow.

Macrolides (50S)

Macrolide’s are a group of bacteriostatic drugs with a wide spectrum of activity. Macrolide’s inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit and blocking translocation. They prevent the release of the uncharged tRNA after it has transferred its amino acid to the growing peptide chain. The donor site remains occupied, a new tRNA cannot attach, and protein synthesis stops.

Clindamycin (50S)

The most useful clinical activity of this bacteriostatic drug is against anaerobes, both gram-positive bacteria such as Clostridium perfringens and gram-negative bacteria such as B. fragilis.

Clindamycin binds to the 50S subunit and blocks peptide bond formation by an undetermined mechanism. Its specificity for bacteria arises from its inability to bind to the 60S subunit of human ribosomes.

Linezolid (50S)

Linezolid is useful for the treatment of vancomycin-resistant enterococci, MRSA, and S. epidermis, and penicillin-resistant pneumococci. It is bacteriostatic against enterococci and staphylococci but bactericidal against pneumococci.

Linezolid binds to the 23S ribosomal RNA in the 50S subunit and inhibits protein synthesis, but the precise mechanism is unknown. It appears to block some early step (initiation) in ribosome formation.

Telithromycin (50S)

Telithromycin (Ketek) is the first clinically useful member of the ketolide group of antibiotics. It is similar to the macrocodes in general structure and mode of action but is sufficiently different chemically such that organisms resistant to macrocodes may be sensitive to telithromycin.

Streptogramins (50S)

Streptogramins cause premature release of the growing peptide chain from the 50S ribosomal subunit. The structure and mode of action of streptogramins is different from all other drug that inhibit protein synthesis, and there is no cross-resistance between streptogramins and these other drugs.

Retapamulin (50S)

Raetapamulin is the first clinically available member of a new class of antibiotics called pleuromutilins. These drugs inhibit bacterial protein synthesis by binding to the 23S RNA of the 50S subunit and blocking attachment of the donor tRNA. Retapamulin is a topical antibiotic used in the treatment of skin infections, such as impetigo, caused by S. progenies and methicillin-sensitive S. aureus.


Inhibition of Nucleic Acid Synthesis

This class of drugs include sulfonamides, trimethoprim, fluoroquinolone, flu cytosine, and rifampin. Mechanisms for the inhibition of nucleic acid synthesis include inhibition of precursor synthesis, inhibition of DNA synthesis, and inhibition of mRNA synthesis.

Sulfonamides

Sulfonamides inhibits the precursor synthesis. The mode of action of sulfonamides is to block the synthesis of tetrahydrofolic acid, which is required as a methyl donor in the synthesis of the nucleic acid precursors adenine, guanine, and thymine. Sulfonamides are structural analogues of p-aminobenzoic acid (PABA), which condenses with a pteridine compound to form dihydropteroic acid, a precursor of tetrahydrofolic acid. Sulfonamides compete with PABA for the active site of the enzyme dihydropteroate synthetase. This competitive inhibition can be overcome by an excess of PABA.

The basis of the selective action of sulfonamides on bacteria is that many bacteria synthesize their folic acid from PABA-containing precursors, whereas human cells require preformed folic acid as an exogenous nutrient because they lack the enzymes to synthesize it. Human cells therefore bypass the step at which sulfonamides act, and similarly, bacteria that can use preformed folic acid are similarly resistant to sulfonamides.

Trimethoprim

Trimethoprim also inhibit the production of tetrahydrofolic, whose mechanism belongs to inhibition of precursors synthesis. However, trimethoprim has a different mode of action compared with sulfonamides that it inhibits the enzyme dihydrofolate reductase. Its specificity for bacteria is based on its much greater affinity for bacterial reductase than for the human enzyme.

Fluoroquinolones

Fluoroquinolone’s are bactericidal drugs that block bacterial DNA synthesis by inhibiting DNA gyrase (topoisomerase).

Flucytosine

Flucytosine is an antifungal drug that inhibits DNA synthesis. It is a nucleoside analogue that is metabolized to fluorouracil, which inhibits thymidylate synthetase, thereby limiting the supply of thymidine.

