Effect of changes in renal perfusion pressure on the suppression of proximal tubular sodium reabsorption due to saline loading

Regulation of Epithelial Cell Functions by the Osmolality and Hydrostatic Pressure Gradients: A Possible Role of the Tight Junction as a Sensor

Epithelia act as a barrier to the external environment. The extracellular environment constantly changes, and the epithelia are required to regulate their function in accordance with the changes in the environment. It has been reported that a difference of the environment between the apical and basal sides of epithelia such as osmolality and hydrostatic pressure affects various epithelial functions including transepithelial transport, cytoskeleton, and cell proliferation. In this paper, we review the regulation of epithelial functions by the gradients of osmolality and hydrostatic pressure. We also examine the significance of this regulation in pathological conditions especially focusing on the role of the hydrostatic pressure gradient in the pathogenesis of carcinomas. Furthermore, we discuss the mechanism by which epithelia sense the osmotic and hydrostatic pressure gradients and the possible role of the tight junction as a sensor of the extracellular environment to regulate epithelial functions.

Dominant factors that govern pressure natriuresis in diuresis and antidiuresis: a mathematical model

We have developed a whole kidney model of the urine concentrating mechanism and renal autoregulation. The model represents the tubuloglomerular feedback (TGF) and myogenic mechanisms, which together affect the resistance of the afferent arteriole and thus glomerular filtration rate. TGF is activated by fluctuations in macula densa [Cl(-)] and the myogefnic mechanism by changes in hydrostatic pressure. The model was used to investigate the relative contributions of medullary blood flow autoregulation and inhibition of transport in the proximal convoluted tubule to pressure natriuresis in both diuresis and antidiuresis. The model predicts that medullary blood flow autoregulation, which only affects the interstitial solute composition in the model, has negligible influence on the rate of NaCl excretion. However, it exerts a significant effect on urine flow, particularly in the antidiuretic kidney. This suggests that interstitial washout has significant implications for the maintenance of hydration status but little direct bearing on salt excretion, and that medullary blood flow may only play a signaling role for stimulating a pressure-natriuresis response. Inhibited reabsorption in the model proximal convoluted tubule is capable of driving pressure natriuresis when the known actions of vasopressin on the collecting duct epithelium are taken into account.

Hormonal regulation of salt and water excretion: a mathematical model of whole kidney function and pressure natriuresis

We present a lumped-nephron model that explicitly represents the main features of the underlying physiology, incorporating the major hormonal regulatory effects on both tubular and vascular function, and that accurately simulates hormonal regulation of renal salt and water excretion. This is the first model to explicitly couple glomerulovascular and medullary dynamics, and it is much more detailed in structure than existing whole organ models and renal portions of multiorgan models. In contrast to previous medullary models, which have only considered the antidiuretic state, our model is able to regulate water and sodium excretion over a variety of experimental conditions in good agreement with data from experimental studies of the rat. Since the properties of the vasculature and epithelia are explicitly represented, they can be altered to simulate pathophysiological conditions and pharmacological interventions. The model serves as an appropriate starting point for simulations of physiological, pathophysiological, and pharmacological renal conditions and for exploring the relationship between the extrarenal environment and renal excretory function in physiological and pathophysiological contexts.

Proximal bicarbonate reabsorption during Ringer and albumin infusions in the rat

Several studies have clearly shown that extracellular volume expansion is associated with suppression of whole kidney bicarbonate reabsorption, although little is known concerning the single nephron correlates of this response. More recently, attention has also been focussed on bicarbonate transport in attempts to identify a possible role for this ion in enhancing the rate of net fluid efflux by proximal tubules. To further explore proximal tubular bicarbonate handling in the rat, we carried out recollection micropuncture studies to assess the effects of infusions of modified Ringer or salt-poor hyperoncotic human albumin. With stable levels of arterial PCO2, plasma [HCO3-] or plasma [K+], marked suppression of fractional HCO3- reabsorption occurred: during Ringer infusion fractional reabsorption fell by 31% (P less than 0.001) while during albumin infusion a decrease of 20% (P less than 0.001) was observed. Despite this, absolute net HCO3- reabsorptive rates did not change significantly. Simple and partial correlation analysis of single tubular responses revealed strong linkage effects between changes in absolute net reabsorptive rates for HCO3- and H2O in both types of infusion; the partial r was 0.91 (P less than 0.001) and 0.94 (P less than 0.001) during Ringer and albumin infusions, respectively. We conclude that under these free-flow conditions, Ringer and albumin infusions do not suppress absolute net HCO3- reabsorption by proximal tubules, and that strongly linked changes in absolute HCO3- and H2O fluxes are characteristic of both protocols.

