Peritubular Environment Research Articles

Medullary and cortical thick ascending limb: similarities and differences.

The thick ascending limb of the loop of Henle (TAL) is the first segment of the distal nephron, extending through the whole outer medulla and cortex, two regions with different composition of the peritubular environment. The TAL plays a critical role in the control of NaCl, water, acid, and divalent cation homeostasis, as illustrated by the consequences of the various monogenic diseases that affect the TAL. It delivers tubular fluid to the distal convoluted tubule and thereby affects the function of the downstream tubular segments. The TAL is commonly considered as a whole. However, many structural and functional differences exist between its medullary and cortical parts. The present review summarizes the available data regarding the similarities and differences between the medullary and cortical parts of the TAL. Both subsegments reabsorb NaCl and have high Na+-K+-ATPase activity and negligible water permeability; however, they express distinct isoforms of the Na+-K+-2Cl- cotransporter at the apical membrane. Ammonia and bicarbonate are mostly reabsorbed in the medullary TAL, whereas Ca2+ and Mg2+ are mostly reabsorbed in the cortical TAL. The peptidic hormone receptors controlling transport in the TAL are not homogeneously expressed along the cortical and medullary TAL. Besides this axial heterogeneity, structural and functional differences are also apparent between species, which underscores the link between properties and role of the TAL under various environments.

Médulla rénale

The ability to produce hyperosmotic urine allows mammals, including humans, to excrete their soluble mineral and organic waste products in the urine with a limited amount of water. The urinary concentrating capacity depends primarily on a special “loop-shaped” architecture of the nephrons and vessels, observed only in mammals. It also depends on the influence of antidiuretic hormone on the permeability to water of the collecting ducts on their entire length, and on the permeability to urea limited to the terminal portion of these ducts in the inner medulla. The ability to concentrate urine also requires (1) an “engine” able to produce an active (energy demanding) transport that generates a transepithelial concentration difference leading to increase the solute concentration in the surrounding interstitium; and (2) the expression of several membrane transporters or channels localized very specifically to limited portions of some nephron segments, collecting ducts and arterial vasa recta. These transporters and channels greatly accelerate the transport of water and some solutes across the cell membranes. Even if some nephron segments and parts of the collecting system are present in both the renal cortex and medulla (namely, the proximal tubule, the thick ascending limb and the collecting duct), their functions in the two renal zones may not be strictly similar, owing to their different peritubular environment, to the composition of the fluid running in their lumen, and to some differences in their epithelium. This paper describes some characteristics and specific functions of these structures in the renal medulla, as opposed to their corresponding structures in the cortex. These specific functions operating in the medulla may lead to various adverse consequences in some pathological situations.

Proximal reabsorption with changing tubular fluid inflow in rat nephrons.

The relative contribution of intraluminal versus peritubular factors in mediating glomerulo-tubular balance (GTB) is still controversial. We modulated the load of tubular fluid to the proximal tubule of single nephrons of rats by injecting oil into the efferent arteries (EAO). In fifty nephrons the changes in reabsorption induced by obstruction occurred in the same direction as, and were significantly correlated with, the simultaneous changes in single nephron glomerular filtration rate (SNGFR) (y = -0.54 + 0.92x, R = 0.91, P < 0.0001). In an additional set of thirty-nine nephrons the load of tubular fluid was changed, during EAO, by partial collection from Bowman's space or from the early proximal convolution. Thus, the rate of tubular fluid delivery along the proximal tubule was changed in an experimental situation that prevented any modification in the oncotic pressure of peritubular capillaries. The changes in proximal deliveries during this experimental condition were significantly correlated with those during reabsorption (y = -2.87 + 0.71x, R = 0.82, P < 0.0001). These data demonstrate that GTB is fully expressed even when the native peritubular environment is kept constant while the rate of perfusion of proximal tubular segments with native tubular fluid is changed.

Regulation of acid-base transport in the rat thick ascending limb.