Rifampin

Rifampin belongs to drugs that inhibits synthesis of mRNA. The selective mode of action of rifampin is based on blocking mRNA by bacterial RNA polymerase without affecting the RNA polymerase of human cells. Rifampin is red, and the urine, saliva, and sweat of patients taking rifampin often turn orange; this is disturbing but harmless.


Alteration of Cell Membrane Function

Two sub-categories of mechanisms are involved with alteration of cell membrane function, including: 1.Alteration of bacterial cell membranes; 2.Alteration of fungal cell membranes.

There are few antimicrobial compounds that act on the cell membrane because the structural and chemical similarities of bacterial and human cell membranes make it difficult to provide sufficient selective toxicity.

Polymyxins

Polymyxins are a family of polypeptide antibiotics of which the clinical most useful compound is polymyxin E (colistin). Polymyxins are cyclic peptides composed of 10 amino acids, 6 of which are diaminobutyric acid. The positively charged free amino groups act like a cationic detergent to disrupt the phospholipid structure of the cell membrane.

Daptomycin

Daptomycin is a cyclic lipopeptide that disrupts the cell membranes of gram-positive cocci. It is bactericidal for organisms such as S. aureus, S. epidermis, S. progenies, Enterococcus faecalis, and E. faecium, including methicillin-resistant strains of S. aureus and S. epidermis, vancomycin-resistant strains of E. faecalis and E. faecium.

Amphotericin B

Amphotericin B disrupts the cell membrane of fungi because of its affinity for ergosterol, a component of fungal membranes but not of bacterial or human cell membranes.

Nystatin

Nystatin is another polyene antifungal agent, which, because of its toxicity, is used topically for infections caused by the yeast Candida.

Terbinafine

Terbinafine blocks ergosterol synthesis by inhibiting squalene epoxidase.

Azoles (Fluconzole, Ketoconazole, Voriconazole, etc.)

They act by inhibiting ergosterol synthesis. The block cytochrome P-450-dependent demethylation of lanosterol, the precursor of ergosterol.


Others

Isoniazid (prodrug probably, see below)

Isoniazid is a bactericidal drug highly specific for Mycobacterium tuberculosis. Isoniazid inhibits mycolic acid synthesis, which explains why it is specific for mycobacteria and relatively nontoxic for humans. The drug inhibits a reductase required for the synthesis of the long-chain fatty acids called mycolic acids that are an essential constituent of mycobacterial cell walls. The active drug is probably a metabolite of isoniazid formed by the action of catalase peroxidase because deletion of the gene for these enzymes results in resistance to the drug.

Metronidazole (prodrug)

Metronidazole is bactericidal against aerobic bacteria (also effective against certain protozoa such as Giardia and Trichomonas). Metronidazole is a prodrug that is activated to the active compound within anaerobic bacteria by ferredoxin-mediated reduction of its nitro group. This drug has two possible mechanisms of action, and it is unclear which is the more important. The first, which explains its specificity for anaerobes, is its ability to act as an electron sink. By accepting electrons, the drug deprives the organism of required reducing power. In addition, when electrons are acquired, the drug ring is cleaved and a toxic intermediate is formed that damages DNA. The precise nature of the intermediate and its action is unknown.

The second mode of action of metronidazole relates to its ability to inhibit DNA synthesis. The drug binds to DNA and causes strand breakage, which prevents its proper functioning as a template for DNA polymerase.

Ethambutol

Ethambutol is a bacteriostatic drug active against M. tuberculosis and many of the atypical mycobacteria. It is thought to act by inhibiting the synthesis of arabinogalactan, which functions as a link between the mycolic acids and the peptidoglycan of the organism.

Griseofulvin

Griseofulvin is an antifungal drug that is useful in the treatment of hair and nail infections caused by dermatophytes. It binds to tubulin in microtubules and may act by preventing formation of the mitotic spindle.

Pentamidine

Pentamidine is active against fungi and protozoa. It hinibits DNA synthesis by an unknown mechanism.