Intrarenal mechanisms of salt retention after bile duct ligation in rats

In order to study renal salt-retaining mechanisms during the early stages of ascites formation, rats were subjected to bile duct ligation. After this procedure, plasma volumes were found to be reduced and hematocrits slightly increased. The whole-kidney glomberular filtration rate and plasma flows were reduced to 59 and 57% of control values, but the filtration fraction was unchanged. Absolute sodium excretion, as well as the fraction of the filtered sodium load excreted, was also significantly reduced. When micropuncture techniques were used to examine the function of single superficial nephrons, the glomerular filtration rate in these nephrons was found to be reduced to 70% of controlled values, and fractional reabsorption was found to be increased at all accessible sites along the nephron. Filtration by intermediate and juxtamedullary nephrons, determined by Hanssen's technique, was reduced to 55 and 48% of control values. By the use of radioactive microspheres, it was demonstrated that blood flow to superficial, intermediate, and juxtamedullary nephrons was reduced to 49, 59, and 73% of control values. Filtration by superficial nephrons decreased much more than plasma flow--a finding which suggests that the measured increase in fractional reabsorption was associated with an increase in the superficial nephron filtration fraction. From this study, it appears that two factors play an important part in the sodium retention observed in the initial stages of ascites formation following bile duct ligation in rats: (a) a decrease in the filtered sodium load and (b) increased fractional reabsorption by the superficial nephrons--the nephrons which show the least decrease in filtration.

The effect on total renal and tubular function and plasma renin of a moderate isotonic saline load in rats anesthetized with amytal and inactin

The renal effects of i.v. saline loading equal to 1% body weight (b.wt.) were studied in 2 groups of rats: group I was anesthetized with Amytal (15 mg/100 g b.wt. plus supplementary doses), group II with Inactin (12.5-15.0 mg/100 g b.wt.). In group I the saline load caused an increase in urine flow (+92%), solute excretion (+67%), inulin clearance (CIN) (+24%), PAH clearance (+31%) and absolute proximal reabsorption rate (+27%). Proximal fractional reabsorption and filtration fraction (FF) remained unchanged, while plasma oncotic pressure (COP) decreased by 10%. Plasma renin fell and there was an inverse relationship between renin concentration and proximal reabsorption rate. In group II urine flow and solute excretion increased after saline (+85% and 110%, respectively); CIN and absolute proximal reabsorption rate was lower than in group I and failed to increase after saline. Proximal fractional reabsorption was also lower and decreased after saline. COP as well as renin decreased as in group I, but no relationship between renin concentration and proximal reabsorption rate could be demonstrated. The data indicate that Inactin depresses both resting proximal tubular reabsorptive capacity and the tubular response to a physiological volume expansion. The results are compatible with the hypothesis that the renin-angiotensin system is significantly involved in regulation of proximal tubular function, while they are incompatible with the idea that peritubular COP plays any major role in this adjustment.

Pressure control of sodium reabsorption and intercellular backflux across proximal kidney tubule

The magnitude of changes in luminal hydrostatic pressure (DeltaP(L)), peritubular capillary hydrostatic pressure (DeltaP(PT)), and peritubular capillary colloid osmotic pressure (Deltapi) was determined in the Necturus kidney during volume expansion (VE). The specific effects of separate changes of each pressure parameter on proximal net sodium transport (J(Na)) were studied in isolated perfused kidneys. The combined effect of DeltaP(L), DeltaP(PT), and Deltapi, of a magnitude similar to that induced by volume expansion, decreases J(Na) by 26% in the perfused kidney. A major portion of the natriuresis in VE is due to changes in intrarenal pressures. The effect of Deltapi on the permeability characteristics of Necturus proximal tubule was studied. With increasing Deltapi, the ionic conductance of the paracellular shunt pathway decreased, since transepithelial input and specific resistance rose significantly, whereas cellular membrane resistance remained unchanged. Transepithelial permeability coefficients for sodium chloride and raffinose changed inversely proportional to transepithelial resistance, indicating an alteration of a paracellular permeation route. Net passive sodium backflux and active transport flux components were calculated. Increased net sodium transport with rising Deltapi is accompanied by a significant drop in passive back diffusion, without an increment in the active flux component. Change in passive sodium ion back diffusion thus appears to be a key physiological factor in the control of transepithelial sodium transport.