The thick ascending limb of the rat influences urinary net acid excretion by reabsorbing both bicarbonate and ammonium. The bicarbonate absorption is mediated predominantly by apical membrane Na+-H+ exchange and occurs at rates that are comparable to or greater than rates measured in cortical and medullary collecting ducts. The ammonium absorption is mediated predominantly by apical membrane Na+-NH4+-2Cl- cotransport and enhances urinary ammonium excretion by promoting countercurrent multiplication of ammonium, which facilitates ammonium secretion into medullary collecting ducts. Studies with medullary thick ascending limbs (MTAL) in vitro have shown that the regulation of these transport processes involves both acute responses to changes in the luminal and peritubular environment and adaptive changes in tubule transport capacity in response to chronic systemic acid-base perturbations. In particular, an increase in potassium concentration inhibits ammonium absorption with no effect on net bicarbonate absorption whereas vasopressin inhibits bicarbonate absorption with no effect on net ammonium absorption. Chronic metabolic acidosis causes an adaptive increase in the ability of the MTAL to reabsorb both bicarbonate and ammonium. These results demonstrate that the MTAL is a site of regulation of renal acid-base transport and that ammonium and bicarbonate transport rates can vary independently in this nephron segment.

Topography of the rat blood-testis barrier after intratubular administration of intercellular tracers

Intratubular injection of electron-opaque tracers (lanthanum hydroxide, peroxidase) by micromanipulation showed that the intercellular spaces of the adluminal compartment are in continuity with the lumen of the rat seminiferous tubule at all stages of the spermatogenic cycle. This continuity involves the intercellular spaces which surround zygotene spermatocytes and late leptotene spermatocytes in stages X–XI. The present observations would seem to cast doubt on the real existence of a third compartment, or intermediate compartment, in the seminiferous epithelium. In the material which, in addition to intratubular administration, had been treated with intravascular tracer perfusion, we found the presence of a parabasal region of interSertoli junctions where the tracer stopped whatever its direction. Freeze-fracture replicas exhibit interSertoli tight junctions arranged, from basal lamina to lumen, in three well-defined patterns: in the most basal area, which is the zone of free penetration of tracers from the peritubular environment, junctions are very sparse or absent; a belt of closely apposed continuous and parallel junctions, also parallel to the basal lamina, is the exclusion zone which prevents tracer penetration from either direction; the most apical membrane areas display irregularly arranged, loosely spaced and frequently interrupted rows of particles, most tending to be perpendicular to the basal lamina. This area corresponds to the territory where the intraluminally injected tracer permeates the interSertoli space.

Electron microprobe analysis of proximal tubule cellular Na, Cl and K element concentrations during acute mannitol-saline volume expansion in rats: evidence for inhibition of the Na pump

It has previously been shown that during mannitol-saline volume expansion (VE) Na transport was inhibited 50% by harvested proximal tubular fluid without a change in paracellular shunt pathway permeability to Na. To determine whether this inhibition was due to changes in cellular entry step or an effect on the pump itself, intracellular element concentrations were measured by electron microprobe X-ray ranalysis in proximal tubular cells of control (non-expanded, NE) and VE rats. Nai, Cli and phosphorusi were increased (mean +/- S.E.) from 19.3 +/- 0.8 to 23.4 +/- 0.6, 15.8 +/- 0.4 to 21.3 +/- 0.4 and 124.3 +/- 2.6 to 138.0 +/- 1.8 mmol . kg-1 wet weight (P less than 0.001) respectively while Ki remained unchanged: 122.9 +/- 2.2 and 124.2 +/- 1.3 mmol . kg-1 wet weight. The increases in Nai and Cli were in excess of cell shrinkage produced by the hyperosmolal peritubular environment while the unchanged Ki in the face of cell shrinkage indicates and actual loss. It is concluded that mannitol-saline VE inhibits the Na pump producing a rise in Nai and a fall in Ki.

Intratubular and peritubular capillary hydrostatic and oncotic pressures after chronic renal sympathectomy in the anaesthetized rat.

