Rate Of Fluid Reabsorption Research Articles

Altered blood pressure, renal tubular function and levels of circulating inhibitors of Na-K-ATPase in mature SHR exposed to low salt diet in the perinatal period.

1. It has been shown previously that low salt diet, confined to the perinatal period only, reduces blood pressure (BP) and sodium retention in spontaneously hypertensive rat (SHR) offspring at maturity when compared with those given high salt diet. 2. In this study it is shown that such high salt diets are associated with an increased level of circulating Na-K-ATPase inhibitor (CINK) activity. 3. Animals given perinatal high salt diet have a significantly greater tubular reabsorptive capacity when compared with those given low salt diet. 4. The finding of a high level of circulating Na-K-ATPase inhibitory material in the face of increased renal tubular capacity and blood pressure suggests that while this inhibitory material may play a role in the elevated blood pressure of animals given high salt diet, it cannot cause the elevated rate of fluid reabsorption.

Effects of β-Adrenergic Blockade on the Glomerular and Tubular Response to Acute Renal Denervation

Using micropuncture techniques in euvolemic adult male Munich-Wistar rats, we assessed the functional role of renal beta-adrenoceptors in mediating neural control of glomerular filtration and proximal tubular reabsorption. The determinants of nephron filtration and rate of proximal tubular reabsorption were measured in two groups of animals before and after acute surgical renal denervation (DNX). Group A animals (n = 6) were pretreated with the beta-adrenoceptor antagonist propranolol (25 mg/kg body weight per day for 4-6 days). Group B animals (n = 7) served as non-beta-blocked controls. Acute renal DNX resulted in no significant change in nephron filtration rate or any of its determinants in either group. Acute DNX caused similar decrements in the rate of fluid reabsorption from the proximal convoluted tubule of beta-blocked and control rats. Loop of Henle fluid reabsorption did not appear to be affected by DNX in either group. Because the effect of denervation on proximal tubular reabsorption was not conditioned by prior beta-blockade, the beta-adrenoceptors present within the proximal convoluted tubule do not appear to be the primary mediators of the adrenergic influence on fluid transport in that segment of the nephron.

Peritoneal transport in CAPD patients with permanent loss of ultrafiltration capacity

During a 10 year period, 14 out of 227 patients (6.2%) undergoing continuous ambulatory peritoneal dialysis (CAPD) developed permanent loss of ultrafiltration capacity (UFC). The risk of UFC loss increased from 2.6% after one year to 30.9% after six years of treatment. A six hour, single dwell study with glucose 3.86% dialysis fluid was carried out in nine of the UFC loss patients and in 18 CAPD patients with normal UFC. Intraperitoneal dialysate volumes were calculated using 131I-tagged albumin (RISA) as volume marker with a correction applied for its elimination from the peritoneal cavity. The RISA elimination coefficient (KE), which can serve as an estimation of the upper limit of the lymphatic flow, was also calculated. Diffusive mass transport coefficients (KBD) for investigated solutes (glucose, creatinine, urea, potassium, total protein, albumin and beta 2-microglobulin) were calculated during a period of dialysate isovolemia. Two patterns of UFC loss were observed: (a) seven patients had high KBD values for small solutes resulting in rapid uptake of glucose, whereas KBD values for proteins were normal; (b) two patients had normal KBD values but a threefold increase both in the fluid reabsorption rate and KE. We conclude that loss of the osmotic driving force (due to increased diffusive mass transport for small solutes) and increased fluid reabsorption (possibly due to increased lymphatic reabsorption) are the two major causes of permanent loss of UFC in CAPD patients.

Osmotic forces driving water reabsorption in the proximal tubule of the rat kidney.

Simultaneous microperfusion of proximal tubules and peritubular capillaries in kidneys of rats anesthetized with Inactin was used to measure reabsorption of fluid in response to an imposed osmotic gradient. The tubular fluid was isotonic and the peritubular capillaries were made hypertonic with NaCl or NaHCO3. Mean gradients and ionic fluxes were measured. When no gradient was imposed tubular fluid became hypotonic and rate of fluid reabsorption was 0.700 nl.mm-1.min-1. Imposition of a 25 mM NaCl gradient increased fluid flux to 3.887 nl.mm-1.min-1, whereas 25 mM NaHCO3 stimulated 5.226 ml/mm fluid reabsorption. This gave a relative reflection coefficient of NaCl:NaHCO3 of 0.73. Apparent water permeability varied with highest values for the smallest gradients. This suggests the possibility of a compartment in the epithelium that is hypertonic to the peritubular capillaries. The hypertonicity required to account for fluid movement was 6-16 mosmol/kg.