The mechanism of decreased intestinal sodium and water absorption after acute volume expansion in the rat

Studies were performed in rat small intestine in vivo to determine the effect of saline infusion on intestinal transport of Na(+) and H(2)O. Saline infusion decreased net Na(+) flux (J(n) (Na)) from 12.7 +/-0.8 to 6.4 +/-1.5 muEq/hr per cm in the jejunum when the intestinal perfusate contained both Na(+) and glucose. A similar fall in J(n) (Na) occurred in ileum. When mannitol was substituted for glucose in the perfusate, control absorption decreased 29% in jejunum and 18% in ileum, but saline infusion still caused a decrease in J(n) (Na) quantitatively similar to that seen when glucose was present. When choline was substituted for Na(+) in the perfusate, there was net movement of Na(+) from blood to lumen during control and this net secretion was increased further after saline infusion. These observations suggest that saline infusion has a similar effect to decrease intestinal J(n) (Na) under three widely different conditions of basal sodium transport. Permeability of intestinal mucosa to inulin was very low under basal conditions but increased fivefold after saline infusion, and the unidirectional flux of Na(+) from blood to lumen doubled. This increase in unidirectional flux of Na(+) was greater than the observed decrease in J(n) (Na).Thus, saline infusion decreased net absorption of Na(+) and H(2)O from small intestine through mechanisms which did not appear to be dependent upon the rate of Na(+) flux from lumen to blood, and in association with an increased flux of inulin and Na(+) into the intestinal lumen. The data suggest that the effect of saline infusion to decrease net absorption from the intestine could be due either to an increase in passive permeability of the epithelium which could disrupt solute gradients within the membrane or to an increase in flow of solution into the intestinal lumen.

Factors involved in antinatriuretic effects of acute constriction of the thoracic and abdominal inferior vena cava

Studies were performed on anesthetized saline-loaded dogs to delineate the factors involved in the antinatriuretic effect of acute constriction of the thoracic (TIVC) and abdominal (AIVC) inferior vena cava. Acute TIVC constriction lowered cardiac output and arterial blood pressure and reduced urinary sodium excretion 48%; the same degree of reduction in renal perfusion pressure by suprarenal aortic constriction diminished sodium excretion only 27%. This finding suggested that factors other than a reduction in renal perfusion pressure are involved in the antinatriuretic effect of acute TIVC constriction. This conclusion was confirmed by studies in which a significant antinatriuretic effect (23%) was observed during acute TIVC constriction while renal perfusion pressure and renal venous pressure were held constant. A similar degree of constriction of the AIVC, as judged by the increase in vena cava pressure, did not cause a decrease in arterial pressure or cardiac output, and the modest antinatriuretic effect of this maneuver was not observed when renal venous pressure was held constant. The antinatriuretic effect of TIVC constriction which occurred in the absence of changes in renal perfusion pressure and renal venous pressure did not correlate with alterations in renal vascular resistance, glomerular filtration rate, and calculated initial postglomerular protein concentration and was not abolished by renal denervation or adrenalectomy. The afferent stimulus for the antinatriuretic effect may be related to the alterations in systemic hemodynamics which occur during acute TIVC, but not AIVC, constriction.

The relative contributions of reabsorptive rate and redistributed nephron filtration rate to changes in proximal tubular fractional reabsorption during acute saline infusion and aortic constriction in the rat

The absolute rate of reabsorption by superficial rat proximal tubules was measured by the in situ microperfusion technique under conditions of hydropenia, infusion of saline, and infusion of saline plus aortic constriction sufficient to decrease whole kidney filtration rate below hydropenic levels. Fractional reabsorption was measured in adjacent filtering nephrons by collecting and recollecting tubular fluid from late proximal convolutions during each experimental condition. During hydropenia, the absolute rate of proximal tubular reabsorption averaged 3.56 +/-0.60 nl/min per mm and late proximal tubular fractional reabsorption averaged 0.56 +/-0.10. From these two measurements and measurements of tubule length to the site of micropuncture, a value for filtration rate was calculated for filtering nephrons. During hydropenia this value averaged 32.9 +/-7.1 nl/min. Saline infusion increased sodium excretion to 5.5% of the filtered load as the absolute rate of proximal tubular reabsorption decreased 38% and fractional reabsorption decreased 45%. Calculated superficial nephron filtration rate increased 21% which on the average was identical with the simultaneously measured increase in whole kidney filtration rate. Similar results were obtained in a separate group of animals by the technique of total collection of late proximal tubular fluid. Aortic constriction during saline infusion decreased whole kidney and calculated nephron filtration rate to the same degree and to values lower than those during hydropenia. Fractional reabsorption increased but not to hydropenic values. The persistent natriuresis during aortic constriction was associated with a continued depression of the absolute rate of proximal tubular reabsorption which was sufficient to maintain an increased delivery of filtrate out of the proximal tubule despite the fall in nephron filtration rate. These results indicate that depressed fractional reabsorption in the proximal tubule during acute saline infusion is due predominantly to a decrease in absolute reabsorptive rate and to a lesser extent to an increase in superficial nephron filtration rate which is proportional to the increase in whole kidney filtration. Continued natriuresis when filtration rate is decreased during saline infusion can be accounted for entirely by the persistent large reduction in the absolute rate of proximal tubular reabsorption.