The possible role of peritubular capillary physical forces in the diuretic-natriuretic effects of chronic renal denervation was investigated in Inactin-anaesthetized nondiuretic control (C) and unilaterally denervated (D) rats. Micropuncture techniques were combined with measurement of intratubular and peritubular capillary hydrostatic pressures and afferent and efferent arteriolar plasma oncotic pressures were determined, as well. Compared to data of C rats and of innervated kidneys, marked denervation diuresis and natriuresis were seen without changes in GFR. Both late proximal and early distal (F/P)In values were significantly lower in D kidneys with similar SNGFR. Afferent (pia) and efferent (pi e) arteriolar oncotic pressures were unchanged by denervation (C-pi a = 23.3 +/- 0.79, pi e = 29.9 +/- 0.87 mm Hg; D-pi a = 23.2 +/- 0.94, pi e = 29.8 +/- 1.04 mm Hg). Proximal intratubular hydrostatic pressure was moderately but significantly higher in D kidneys (C = 11.9 +/- 0.5, D = 13.7 +/- 0.3 mm Hg, P less than 0.01), while peritubular capillary pressures were: efferent arteriole (C = 13.9 +/- 0.5, D = 13.4 +/- 0.6 mm Hg, NS). It is concluded that the tubular effects of chronic renal sympathectomy are not dependent on changes in Starling forces of the peritubular environment.

Membrane mechanisms for transepithelial amino acid absorption and secretion.

Although amino acids are normally almost completely absorbed by the proximal nephron, specific amino acids may also exhibit active secretion under the appropriate circumstances. This brief review examines details of the influx and efflux transport processes across both lumina and basolateral membranes of the proximal tubule and through the paracellular pathway as determinants of the direction and rate of transepithelial amino acid transport. Net absorption depends upon active uptake from the lumen into tubular cells by cotransport with Na+. At least some proximal tubule cells also actively accumulate amino acids from their peritubular environment by active transport across basolateral membranes. The net direction of transepithelial movement depends on the relative rates of passive amino acid exit from the cell across luminal and basolateral membranes. Since exit across the basolateral membrane appears to be facilitated whereas luminal exit is not, normally net absorption occurs. The minimum luminal amino acid concentration and excretion rate are also dependent on a relatively low paracellular amino acid permeability, especially in the latest segments of the proximal nephron where the transepithelial concentration gradient is the greatest.

Functional profile of the isolated uremic nephron. Role of compensatory hypertrophy in the control of fluid reabsorption by the proximal straight tubule.

An in vitro approach to the study of single nephron function in uremia has been employed in evaluating the control of fluid reabsorption by the renal superficial proximal straight tubule (PST). Isolated segments of PSTs from the remnant kidneys of uremic rabbits (stage III) were perfused in vitro and their rate of fluid reabsorption compared with normal PSTs and with PSTs derived from the remnant kidneys of nonuremic rabbits (stage II). All segments were exposed to a peritubular bathing medium of both normal and uremic rabbit serum thereby permitting a differentiation to be made between adaptations in function which are intrinsic to the tubular epithelium and those which are dependent upon a uremic milieu.Compared with normal and stage II PSTs, there was significant hypertrophy of the stage III tubules as evidenced by an increase in length and internal diameter, and a twofold increase in the dry weight per unit length. Fluid reabsorption per unit length of tubule was 70% greater in stage III than in normal and stage II PSTs, and was closely correlated with the increase in dry weight. Substitutions between normal and uremic rabbit serum in the peritubular bathing medium did not affect fluid reabsorption significantly in any of the three groups of PSTs. Perfusion of the tubules with an ultrafiltrate of normal vs. uremic serum likewise failed to influence the rate of net fluid reabsorption. It has previously been observed that net fluid secretion may occur in nonperfused or stop-flow perfused normal rabbit PSTs exposed to human uremic serum. Additional studies were thus performed on normal and stage III PSTs to evaluate whether net secretion occurs in the presence of rabbit uremic serum. No evidence for net secretion was found. These studies demonstrate that fluid reabsorption is greatly increased in the superficial PST of the uremic remnant kidney and that this functional adaptation is closely correlated with compensatory hypertrophy of the segment. Humoral factors in the peritubular environment do not appear to be important mediators of the enhanced fluid reabsorption.