New evidence for active sodium transport from fluid-filled rat lungs.

The hypothesis that fluid reabsorption from the air spaces is mediated at least in part by active transport of Na+ was investigated in six sets of experiments conducted in isolated fluid-filled rat lungs. Fluid reabsorption was monitored by following the changes in the air space concentration of labeled albumin. We found that incorporation of bicarbonate rather than a nonvolatile buffer (N-2-hydroxy-ethylpiperazine-N'-2-ethanesulfonic acid) in the air space solution more than doubled the rate of fluid reabsorption. Addition of 10(-4) M amiloride to the air space solution reduced the rate of fluid reabsorption over a 2-h experiment from 1.2 +/- 0.1 to 0.7 +/- 0.1 ml and decreased reabsorption of both labeled and unlabeled Na+ from the air spaces. To show that Na+ could be reabsorbed from the air spaces even if the concentrations of Na+ in the perfusate increased above those in the air space, mannitol (150 mM) was added to the perfusate and air space solutions and the concentrations of Na+ and Cl- were reduced to 90 and 60 mM, respectively. Mannitol diffuses across the pulmonary epithelium very slowly, and it osmotically restrained the movement of water out of the air spaces. Na+ concentrations in the perfusate increased by 10 +/- 2 mM, but concentrations in the air space remained unchanged. Despite an increasingly unfavorable concentration gradient for Na+, 0.2 mmol Na+ and 0.6 ml water were reabsorbed from the air spaces in 2 h. Ouabain (10(-4) M) did not appear to slow fluid reabsorption in the presence of mannitol, but it reduced K+ secretion into the air spaces and increased K+ appearance in the perfusate in a manner consistent with inhibition of Na+-K+-adenosinetriphosphatase at the basolateral surface of the epithelial cells. Fluid reabsorption was not altered when the lungs were exposed to a hypotonic solution (185 mM), but secretion of K+ into the air spaces was accelerated and K+ was lost from the perfusate. These experiments are consistent with active Na+ transport from the air spaces.

Peritoneal Ultrafiltration and Fluid Reabsorption During Peritoneal Dialysis

Peritoneal ultrafiltration and fluid reabsorption characteristics for 18 patients undergoing continuous ambulatory peritoneal dialysis (CAPD) were investigated in single dwell studies of 6 h duration with 21 of 3.86% glucose dialysis fluid. Dialysate volumes were determined in situ using radioiodinated serum albumin (RISA) as volume marker with a correction applied for the total elimination of RISA from the peritoneal cavity. The RISA elimination rate was calculated as 2.1 +/- 0.5 ml/min. The true dialysate volume after 360 min was on average 28% less than the apparent volume calculated without correction for the elimination of RISA. The mean maximum true volume plus sampling losses was 3255 ml at 240 min, corresponding to a mean net ultrafiltration volume of 762 ml between 3 min and 240 min. The mean net fluid reabsorption rate between 240 min and 360 min was 1.2 +/- 0.7 ml/min. This study of standard dialysate volume/time curves in clinically stable CAPD patients using hypertonic dialysis fluid shows that about 90% of the total net ultrafiltration is achieved during the first 90 min of the dwell. After an extended period of dialysate isovolaemia, usually lasting as long as between 120 min and 240 min, fluid reabsorption is observed in all patients.

Changes in glomerular filtration rate, lithium clearance and plasma protein clearances in the early phase after unilateral nephrectomy in living healthy renal transplant donors.

1. Glomerular and tubular function was studied before and 2 months after unilateral nephrectomy in 14 healthy kidney donors by measurement of the clearances of 51Cr-labelled ethylenediaminetetra-acetate, lithium, beta 2-microglobulin, albumin and immunoglobulin G. 2. The glomerular filtration rate (GFR) of the kidney that remained in the donor rose from 45 +/- 10 (mean +/- SD) to 59 +/- 10 ml/min (P less than 0.01) 5 days after contralateral nephrectomy and remained at this level through the observation period. 3. The lithium clearance (CLi) of the remaining kidney rose from 11.6 +/- 3.7 to 20.5 +/- 8.2 ml/min (P less than 0.01) and remained significantly elevated throughout the observation period. 4. Absolute proximal fluid reabsorption rate (APR), which was estimated as GFR minus CLi, was unchanged 5 days after contralateral nephrectomy, but then rose gradually to reach significantly elevated levels after 4 weeks. 5. Fractional proximal reabsorption (FPR; APR/GFR) fell from 0.75 +/- 0.06 to 0.66 +/- 0.11 (P less than 0.01) but subsequently rose to levels not significantly decreased from normal. 6. Twenty-four hour fractional clearances of beta 2-microglobulin, albumin and immunoglobulin G rose markedly on the day of nephrectomy, peaked at 2-3 days and subsequently fell to moderately elevated levels. 7. Both the CLi and the plasma protein clearance studies demonstrate that the early response of the remaining kidney to contralateral nephrectomy in man is an increase in GFR, an unchanged APR and a fall in FPR. The proximal tubules thus initially handle the increased filtrate load by passing it on to more distal nephron segments.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural Differentiation and Fluid Reabsorption in the Ductuli Efferentes Testis of the Rat