Accelerated reabsorption in the proximal tubule produced by volume depletion

The renal response to chronic depletion of extracellular volume was examined using the techniques of micropuncture. Depletion of salt and water was produced by administration of furosemide to rats maintained on a sodium-free diet. There was a marked fall in body weight, plasma volume, and glomerular filtration rate. The intrinsic reabsorptive capacity of the proximal tubule, measured by the split-droplet technique, was greatly enhanced. The acceleration of proximal fluid reabsorption could not be accounted for by changes in filtration rate, tubular geometry, or aldosterone secretion. The half-time of droplet reabsorption in the distal tubule was not altered by sodium depletion. An increase in the reabsorption of fluid in the proximal tubule, as demonstrated directly in the present experiments, provides an explanation for a variety of clinical phenomena associated with volume depletion.

Effect of saline infusions on intrarenal distribution of glomerular filtrate and proximal reabsorption in the dog

The effect of acute extracellular volume expansion with saline on the intrarenal distribution of glomerular filtrate, was studied in dogs utilizing micropuncture techniques in which samples were obtained by both recollection and from new tubules. Recollection was examined in seven dogs during continuous hydropenia and in five dogs during continuous saline diuresis. Recollection was associated with an increase in nephron flow rate of 8% during hydropenia and 27% during saline diuresis. In addition, during continuous saline diuresis, shortened transit times and lowered intratubular pressures were recorded in previously punctured tubules. Despite increased tubular flow, fractional reabsorption was unchanged. Nephron glomerular filtration rates (gfr) were measured during hydropenia and then after acute volume expansion in 10 dogs. In the repunctured tubules gfr rose 38% more than total glomerular filtration rate (GFR). In contrast, when new tubules were punctured during volume expansion, nephron gfr and total GFR changed proportionately. The disproportionate rise in nephron gfr after volume expansion noted with the recollection technique appears to be artifactual when contrasted to micropuncture of new tubules. With acute volume expansion, fractional reabsorption decreased 15% in recollected samples and 16% in newly sampled tubules. Increased nephron gfr cannot account for the fall in fractional reabsorption. It is concluded that in dogs, saline diuresis is not associated with redistribution of filtrate from deep to superficial nephrons, and that the fall in proximal fractional reabsorption is caused by diminished absolute reabsorption.

A microperfusion study of sucrose movement across the rat proximal tubule during renal vein constriction

Constriction of the renal vein has been shown to inhibit net sodium and water reabsorption by the rat proximal tubule. The mechanism is unknown but might be the result of inhibition of the active sodium pump induced by changes in the interstitial fluid compartment of the kidney, or to enhanced passive backflux of sodium and water into the cell or directly into the tubular lumen. Since passive movement of solutes across epithelial membranes is determined in part by the permeability characteristics of the epithelium, an increase in the permeability of the proximal tubule during venous constriction would suggest that enhanced passive flux is involved in the inhibition of reabsorption. In the present experiments, isolated segments of rat proximal convoluted tubules were microperfused in vivo with saline while the animals were receiving (14)C-labeled sucrose intravenously. In normal control animals, no sucrose was detected in the majority of the collected tubular perfusates. In rats with renal vein constriction (RVC), however, sucrose consistently appeared in the tubular perfusates. The rate of inflow of sucrose correlated with the length of the perfused segment, estimated by fractional water reabsorption. In another group of animals with renal vein constriction, inulin-(14)C was given intravenously and the proximal tubules similarly microperfused. Inulin did not appear in the majority of collected perfusates in these animals. These observations indicate that a physiological alteration in the permeability of the proximal tubule occurs during RVC. Such an increase in permeability is consistent with the view that enhanced passive extracellular back-flux plays a role in the reduction of net sodium and water reabsorption in this experimental condition.

