Proximal Tubule

Regulation of Sodium Excretion

June 25, 2016 Cardiology, Critical Care, Nephrology, Physiology and Pathophysiology No comments , , , , , , , , , , , , , , , , , , , , , , , , ,

Percentage of Sodium Reabsorbed

Screen Shot 2016-07-31 at 2.12.05 PM

The Goals of Regulation

The overriding goals of regulating sodium and water excretion are to support the requirements of the cardiovascular system. This is manifested in 3 ways: 1.the kidneys maintain a sufficient ECF volume to fill the vascular space (mean circulatory filling pressure); 2.keep the osmolality of the ECF at a level consistent with cellular health; and 3.limit the changes in renal blood flow (RBF) and GFR that might otherwise reach deleterious levels. The kidneys and the CV system work cooperatively to ensure that peripheral tissue is sufficiently perfused. An adequate circulating volume is one of the essential requirements for tissue perfusion and it is the kidneys that control this volume. Osmolality is the ratio of solute content to water content. Sodium and chloride together account for 80% of the normal extracellular solute; thus the excretion of sodium and water by the kidneys regulates osmolality in the tight range that is needed for the health of tissue cells. There is a separate goal of regulation that differs from those stated above. Variations in RBF and GFR are major means of regulating sodium excretion. However, the kidney cannot change blood flow and filtration to such extreme values that they compromise the metabolic health of the kidneys or interfere with the excretion of substances other than sodium, particularly organic waste.

Formulas for ECF Volume

There are some formulas showing the relationship between ECF solute content, ECF osmolality, and ECF volume. Since almost all of the ECF solute is accounted for by sodium and an equivalent number of anions (mostly chloride and bicarbonate), the amount of ECF solute is approximately twice the sodium content.

ECF osmolality = ECF solute content / ECF volume (Equation 7-1)

ECF volume = ECF solute content / ECF osmolality (Equation 7-2)

ECF volume ≈ 2 x Na content / ECF osmolality (Equation 7-3)

Therefore, in the face of tightly controlled ECF osmolality, ECF volume varies directly with sodium content. But how do the kidneys know how much sodium there is in the ECF? The detection of sodium content is indirect, based on a combination of assessing sodium concentration and vascular pressures. Glial cells in regions of the brain called the circumventricular organs have sensory Na+ channels that respond to and act as detectors of extracellular sodium concentration. The glial cells modulate the activity of nearby neurons involved  in the control of body sodium. There are also neurons in the hypothalamus contain the sensory Na+ channels that respond to the sodium concentration in the cerebrospinal fluid. Thus cells in or near the hypothalamus monitor extracellular sodium concentration.

The volume affects pressure in different regions of the vasculature. It is the presssure baroreceptors in these regions of the vasculature detect the vascular pressures.

Major Controllers of Sodium Excretion

Sympathetic Stimulation 

Vascular pressures are so important in regard to sodium excretion and because volume affect pressure in different regions of the vasculature, so the changes in ECF affects pressures (arterial and/or venous) and changes in pressure affect sodium excretion (Thread "Regulation of Arterial Pressure" at http://www.tomhsiung.com/wordpress/2016/06/physiology-regulation-of-arterial-pressure/ and thread "Mean Circulatory Filling Pressure and CVP" at http://www.tomhsiung.com/wordpress/2016/06/mean-circulatory-filling-pressure-and-cvp/).

The vasculature and tubules of the kidney are innervated by postganglionic sympathetic neurons that release norepinephrine. In most regions of the kidney, norepinephrine is recognized by alpha-adrenergic receptors. In the renal vasculature activation of alpha1-adrenergic recpetors causes vasoconstriction of afferent and efferent arterioles. This reduces RBF and GFR.

GFR is a crucial determinant of sodium excretion. However, except in body emergencies such as hypovolemic shock, GFR is kept within rather narrow limits due to autoregulatory processes (detail for vascular autoregulatory regulation http://www.tomhsiung.com/wordpress/2015/07/arteriolar-tone-and-its-regulation-local-mechanisms/). Thus although neural control does affect GFR, this component of sympathetic control is probably less important in normal circumstances than its effect on sodium reabsorption. Neural control of the renal vasculature is exerted primarily on blood flow in the cortex, allowing preservation of medullary perfusion even when cortical blood flow is reduced.

The proximal tubule epithelial cells are innervated by alpha1- and alpha2-adrenergic receptors. Stimulation of these receptors in the proximal tubule by norepinephrine activates both components of the main transcellular sodium reabsorptive pathway, that is, the sodium-hydrogen antiporter NHE3 in the apical membrane and the Na-K-ATPase in the basolateral membrane. The effects of sympathetic stimulation on cells in the distal nephron are less straightforward. However, the overall outcome of sympathetic stimulation of the kidney is clearly reduced sodium excretion.