The ductuli efferentes testis of the rat form a cord which is embedded in adipose tissue. The cord is anatomically differentiated into a proximal cylindrical region, the initial zone, and an ampulla, the coni vasculosi. The initial zone contains six or seven ductuli which leave the rete testis and run in a sinuous path, roughly parallel with one another. However, the ductuli in the coni vasculosi are more sinuous than in the initial zone and they anastomose; pairs join together to form ultimately a single, common ductulus efferens. Stereological studies of paraffin sections and electron micrographs showed that the differentiation of the ductuli into two parts can be recognized at tissue and cellular levels of organization. Stereological and micropuncture studies showed that the ductuli efferentes reabsorb most of the fluid leaving the testis and it was concluded that most reabsorption occurred in the initial zone. It was estimated that the rate of fluid absorption is greater in the ductuli efferentes than in the proximal convoluted tubules of the kidney. The mechanism of fluid transport across the mucosa of the ductuli is considered in the Discussion. It is concluded that transport in vesicles and vacuoles could not account for the rate of fluid reabsorption and that the main mechanism of transport probably involves the coupling of water and active salt transport.

Application of cold flush preservation to in vitro microperfusion studies of kidney tubules

The technique of microperfusion in vitro of isolated segments of the nephron, first described by Burg eta! [1], has led to major advances in our understanding of nephron function. The technique has been applied to all segments of the nephron, many of which are inaccessible to other forms of study (see Kidney mt 22(5), 1982). Two limiting factors that have influenced the application of this technique are: (1) the ease with which a tubule segment can be dissected from kidney tissue and (2) the viability of the tubule segment during perfusion, particularly when proximal segments are perfused. Many laboratories have limited their observations to New Zealand white rabbits, which possess kidneys that are more readily dissectable than those of other mammals; even so, the occurrence of readily dissectable kidneys varies considerably. Kidney tissue is customarily cooled for dissection to minimize the development of ischemic tubule damage [21; nevertheless, it is common practice to use another rabbit if the dissection time exceeds 30 to 60 mm. We have recently made use of a technique that permits extension of the dissection time and provides several other practical advantages. The technique involves the application of a modified cold preservation procedure similar to that used in kidney transplantation. It arises from a new type of cold flush solution which consists of a phosphate buffer with added sucrose (PBSuc) shown to provide excellent preservation of tubule morphology in rat kidneys [3]. Our observations suggest that it also offers effective preservation of function as judged by measurements of fluid reabsorption rate (Jv), transmural potential difference (PD), leak of iothalamate from lumen to bath, and the responses to removal of intraluminal glucose and alanine.

Epithelial water transport in a balanced gradient system

The relationship between epithelial fluid transport, standing osmotic gradients, and standing hydrostatic pressure gradients has been investigated using a perturbation expansion of the governing equations. The assumptions used in the expansion are: (a) the volume of lateral intercellular space per unit volume of epithelium is small; (b) the membrane osmotic permeability is much larger than the solute permeability. We find that the rate of fluid reabsorption is set by the rate of active solute transport across lateral membranes. The fluid that crosses the lateral membranes and enters the intercellular cleft is driven longitudinally by small gradients in hydrostatic pressure. The small hydrostatic pressure in the intercellular space is capable of causing significant transmembrane fluid movement, however, the transmembrane effect is countered by the presence of a small standing osmotic gradient. Longitudinal hydrostatic and osmotic gradients balance such that their combined effect on transmembrane fluid flow is zero, whereas longitudinal flow is driven by the hydrostatic gradient. Because of this balance, standing gradients within intercellular clefts are effectively uncoupled from the rate of fluid reabsorption, which is driven by small, localized osmotic gradients within the cells. Water enters the cells across apical membranes and leaves across the lateral intercellular membranes. Fluid that enters the intercellular clefts can, in principle, exit either the basal end or be secreted from the apical end through tight junctions. Fluid flow through tight junctions is shown to depend on a dimensionless parameter, which scales the resistance to solute flow of the entire cleft relative to that of the junction. Estimates of the value of this parameter suggest that an electrically leaky epithelium may be effectively a tight epithelium in regard to fluid flow.

