Active Sodium Reabsorption Research Articles

Bicarbonate and chloride transport in relation to the acidification or alkalinization of the urine.

Although the electrogenicity of the active reabsorption of sodium and bicarbonate (or secretion of protons) has been well-established in the short-circuited turtle bladder preparation, the nature of the active reabsorption of chloride and secretion of bicarbonate has been controversial. These processes have been ascribed to the separate actions of two discrete electrogenic pumps or to the single action of a metabolically driven electroneutral anion exchange mechanism. The present report deals with these transport processes per se; with the relations among serosal bicarbonate,glucose, and the reabsorption of Na and Cl; and finally with the application of the Heinz model for inversion of an active transport as a tentative single mechanism for the alkalinization and acidification of the urine.

Effects of Anion-Transport Inhibitors on NaCl Reabsorption in the Rat Superficial Proximal Convoluted Tubule

The effects of anion-transport inhibitors on volume reabsorption, and total CO(2) concentrations were examined by in vivo microperfusion of superficial proximal convoluted tubules of rats. The luminal perfusion solution was a high-chloride, low-bicarbonate solution like that in the in vivo late proximal tubule. The anion-transport inhibitors were only added to the luminal perfusion solutions. In tubules perfused with the control high-chloride solution, the rate of volume reabsorption (J(v)) was 2.3+/-0.2 nl/mm.min (n = 18), and the collected total CO(2) concentration was 4.0+/-0.3 mM. Furosemide (3 mM) caused a marked reduction in volume reabsorption to 0.8+/-0.3 nl/mm.min (n = 20) and only a slight increase in the total CO(2) concentration of collected samples of perfusate (7.8+/-0.5 mM). 0.8 mM acetazolamide caused a more pronounced rise in the collected total CO(2) concentrations to 10.7+/-0.5 mM but only a slight fall in J(v) to 1.7+/-0.3 nl/mm.min (n = 19). Hence, we inferred that inhibition of carbonic anhydrase only partially accounted for the inhibition of J(v) by furosemide. 4-acetamido-4'-iso-thiocyanato-stilbene-2,2'-disulphonic acid (0.1 mM), a well-characterized inhibitor of erythrocyte anion exchange mechanisms, also reduced J(v) to 1.6+/-0.3 nl/mm.min (n = 15) without changing the total CO(2) concentrations of the collected perfusates (3.6+/-0.4 mM). The effect of 4-acetamido-4'-iso-thiocyanato-stilbene-2,2'-disulphonic acid on volume reabsorption could not be explained by carbonic anhydrase inhibition because there was no increase in the total CO(2) concentration of the collected fluids. Furosemide did not significantly inhibit the rate of tracer glucose efflux out of the tubules, which suggests that the effect of furosemide on volume reabsorption was not a result of some nonspecific depression of active sodium transport. These results are discussed with respect to the possible effects of anion-transport inhibitors on the paracellular shunt pathway, active sodium reabsorption, and neutral sodium chloride transport.

Organic mercurial diuresis: Inhibition of glutamine utilization in the acidotic rat

Organic mercurials inhibit mitochondrial glutamine metabolism in vitro while metabolic acidosis, a condition in which the predominant renal fuel is glutamine, potentiates mercurial diuresis. The following studies were undertaken to determine whether potentiation of diuresis reflects mercurial inhibition of glutamine utilization. (1) All three mercurials employed (mersalyl, chlormerodrin, and p-chloromercuribenzoate) are diuretics in the rat and this effect was potentiated by NH4Cl. (2) Despite reabsorbing less sodium, mercurial-treated rats had lower kidney ATP content (4.35 +/- 0.26 and 3.84 +/- 0.43 mumol/g dry weight (mercurial plus NH4Cl) than did controls (4.95 +/- 0.31 and 4.87 +/- 0.39 mumol/g dry weight (NH4Cl). (3) Isolated kidneys from NH4Cl and NH4Cl plus mercurial treated rats were perfused with 1 mM L-[U-14C]glutamine to determine rates of extraction and oxidation. Mercurial-treated acidotic rat kidneys had a reduced rate of glutamine uptake (40.8 +/- 7.4 vs. 64.8 +/- 5.8 mumol/h per kidney), a diminished rate of glutamine conversion to CO2 (14.8 +/- 3.6 vs. 26.4 +/- 5.2 mumol/h per kidney), and a reduction in glucose production (16 +/- 5 vs. 27 +/- 4 mumol/h per kidney). These results are consistent with an effect of organic mercurials upon glutamine utilization, limiting ATP availability, and thereby reducing tubular active sodium reabsorption.