The Renin-Angiotensin System

AII's function

  • Reduces the RBF and GFR
  • Stimulation of sodium tubular reabsorption
  • Stimulation of the CNS: salt appetite, thirst, and sympathetic drive
  • Stimulation of aldosterone secretion

The major determinant of circulating AII is the amount of renin available to form angiotensin I.

PS: "Control of the Circulating RAAS" is ready at http://www.tomhsiung.com/wordpress/2016/06/control-of-the-circulating-raas/

AII is a potent vasoconstrictor, acting on the vasculature of many peripheral tissues, the effect of which is to raise arterial pressure. It also vasoconstricts both cortical and medullary vessels in the kidney. This reduces total RBF and reduces GFR, thus decreasing the filtered load of sodium.

AII stimulates sodium reabsorption in both the proximal tubule and distal nephron. In the proximal tubule it stimulates the same transcellular transport pathway as does norepinephrine, namely NHE3 sodium/hydrogen antiporter in the apical membrane and the Na-K-ATPase in the basolateral membrane. In the distal tubule and connecting tubule, it stimulates the activity of sodium/chloride symporters and sodium channels (ENaC) that reabsorb sodium.

AII stimulates behavioral actions in response to fluid loss that increase salt appetite and thirst. AII acts on the circumventricular organs in the brain. These function as detectors of many substances in the blood and convey information to various areas of the brain. In situations of volume depletion and low blood pressure, when circulating levels of AII are high, a key effect, in addition to vascular and tubular actions is increased thirst and salt appetite. These pathways also increase sympathetic drive.

Aldosterone is a major stimulator of sodium reabsorption in the distal nephron, that is, regions of the tubule beyond the proximal tubule and loop of Henle. We focus here on the role of aldosterone in sodium reabsorption, but aldosterone has many other important actions, including stimulation of potassium excretion and acid excretion. The most important physiological factor controlling secretion of aldosterone is the circulating level of AII, which stimulates the adrenal cortex to produce aldosterone. But keep in mind that elevated plasma potassium concentration, atrial natriuretic factors are other stimulators of aldosterone secretion. The aldosterone has enough lipid character to freely cross principal cell membrane in the collecting ducts, after which it combines with mineralocorticoid receptors (aldosterone receptors) in the cytoplasm. After being transported to the nucleus, the receptor acts as a transcription factor that promotes gene expression of specific proteins. The effect of these proteins is to increase the activity or number of luminal membrane sodium channels (ENaCs) and basolateral membrane Na-K-ATPase pumps.

Dopamine

Dopaimine inhibits sodium reabsorption in the kidney. The dopamine that acts in the kidney is not released from neurons; rather it is synthesized in proximal tubule cells from the precursor l-DOPA. l-DOPA is taken up from the renal circulation and glomerular filtrate and converted to dopamine in the proximal tubule epithelium, and then released to act in a paracrine manner on nearby cells. Although the signaling path is not clear, it is known that increases in sodium intake lead to increased production of intrarenal dopamine. Dopamine has 2 actions, both of which reduce sodium reabsorption. First, it causes retraction of NHE antiporters and Na-K-ATPase pumps into intracellular vesicles, thereby reducing transcellular sodium reabsorption. Second, it reduces the expression of AII receptors, thereby decreasing the ability of AII to stimulate sodium reabsorption.

Other Controllers of Sodium Excretion

ADH

When ADH binds to V2 reecptors in tubular cells, it increases the production of c-AMP. This results in increased activity of the NKCC multiporter in the thick ascending limb and increased sodium channel (ENaC) presence in principal cells of the distal nephron, thereby increasing the uptake of sodium that, in both regions, is actively transported into the interstitium by the Na-K-ATPase. Interestingly, in the distal nephron the mechanism proceeds, not by moving ENaCs into the membrane, but rather by decreasing their removal and degradation.

Glomerulotubular Balance

Glomerulotubular balance (not to be confused with TG feedback described previously) refers to the phenomenon whereby sodium reabsorption in the proximal tubule varies in parallel with the filtered load, such that approximately two thirds of the filtered sodium is reabsorbed even when GFR varies. The mechanism by which reabsorption varies with filtered load appears to be via mechanotransduction by the microvilli on the apical surface of the proximal tubule cells, similar in principle to mechanotransduction by primary cilia in the macula densa. As flow changes, the amount of bending of the microvilli changes, and this is converted by cellular mechanisms into changes in transport.