The rational approach to the management of oedema

Oedema results from excessive reabsorption of filtered sodium for the renal tubules. The reabsorption is not due to intrinsic alterations in tubular function but secondary to changes in the peritubular environment. Diuretics however act by blocking reabsorption in the tubules themselves. The mechanisms by which they produce their effects are still poorly understood. Advances in knowledge of the patterns of sodium transport at different parts of the nephron have enabled us to localize their sites of action in man. Application of this knowledge in patients with resistant oedema enables us to decide how to alter the type or dose of diuretic or diuretic combination.

Variation of proximal tubular reabsorptive capacity by volume expansion and aortic constriction during constancy of peritubular capillary protein concentration in rat kidney.

The present study was undertaken in anesthetized rats to examine the effect of aortic constriction and volume expansion on proximal tubular sodium reabsorption during constancy of flow and fluid composition in the peritubular circulation of the kidney. Efferent arterioles and branch capillaries were perfused at 625 nl/min with an artificial perfusate containing 9 g per oiter of albumin both before and during either aortic constriction or saline infusion. Results of recollection micropuncture studies in those tubules surrounded by artificially perfused capillaries were compared with results in control tubules in which the peritubular capillary flow and fluid composition were allowed to change during aortic constriction or volume expansion following saline infusion. Changes in single nephron filtration rate, fractional and absolute reabsorption induced by both aortic constriction and saline infusion were found to be qualitatively and quantitatively comparable in tubules with constant peritubular capillary microperfusion and in the control tubules with changing peritubular capillary environment due to the experimental maneuvers. Taken together, therefore, the present results indicate that with the use of the peritubular microperfusion technique no evidence is found to support a role of alteration in the peritubular environment in modulating the effect of aortic constriction or saline infusion on tubular sodium reabsorption in the rat nephron. Rather, these results provide indirect evidence in support of intraluminal factors as mediating these responses in tubular reabsorption to volume expansion and aortic constriction.

A Model of Peritubular Capillary Control of Isotonic Fluid Reabsorption by the Renal Proximal Tubule

A mathematical model of peritubular transcapillary fluid exchange has been developed to investigate the role of the peritubular environment in the regulation of net isotonic fluid transport across the mammalian renal proximal tubule. The model, derived from conservation of mass and the Starling transcapillary driving forces, has been used to examine the quantitative effects on proximal reabsorption of changes in efferent arteriolar protein concentration and plasma flow rate. Under normal physiological conditions, relatively small perturbations in protein concentration are predicted to influence reabsorption more than even large variations in plasma flow, a prediction in close accord with recent experimental observations in the rat and dog. Changes either in protein concentration or plasma flow have their most pronounced effects when the opposing transcapillary hydrostatic and osmotic pressure differences are closest to equilibrium. Comparison of these theoretical results with variations in reabsorption observed in micropuncture studies makes it possible to place upper and lower bounds on the difference between interstitial oncotic and hydrostatic pressures in the renal cortex of the rat.

Role of distal reabsorption and peritubular environment in glomerulotubular balance.

Total kidney glomerulotubular balance was examined during aortic constriction and release in saline-loaded dogs and in dogs undergoing water diuresis. Aortic constriction lowered the glomerular filtration rate by 45% in both groups, and glomerulotubular balance, as judged by changes in absolute sodium reabsorption, was also comparable. During water diuresis, a linear relationship was observed between free water clearance and urine flow during all maneuvers, suggesting that distal sodium reabsorption is related primarily to distal delivery. The results suggest that if alterations in the peritubular environment are responsible for the changes in tubular sodium reabsorption during aortic constriction in the saline- or water-loaded dog, then a change in renal plasma flow, and presumably delivery rate of oncotic force, may be the most likely mediator.