Influence of cystic fibrotic and heterozygous serum on rat jejunum.

SummaryThe rat jejunum bioassay has been shown to be capable of identifying both the cystic fibrotic homozygote and heterozygote and therefore would appear to have great potential as a screening test for the latter. It is suggested that the cystic fibrosis factor does not influence sodium reabsorption by inhibiting an active sodium reabsorption mechanism but rather by inhibiting passive sodium entry into the cell.

Effect of intraluminal amiloride on Na transport in the rat proximal tubule.

Amiloride (3,5-diamino-6-chloropyrazinoyl-guanidine) was found to inhibit the unidirectional flux of 22Na in perfused proximal tubules of rats when the drug was present in the blood (1). The drug did not inhibit the net water reabsorption, therefore, it apparently did not have an affect on the active reabsorption of sodium. Since amiloride has been found to be a potent inhibitor of sodium reabsorption in the amphibian bladder and skin (2, 3) and since it has been found by the same authors that the inhibition of sodium transport is about one thousand times higher when the drug is on the source than when it is on the sink site of the sodium transport, experiments were designed to test the effect of the drug when present in the luminal and not in the interstitial site of the tubule cell. Methods. Sprague-Dawley rats weighing about 200 g were anesthetized with Inactin (100 mg/kg body wt.) and then infused through the right jugular vein with Ringer's solution at a rate of 30 ml/hr-kg. Using a modification of the microperfusion technique and apparatus of Sonnenberg and Deetjen (4), proximal tubules were perfused at a rate of 20 nanoliters per min. Two pumps were used, one contained a control solution (Ringer's with 22Na and 14Cinulin) and the other contained the same solution plus 10-4 amiloride. A selected proximal tubule was first perfused with the control solution for 30-40 rnin, and 2-4 samples were obtained during this period. Then the control pump was withdrawn and substituted by the “amiloride” pump. Perfusion was restarted at exactly the same location. The amiloride perfusion was administered for 45-60 min during which time 2-4 samples were obtained. The use of the 2 pumps was randomized with reference to their use for control or for amiloride perfusions.

Computer simulation of osmotic gradient without active transport in renal inner medulla

Computer simulation of osmotic gradient without active transport in renal inner medulla. It has been generally supposed that the osmotic gradient observed in the inner medulla of mammalian kidneys during antidiuresis results from countercurrent multiplication of active sodium reabsorption from thin ascending limbs of Henle. However, disagreement persists on whether the thin ascending limb can transport sodium actively. An alternative model has been proposed by Kokko and Rector. In this model water reabsorption from the descending limb of Henle creates a high tubular concentration of sodium at the bend of the loop, so that sodium can move passively out of the ascending thin limb. The present report shows that computer simulation of this model can indeed result in an osmotic gradient in the inner medulla, on condition that sodium movement out of the thin ascending limb results in the tubular fluid becoming hyposmotic to adjacent interstitium. This can be achieved if the descending limb is permeable to water but relatively impermeable to urea and electrolytes, and the ascending limb is less permeable to water and urea than to electrolytes. In this model, a key role is played by urea which provides a driving force for water reabsorption from the descending limb. The importance of urea is illustrated in the model, firstly by simulation of a low-protein diet, and secondly by simulation of water diuresis. In both situations, reduced reabsorption of urea from the late collecting ducts causes a reduced interstitial urea concentration, which in turn causes a severe reduction in interstitial electrolyte concentration.

Potassium and sodium transport across single distal tubules of Amphiuma.