Pressure Natriuresis and Diuresis

Because the kidneys are responsive to arterial pressure, there are situations in which elevated blood pressure can lead directly to increased excretion of sodium. This phenomenon is called pressure natriuresis, and because natriuresis is usually accompanied by water, it is often called pressure diuresis. This is an intrarenal phenomenon, not requiring external signaling. However, external signals normally override pressure natriuresis.

Natriuretic Peptides

Several tissues in the body synthesize members of a hormone family called natriuretic peptides. Key among these are atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP). The main source of both natriuretic peptides is the heart. The natriuretic peptides have both vascular and tubular actions. The relax the afferent arteriole, thereby promoting increased filtration, and act at several sites in the tubule. They inhibit release of renin, inhibit the actions of AII that normally promote reabsorption of sodium, and act in the medullary collecting duct to inhibit sodium absorption. The major stimulus for increased secretion of the natriuretic peptides is distention of the atria, which occurs during plasma volume expansion. This is probably the stimulus for the increased natriuretic peptides that occurs in persons on a high salt diet.

Proximal Tubule Reabsorption and Secretion

November 4, 2015 Nephrology, Physiology and Pathophysiology No comments , , , , ,

dialysis-patient

 

Basic Transport Mechanisms

A similar discussion of this topic is available at http://www.tomhsiung.com/wordpress/2015/10/basic-mechanisms-of-renal-transepithelial-transport/

The kidneys are transport machines, moving a large arrary of substances between the renal tubules and the nearby network of blood vessels. The basic process of moving these substances (secretion and reabsorption) requires that solutes and water cross 2 cell layers: 1.the epithelium that makes up the walls of the tubules and 2.the endothelium that makes up the vascular walls. Substances must also traverse the thin region of interstitial fluid between them. In the cortex, where the fluxes of many filtered substances are enormous, the vascular entothelium (peritubular capillaries) is fenestrated. The fenestrae and the loose underlying basement membrane offer virtually no resistance to the passive movement of water and small solutes. This facile permeation has 2 consequences. First, the rate of transport is governed almost exclusively by events in the tubular epithelium rather than the vascular endothelium; second, the cortical interstitium, which is the medium faced by the basolateral membranes of the tubular epithelia, has an osmolality and concentration of small solutes very close to those in plasma.

In contrast, both blood flow and transport events are less rapid in the medulla. Only some regions of the medullary vasculature are fenestrated (i.e., ascending vasa recta), so that 1.overall transport depends on both the properties of the vascular endothelium and tubular epithelium, and 2.the medullary interstitium is most definitely not plasma-like in its composition.

Crossing the tubular epithelium can occur through the cells or around the cells (both reabsorption and secretion). The paracellular route is when the substance goes around the cells, that is, through the matrix of the tight junctions that link each epithelial cell to its neighbor. In most cases, however, a substance takes the transcellular route, a 2-step process through the cells. For reabsorption, this is entrance across the apical membrane facing the tubular lumen, through the cell cytosol, and then exit across the basolateral membrane facing the interstitium. For secretion the process is reversed.

Screen Shot 2016-07-08 at 9.11.44 PMAn array of mechanisms exists by which substanes cross the various barriers. These are no different from transport mechanisms used elsewhere in the body (Figure 4-2). The presence or absence of a given transport protein endows the tubular epithelium with selectivity, that is, the ability to choose which substance is permitted to move. Selectivity, obviously, applies to cell membranes containing different transport proteins. It also applies to paracelluar flux through tight junctions. Key tight junction proteins, members of the claudin family, determine the degree to which various substances can travel paracellularly.

Specific Transport Mechanisms (both paracellular route and transcellular route)

Movement by Diffusion

Diffusion is the frenzied random movement of free molecules in solution (like the Ping-Pong balls in a lottery drawing). Net diffusion occurs across a barrier (i.e., more molecules moving one way than the other) if there is driving force (a concentration gradient or, for charged molecules, a potential gradient) and if the barrier is permeable. This applies to almost all substances crossing the endothelial barrier lining the peritubular capillaries. It applies to substances taking the paracellular route around the tubular epithelium and to some substances taking the transcellular route through membranes. Small neutral molecules that are lipid soluble, such as the blood gases, alcohol, and steroids, can diffuse directly through the lipid bilayer.