The renal regulation of sodium transport

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Effects of hematocrit on renal hemodynamics and sodium excretion in hydropenic and volume-expanded dogs

The effects of hematocrit on renal hemodynamics and sodium excretion were studied in anesthetized dogs during both hydropenia and volume expansion. The hematocrit was decreased by isovolemic exchange with the animal's own previously harvested plasma and increased by isovolemic exchange with fresh, washed red blood cells. Renal perfusion pressure was maintained constant throughout the experiments by the adjustment of a suprarenal aortic clamp. During hydropenia, a decrease in hematocrit was associated with an increase in sodium and potassium excretion and solutefree water reabsorption. These changes were accompained by an increase in renal plasma flow and renal blood flow and a decrease in renal vascular resistance. Glomerular filtration rate was unchanged and filtration fraction was significantly decreased as hematocrit was lowered. Increasing hematocrit during hydropenia had the opposite effects on electrolyte excretion, solute-free water reabsorption, and renal hemodynamics. In another group of animals, hematocrit was lowered during volume expansion with either saline or plasma, then returned to the control level by isovolemic exchange with washed red blood cells. This increase in hematocrit during volume expansion had a similar effect on electrolyte excretion, solute-free water reabsorption, and renal hemodynamics as during hydropenia. These results therefore suggest that acute changes in hematocrit may significantly affect sodium excretion and renal hemodynamics during both hydropenia and volume expansion. The changes in solute-free water reabsorption and potassium excretion suggest that the alterations in hematocrit may affect primarily the reabsorption of sodium in the proximal tubule. The concommitant effects of hematocrit on renal vascular resistance and filtration fraction may mediate this change in sodium reabsorption by altering hydrostatic and oncotic pressures in the peritubular circulation.

Natriuretic activity in plasma and urine of salt-loaded man and sheep

The present study was designed to examine the question of whether or not there is a natriuretic hormonal substance involved in the renal regulation of sodium balance. For this purpose, procedures for concentration and fractionation of plasma and urine samples and a sensitive bioassay for demonstrating changes in renal sodium excretion were developed. The natriuretic assay utilized rats with mild diabetes insipidus which were maintained in salt and water balance. Using these approaches a natriuretic humoral substance was demonstrated in plasma and urine from normal man and sheep, and in patients with primary aldosteronism or essential hypertension. It seems likely that this substance participates in day to day regulation of sodium balance because it was not detectable in sodium-depleted subjects and it consistently appeared in the sodium-loaded subjects. The hormonal agent may not act immediately and its activity can be apparent for up to 3 hr. Full expression of its activity requires that the assay animals be appropriately volume expanded. This suggests that the increases in sodium excretion mediated by this hormonal substance depend in part on the coparticipation of other physical and perhaps humoral factors. This natriuretic substance appears to be of large molecular weight or carried by a large molecule. The data suggest that it acts, at least in part, to block sodium reabsorption in a more distal portion of the tubule.

Increased sodium reabsorption in the proximal and distal tubule of caval dogs

The effects of water diuresis, hypotonic NaCl, and hypotonic mannitol diuresis on renal sodium and water excretion were examined in normal dogs and in dogs with chronic constriction of the thoracic inferior vena cava and ascites (caval dogs). During all three diuretic states, the capacity to excrete solute-free water relative to the supply of sodium to the water clearing segment of the nephron was significantly greater in the caval dog. This finding was most evident during hypotonic NaCl diuresis but was also striking during hypotonic mannitol diuresis despite the more unfavorable gradient for sodium reabsorption at the distal tubule produced by this agent in caval dogs. In addition, fractional distal sodium load was significantly smaller in caval dogs during water diuresis and could not be increased as readily as in normal dogs by hypotonic NaCl or mannitol infusion. The data indicate that fractional sodium reabsorption is increased at the water clearing segment and the proximal tubule in caval dogs. The differences in the pattern of free water clearance and tubular sodium transport between normal and caval dogs could not be easily explained by alterations in renal hemodynamics or aldosterone secretion. It is suggested that in the caval dog an alteration occurs in other factors which might influence renal tubular sodium transport, such as intrarenal hemodynamics, renal interstitial volume or pressure, or a natriuretic hormone, leading to increased tubular sodium reabsorption.