The transport properties of potassium (K) and sodium (Na) were studied in single distal tubules of Amphiuma using free-flow micropuncture techniques and stationary microperfusion methods. The transepithelial movement of labeled potassium was measured utilizing a three-compartment system in series in which the time course of tracer disappearance from the lumen was followed. Under control conditions, in blood- and doubly-perfused kidneys, extensive active net reabsorption of sodium and potassium obtains along single distal tubules. Tubular potassium reabsorption is abolished by ouabain at a concentration of 5 x 10(-6)M. Significant net secretion of K can be induced by exposing Amphiuma to a high K environment (100 mM KCl) or by adding acetazoleamide (1 x 10(-4)M) to the perfusion fluid. Transepithelial movement of potassium involves mixing with only a small fraction of total distal tubular cell potassium. This transport pool of potassium increases significantly with the transition from tubular net reabsorption to net secretion. Indirect evidence is presented which indicates that increased active K uptake across the peritubular cell boundary may be of prime importance during states of net K secretion.

Micropuncture study of sodium and potassium excretion in the rat parotid saliva.

Having demonstrated that the rat parotid gland is able to excrete a hypotonic saliva, we studied the formation of saliva in the different segments of the duct system, through micropuncture and microanalytical techniques. The duct system of the gland is composed of three functionally different segments, acini-intercalated ducts, striated ducts and excretory ducts. a) Acini-intercalated ducts secrete a primary fluid with plasma-like osmolality and sodium and potassium concentration. b) Striated ducts modify the primary fluid by active reabsorption of sodium in excess of water, thus producing hypotonicity. c) Excretory ducts serve as conduits of the saliva to the mouth. A process of reequilibration of the duct fluid to isotonicity occurs in the excretory ducts of the resting gland. Secretion of small amounts of potassium also appears to take place in these ducts.

Sodium transport and oxygen consumption in the mammalian kidney.

IN the isolated frog skin, it was found by Zerahn1 and by Leaf and Renshaw2 that the amount of sodium actively transported had a fixed relationship to the net oxygen consumption (sodium/net oxygen ratio), as about 18 sodium ions were transported per molecule of oxygen consumed under a variety of experimental conditions. Net oxygen consumption was defined as the difference between the total oxygen uptake and the basal uptake, that is, the oxygen uptake when no sodium transport took place. In the mammalian kidney, active re-absorption of sodium (anions and water following passively) represents on a molar basis the overwhelming part of all tubular transport processes. On the assumption that sodium re-absorption does therefore require the bulk of the oxidative energy supplied, we have determined the sodium/net oxygen ratio in the intact kidney of the dog.

Water diuresis stop flow analysis in the dog.

SummaryDuring water diuresis stop flow-analysis the nephron volume is less than during osmotic diuresis stop flow analysis, there is a demonstrable increase in osmolal U/P in the distal nephron samples, and sodium U/P minimum occurs near the PAH peak. During water diuresis stop flow analysis active proximal reabsorption of sodium is demonstrable or para-aminohippurate is not a valid marker of proximal nephron samples.

Localization of renal calcium transport; effect of calcium loads and of gluconate anion on water, sodium and potassium.

We have performed a first series of experiments using our stop flow technique to localize renal transport of calcium in dogs. Calcium is found to be actively reabsorbed in a far distal area but does not appear to be actively lowered in concentration in proximally derived samples. In a second series of dogs, elevation of plasma calcium by infusion of the salt CaCl2 caused a 43% reduction in the additional volume of water reabsorbed by the proximal tubule during stop flow. Calcium does not interfere with the active reabsorption of sodium in the distal area and does not cause significant increases in sodium excretion. In contrast calcium causes a considerable increase in potassium secretion at a far distal site. When calcium gluconate was employed, severe impairment of distal sodium and calcium reabsorption occurred in addition to the increased potassium secretion. We suggest that the gluconate anion restricts the movement of sodium, calcium and potassium electrostatically thus obligating cations to remain in the tubular urine depending upon their reabsorption potential.