Movement Through Channels

Most substances of biological importance cannot penetrate lipid membranes fast enough to meet cellular needs. To speed up the process their transmembrane flux is mediated by integral membrane proteins, which are divided into categories of channels and transporters. Channel and transporter proteins are not permanent fixtures in the membrane. Their lifetimes in the membrane are generally in the range of a few hours.

PS: Transmembrane movement via membrane proteins

Channels
1.Mechanosensitive channels
2.Voltage-sensitive channels
3.Chemosensitive channels
4.Others (e.g., channels that usually stay open that maintain resting potential, water channels, cell-cell channels that connect the cytoplasm of one cell with the cytoplasm of another)
Pumps
1.Na/K pump
2.Ca pump
3.H/K pump
4.F-type H pumps
5.V-type H pumps
Transporters
1.Uniporters
2.Symporters/Cotransporters
3.Antiporters/Exchangers
4.ABC transporters (active transport)

Channels represent a mechanism for rapid movement of specific substances across membrnes, which would otherwise diffuse slowly or not at all. Channels are small pores (proteins with a "hole" through the interior of the protein) that permit, depending on their structure, water or specific solutes to diffuse through them. Thus, we use the terms sodium channel and potassium channel to designate channels that permit diffusion of these molecular species. Aquaporins are channels that are permeable to water.

Channels typically flicker open and closed like camera shutters, so that the permeability of a membrane containing many channels is proportional to the number of channels and the probability of their being open. Movement through channels is passive, that is, no external energy is required. The energy to drive the diffusion is inherent in electrochemical gradient.

A characteristic of channels critical for renal function is the regulation of their permeability by a number of environmental factors and signaling cascades. First, many channel types can be gated, meaning that the probability that the channel is open can be increased or decreased. Second, many channel types have phosphorylation sites such that phosphorylation either locks the cahnnel shut or allows it to be gated by ligand, voltage, or stretch. Third, some channel species can be moved back and forth between the surface membrane and intracellular vesicles, thereby regulating the number of the existing channels actually functioning as permeability pathways. Fourth, and on a slower time scale, the genomic expression of channels is regulated so that the total number of channels, whether in the membrane or sequestered in vesicles, is altered up or down.

Movement by Transporters

Our genome codes for a large array of proteins that function as transporters, all with names and acronyms that suffuse the physiological literature. Transporters, like channels, permit the transmembrane flux of a solute that is otherwise impermeable in the lipid bilayer. Channels can move large amounts of materials across membranes in a short period of time, but most transporters have a lower rate of transport because the transported solutes bind much more strongly to the transport protein. Furthermore, the protein must undergo a more elaborate cycle of conformational change to move the solute from one side of the membrane to the other. However, overall flux rate depends not only on the kinetics of individual transporters, but also on the density of transporters in the membrane. Totoal flux via transporters can be very high if the transporter density is high. As is the case for channels, the amount of substance moved via transporters is highly regulated. The regulation includes changes in phosphorylation of the transporter (thereby turning its activity on or off), sequestration into vesicles, and of course, changes in genomic expression.

Receptor-Mediated Endocytosis and Transcytosis

In the case of receptor-mediated endocytosis and transcytosis, a solute, usually a protein binds to a site on the apical surface of an epithelial cell, and then a patch of membrane with the solute bound to it is internalized as a vesicle in the cytoplasm. Subsequent processes then degrade the protein into its constituent amino acids, which are transported across the basolateral membrane and into the blood.

For a few proteins, particularly immunoglobulins, endoctosis can occur at either the apical or basolateral membranes, after which the endocytic vesicles remain intact and are transported to the opposite cellular membrane, where they undergo exocytosis to release the protein intact.


Proximal Tubule

Almost all solutes (except large plasma proteins) are filtered from plasma into Bowman's space in the same proportion as water; thus, their concentrations in the glomerular filtrate are the same as in the plasma. By the end of the proximal tubule, about two third of the water and solutes have been reabsorbed. The rates of reabsorption, and thus concentrations in the lumen at the end of the proximal tubule, vary from solute to solute, but the summed total of solutes (osmoles) reabsorbed is proportional to water reabsorbed. This is called iso-osmotic reabsorption. In the later portions of the nephron, beyond the proxial tubule, reabsorption is generally not iso-osmotic, meaning that water and total solute reabsorption are usually not proportional. This is crucial for our ability to independently regulate solute and water balance.