Cortical and papillary absorptive defects in gentamicin nephrotoxicity

Renal function was examined in rats given daily injections of gentamicin (100 to 150 mg/kg) for 10 to 14 days. Whole kidney inulin clearance fell and urine volume increased. Single nephron GFR of surface nephrons varied. Some nephrons had no filtration, some had low rates, and some had high rates. Abnormal renal tubular epithelial inulin permeability was demonstrated by microinjection. Micropuncture of individual nephrons early and later in their course demonstrated reduced fluid reabsorption along the proximal convoluted tubule of superficial nephrons. Rates of fluid delivery to the late proximal and distal tubule were elevated. The rate of fluid reabsorption in the superficial loop of Henle was increased. Maximal urine osmolality and papillary tissue content of urea was reduced. The polyuria, therefore, results from decreased fluid reabsorption by proximal tubules and, probably, by papillary collecting ducts. The decrease in proximal fluid reabsorption is probably secondary to impaired solute reabsorption. A decrease in collecting duct fluid absorption can be attributed to the observed decrease in papillary solute concentration.

The effect of peritubular protein upon fluid reabsorption in rabbit proximal convoluted tubules perfused in vitro.

This paper describes an investigation of the effects of varying the peritubular protein concentration upon the rate of fluid reabsorption of proximal convoluted tubule segments of the rabbit kidney, isolated and perfused in vitro. Eleven moderately distended tubules, bathed with rabbit serum (60 g l.-1 protein) and perfused with an ultrafiltrate of serum (ca. 0 g l.-1) protein, reabsorbed fluid at a rate of 1.0 +/- 0.07 nl. mm-1 min-1. When the protein of the bathing solution was reduced by replacing the serum with ultrafiltrate there was little change in fluid reabsorption rate. A further eleven moderately distended tubules, bathed with physiological saline containing bovine serum albumin (60 g l.-1) and perfused with saline, reabsorbed fluid at 1.0 +/- 0.06 nl. mm-1 min-1. There was little change in fluid reabsorption when the protein concentration was reduced to either 20 or 0 g l.-1; these tubules responded to 10(-5) M-ouabain in the bath, or a temperature of 25 degrees C, by reducing their absorption rate to 0.13 +/- 0.05 nl. mm-1 min-1. Nine minimally distended tubules reabsorbed fluid at 0.8 nl. mm-1 min-1 when bathed with physiological saline containing bovine serum albumin (60 g l.-1) and perfused with saline. Under these circumstances reabsorption rate fell by 26% when protein was removed from the bath. A study of the pressure-diameter relationship was made for eight tubules typical of those used in this laboratory. The probable range of lumen hydrostatic pressures was discussed for the distended and undistended states. We conclude that the effect of peritubular protein concentration upon fluid reabsorption in proximal convoluted tubule segments perfused in vitro is dependent upon some property of the tubule wall that is changed when distension occurs.

A mathematical model of fluid transport in the kidney.

A mathematical model of the rat kidney is developed from glomerular and tubular submodels. It is assumed that all nephrons are identical, that the hydraulic pressure in the tubules obeys Hagen-Poiseuille's law, that the rate of fluid reabsorption depends on the flow rate of tubular fluid, and that the tubules are distensible. The independent variables of the model are selected to comply with experimental measurements in the hydropenic rat. The model is used to evaluate the mechanism of glomerulotubular balance: changing the mean ultrafiltration pressure to the glomerular capillaries has a substantial influence on glomerular filtration rate (GFR). A change in the rate of fluid reabsorption in the proximal tubules has a strong influence on GFR notwithstanding that the change in GFR is smaller than that in the rate of fluid reabsorption. The calculated values for the hydraulic pressure profile in the tubular system and the interstitial pressure during ureteral obstruction are in close agreement with experimental measurements. Increasing the arterial haematocrit above normal causes a substantial decrease in GFR, whilst reducing it below normal has only a small effect on GRF.