Development of an isolated perfused mammalian kidney

This study describes the development and functional evaluation of an artificial heart-lung perfusion system in which whole blood is used to perfuse the isolated rabbit kidney. Data are presented which indicate that this preparation is capable of attaining normal or near normal function at some time during the course of the experiment in at least 40% of the last 26 experiments. Functional evaluation of the perfused kidney is made by measuring the observed renal blood flow, rate of urine flow, and extraction ratios and clearances of creatinine and PAH. In a limited number of experiments, it has been shown that in the isolated perfused kidney, there is considerable active reabsorption of glucose and sodium. Furthermore, the same response to drugs and hormones, known to affect kidney function in the intact animal (papaverine, renin and Pitressin), has been demonstrated.

Glutaminase Activity of the Fœtal Membranes and Kidneys of Pigs

AT a Ciba Foundation Colloquium on the effects of ageing on electrolyte metabolism, Widdowson and McCance1 reported the pH of allantoic fluid to be between 5.5 and 6 at 46 and 67 days of gestation. Ammonia accounted for about 25 per cent of the “titratable acidity + ammonia” of these fluids. The total osmolar concentration of the allantoic fluid had been shown before to be only one-third to one-half that of fœtal serum at the same times of gestation, mainly due to the low concentration of sodium in the fluid2. When, therefore, Widdowson and McCance found that the acidity of allantoic fluid was largely due to high concentrations of carbonic acid, they suggested that active sodium re-absorption by hydrogen-ion exchange might be occurring in the fœtal membranes and kidneys. In support of this, they reported considerable carbonic anhydrase activity in the fœtal kidneys and membranes lining the allantoic sac.

Single proximal tubules of the Necturus kidney. II. Effect of 2, 4-dinitro-phenol and ouabain on water reabsorption.

The effect of ouabain and 2,4-dinitrophenol on water reabsorption from the proximal tubule of the kidney of Necturus maculosus has been studied using the collection and stopped flow perfusion techniques previously described. The intravenous administration of ouabain in collection experiments results in a decrease in water reabsorption from 32.7% (normal controls) to 15.4%. In stopped flow perfusion experiments on control animals, water reabsorption after 20 minutes is 27.2%. When 2,4-dinitrophenol or ouabain is added to the perfusion fluids, values of 10.2% and 9.8%, respectively, are observed. The experimental findings demonstrate that water reabsorption in the proximal tubule is inhibited by these agents. The results are in agreement with the hypothesis of active reabsorption of sodium from the proximal tubule of the kidney of Necturus.

Graphical placement of transport segments along the nephron from urine concentration pattern developed with stop flow technique.

The authors characterize transport within segments at points along the nephron by the concentration pattern developed along the renal tubule during stopped flow (brief ureteral occlusion during osmotic diuresis in dogs). The pattern is caught in serial urine samples on reinstatement of flow. They here develop a system of graphical construction which places active transport segments of proper length and position to account for the concentration patterns developed for the substances: Sodium, glucose, phosphate and para-aminohippurate (PAH). The system is based on mixing phenomena as nephrons of different length deliver their fluid content into mixed samples. Glucose reabsorption, phosphate reabsorption and PAH secretion occur in identical proximal segments, containing 45% of the total stop flow nephron volume. A far distal area for very effective active sodium reabsorption contains about 29% of the occluded nephron volume. The sodium concentration pattern does not delineate its own proximal segment but forms a plateau in that area. This results from the fact that proximal concentrations are not lowered during stop flow but remain the same as in new flow and free flow proximal fluid, no lower than the 30–60 mm concentration of free flow urine itself, considerably higher if distal removal of Na occurs during the new outflow.

The Composition and Origin of the Foetal Fluids of the Pig

ABSTRACT Many of the unicellular organisms have a greater concentration of osmotically active material within their body fluids than there is in the medium surrounding them, and the higher animals have glands like the kidney and sweat glands which are able to elaborate fluids with much lower concentrations of sodium and chloride than the body-fluids from which they were derived. The concentrations of sodium and chloride in hypotonic urine and sweat are much lower than those in the serum, and consequently the total osmolar concentrations are also lower, but the urea is characteristically higher. There is no proof that the process is one involving the secretion of water; and, indeed, the kidney is now thought to produce a hypotonic urine by the active reabsorption of sodium from the distal tubule, the walls of which are impermeable to water unless there is posterior pituitary hormone in the circulation.