Sodium and Water

Sodium accounts for nearly half of the total solute load appearing in the glomerular filtrate, and most of the rest consists of the anions (primarily chloride and bicarbonate) that must accompany sodium to maintain electroneutrality. Similarly, sodium and its accompanying anions account for the vast majority of solutes reabsorbed in the proximal tubule. The proximal tubule epithelium is very permeable to water, which follows the osmoles accross in equal proportions. Thus, both the fluid removed from the lumen and that remaining behind are essentially iso-osmotic with the original filtrate. We say "essentially" because there must be some difference in osmolality to induce water movement, but for an epithelial barrier like the proximal tubule that is very permeable to water, a difference of less than 1 mOsm/kg is sufficient to drive reabsorption of water (1 mOsm/kg is equal in driving force to 19.3 mm Hg of hydrostatic pressure). Once in the interstitium, the solutes and water move from interstitium into the peritubular capillaries and are returned to the systemic circulation.

Epithelial transprot requires that the cells be polarized, that is, the proteins present in the apical and the basolateral membranes are not the same. In the case of sodium, polarization of the proximal tubule epithelium promotes the net flux from lumen to interstitium. Movement of sodium is the linchpin around which the transport of virtually every other substance depends. Step 1 is the active extrusion of sodium from epithelial cell to interstitium across the basolateral membrane. Step 2 is the passive entrance of sodium from the tubular lumen across the apical membrane into the cell to replace the sodium removed in step 1. Step 3 is the parallel movement of anions that must accompany the sodium to preserve electroneutrality. Step 4 is the osmotic flow of water from tubular lumen to interstitium. Finally, step 5 is the bulk flow of water and solute from interstitium into the peritubular capillary.

Screen Shot 2015-10-23 at 9.36.08 PMOther Solutes

As water follows sodium and its anions across the epithelium, the luminal volume decreases, thereby concentrating all remaining solutes. If two thirds of the water is removed, any solute not previously removed will increase in concentration by a factor of 3. As its luminal concentration rises, this generates a concentration gardient across the tight junctions between the lumen and the interstitium (The interstitial concentration of transported substances is essentially clamped to the plasma value because of the high peritubular blood flow and high permeability of the fenestrated capillaries). If the tight junctions are permeable to the substance in question, the substance diffuses from the lumen to the interstitium and then into the peritubular capillaries along with sodium and water. This is precisely what happens to many solutes (e.g., urea, potassium, calcium, and magnesium) in the proximal tubule. The exact fractions that are reabsorbed depend on the permeabiltiy of the tight junctions, but are generally in the range of one half to two thirds.

Limits on Rate of Transport

Even though the transport capacity of the renal tubules is huge, it is not infinite. There are upper limits to the rate at which any given solute can be reabsorbed or secreted. In situations in which unusually large amounts of a substance are filtered, these limits are reached with the consequence that larger than normal amounts of solute are not reabsorbed. In general, transporter mechanisms can be classified by the properties of these limits as either 1.tubular maximum-limited systems or 2.gradient-limited systems.

The classification is based on the leakiness of the tight junctions. Consider first gradient-limited system. When the tight junctions are very leaky to a given substance, for example, sodium, it is impossible for the removal of the substance from the lumen to reduce its luminal concentration very much below that in the cortical interstitium. As the substance is removed and the luminal concentration starts to fall, the gradient between these 2 media increases, causing the substance to leak back as fast as it is removed. Thus for sodium and all other substances whose reabsorption is characterized by a gradient-limited system, the luminal concentration remains close to the interstitial concentration. Be aware that the existence of a limiting rate does not stop reabsorption in normal circumstances because water is being reabsorbed simultaneously. Even though the luminal concentration does not decrease very much, large amounts are stilling being removed. In contrast, if unusual osmotic conditions retard water reabsorption, then removal of the substance is not accompanied by a corresponding amount of water. Consequently its concentration does decrease, the limiting gradient is indeed reached, and now the substance leaks back, leaving unusually high amounts of the substance in the large volume of unreabsorbed water.

In the case of tubular maximum-limited systems the tight junctions are impermeable to the solutes in question. There is no back leak and no limit on the size of the difference in concentration between lumen and interstitium. The limit on transport rate instead is placed on the capacity of the transporters to remove the substance. As the filtered load rises, the amount reabsorbed increases in parallel up to the point of saturating the transporters. For loads below the Tm, virtually all is reabsorbed. But any increase in filtered load above the Tm does not increase transport out of the lumen. Consequently, the excess is left behind. In most cases, the amount that escapes reabsorption in the proximal tubule is excreted.

The functional reason for differentiating between Tm and gradient-limited systems is that solutes handled by Tm systems will, if the filtered load is below the Tm, be reabsorbed essentially completely, whereas solutes handled by gradient-limited systems are never reabsorbed completely, that is, a substantial amount always remains in the tubule to be passed on to the next nephron segment.