A Micropuncture Study of Transepithelial Water Reabsorption in Rat Ductuli Efferentes

Micropuncture techniques were used to study transepithelial water reabsorption in single tubular sections of rat ductuli efferentes in vivo and in situ. Fluid reabsorption was found to be secondary to an electrolyte transport process. It was completely abolished by intraluminal application of 10(-4) M amiloride and by an elevated local temperature of 38 degrees C. The fluid reabsorption rate was modified by the intraluminal electrolyte concentration, whereas various local temperatures from 37 degrees C to room temperature had no effect. This finding led to the conclusion that the function of the ductuli efferentes may require temperatures lower than body temperature, which is in accordance with the well-known behavior of mammalian testes. It was further suggested that the described fluid reabsorption leads to concentrations of spermatozoa and of substrates which spermatozoa may need to develop and mature.

Effect of catecholamines on fluid reabsorption by the isolated proximal convoluted tubule.

The effect of L-norepinephrine and isoproterenol on fluid transport was studied in superficial convoluted segments and straight portions of the rabbit proximal tubule (PCT and PR, respectively) by the technique of microperfusion in vitro. In PCT, L-norepinephrine (10(-6) M) added to the bath stimulated reversibly fluid reabsorption (Jv) by about 67%. In the presence of propranolol (10(-4) M) in the bath, L-norepinephrine caused a small (about 19%) but significant decrease of Jv. Phentolamine and isoproterenol added simultaneously to the bath also increased fluid reabsorption, an effect that was abolished by propranolol. Norepinephrine had no effect on Jv when added to the perfusate in PCT. No change of Jv was observed after addition of norepinephrine to the bath in PR. The effects on fluid reabsorption rate observed in PCT are consistent with a physiologic role of the sympathetic nervous system in the modulation of PCT fluid transport.

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.

EFFECTS OF CATECHOLAMINES AND ADRENERGIC BLOCKADE ON FLUID REABSORPTION IN ISOLATED RAT CAUDA EPIDIDYMIDIS

The rate of fluid reabsorption in the cauda epididymidis of rat has been measured in vitro. Both adrenaline and isoprenaline produced a prompt, reversible and dose dependent increase in the reabsorption rate. These effects were completely blocked by propranolol. The response to noradrenaline consisted of two components. In the presence of an alpha blocker, noradrenaline caused an increase, while in the presence of a beta blocker, it produced an inhibition in the rate of fluid reabsorption. The effects of these adrenergic agents were only observed when sodium ions were present in the intraluminal fluid, suggesting that they only affect the Na+-dependent component of fluid reabsorption. The possibility that they may affect the active transport of sodium in terms of the presence of alpha and beta receptors in the epithelium of the rat cauda epididymidis.

Hormonal regulation of fluid reabsorption in isolated rat cauda epididymidis.

The hormonal dependence of fluid reabsorption has been studied in isolated rat cauda epididymidis. In a group of experiments, rats were castrated by removal of the testes but leaving the epididymis with the blood supply intact. The rate of fluid reabsorption measured 3 weeks after castration fell from the control value of 2.10 ± 0.10 μl/cm2/30 min to 0.82 ± 0.12 μl/cm2/min (P < 0.001). When testosterone propionate (2 mg/kg/day) was injected into castrated rats for 7 days, the rate of fluid reabsorption was restored. This suggests that fluid reabsorption in rat cauda epididymidis is dependent on the presence of circulating androgen. Adrenalectomy in rats did not affect the rate of fluid reabsorption or did treatment of the epididymal ducts with aldosterone (10−6M) in vitro. Both LH (5 μg/ml) and FSH (5 μg/ml) produced a small but significant increase in the rate of fluid reabsorption. Of the other hormones studied, vasopressin (10 mU/ml) increased the reabsorption rate by 15%. Acetylcholine (10−5M) produce...

Fluid reabsorption in the isolated duct of the rat cauda epididymidis

A technique is described fro measurement of rate of fluid reabsorption in a segment of the rat cauda epididymidis in vitro. The basal rate was 2-63 +/- 0-22 mul/cm2/30 min and was dependent on temperature and inhibited by 2,4-dinitrophenol, suggesting that fluid reabsorption is an energy-dependent process. About 50% of the fluid reabsorption was dependent on intraluminal sodium ions. This Na+-dependent component was inhibited by addition of amiloride to the intraluminal fluid and ouabain to the peritubular fluid.

Inhibition by amiloride of sodium-dependent fluid reabsorption in the rat isolated caudal epididymis.

The rate of fluid reabsorption was studied in the rat isolated caudal epididymal sac in vitro. 2 Part of the fluid reabsorption was found to be dependent on intraluminal Na+. Amiloride (0.1 mM) completely inhibited this component of fluid reabsorption. 3 The log dose-inhibition curve to amiloride was sigmoid and the IC50 value was found to be 1.6 muM.