Net Sodium Transport Research Articles

Inhibition of sodium transport by prostaglandin E2 across the isolated, perfused rabbit collecting tubule.

This study was designed to examine whether prostaglandin E2 can directly affect sodium transport across isolated perfused rabbit renal collecting tubules. Changes in transepithelial potential and isotopic sodium fluxes in response to peritubular prostaglandin E2 were measured. In addition, changes in transepithelial potential of the outer medullary collecting tubule in response to prostaglandin E2 were also measured. With few exceptions, all rabbits received 5 mg/day desoxycorticosterone acetate for 4-11 days before experimentation. The results of the experiments show that: (a) prostaglandin E2 inhibits the negative transepithelial potential in the cortical collecting tubule as well as the outer medullary collecting tubule; (b) prostaglandin E2 inhibits net sodium transport out of the lumen by inhibiting efflux while backflux is unaffected; (c) prostaglandin E2 produces this inhibition within 15 min, and the effects are dose dependent and reversible. These results suggest that prostaglandin E2 may modulate sodium transport in vivo and may contribute to the final regulation of sodium excretion.

Bicarbonate ions in active sodium transport across toad bladder

The conductance, ga, and electromotive force, E, of active ion transport across toad bladders mounted as sacs were estimated from electrical measurements made before and after addition of sufficient ouabain (1.89 X 10(-3) M) to eliminate spontaneous potential. The ratio of net sodium transport (estimated from bidirectional fluxes) to external current in bladders voltage clamped to 0 mV was significantly less than unity in a normal medium containing HCO3- and Cl- ions, and also when Cl- was replaced with SO42-. However, when acetazolamide was added or when HCO3- was replaced by phosphate, short-circuit current and net sodium transport became equal. Spontaneous potential, E, and ga were all reduced about 20% by these maneuvers. The response of bidirectional sodium fluxes to voltage clamping at 0 mV or 120 mV in a bicarbonate-free medium was otherwise similar to that observed in a normal medium: net flux varied linearly with potential and calculated fluxes in the active transport path indicated a value significantly greater than unity for the empirical constant Q in the equation for change in the flux ratio, f, with change in potential, psi, viz., delta ln f = Q(ZF/RT)delta psi, similar to the high value for this constant that we have found in a bicarbonate-containing medium. We conclude that bicarbonate ions facilitate active sodium transport and also may be actively transported from serosa to mucosa in Dominican toads. However, coupling between bicarbonate and sodium fluxes does not account for the high value for Q for sodium in the active transport path.

Resolution of parameters in the equivalent electrical circuit of the sodium transport mechanism across toad skin.

In amphibian epithelia, amiloride reduces net sodium transport by hindering the entry of sodium to the active transport mechanism, that is, by increasing the series resistance (Rser). Theoretically, therefore, analysis of amiloride-induced changes in potential differences and short-circuit current should yield numerical estimates of all the parameters in the equivalent electrical circuit of the sodium transport mechanism. The concept has been explored by analysis of such changes in toad skins (Xenopus laevis) bathed in hypotonic sulphate Ringer's, after exposure to varying doses of amiloride, or to amphotericin, dinitrophenol or Pitressin. The estimated values of Rser, of the electromotive force of the sodium pump (ENa), and of the shunt resistance (Rsh) were independent of the dose of amiloride employed. Skins bathed in hypotonic sulphate Ringer's exhibited a progressive rise in ENa. Amphotericin produced a fall in Rser, while dinitrophenol caused a fall in ENa; washout of the drugs reversed these effects. Pitressin produced a fall in both Rser and Rsh, with a rise in ENa. These results are in accord with earlier suggestions regarding the site(s) of action of these agents.

Localization of K+-Stimulated p-NPPase in the lachrymal ?Salt? gland of Malaclemys, using cytochemical and autoradiographical techniques

Using two independent techniques, histochemistry and autoradiography, an enzyme (E.C. 3.6.1.3.) has been localized on basolateral cell membranes of salt secreting cells in the lachrymal gland of Malaclemys. This enzyme is ouabain sensitive. In addition an L-tetramisole sensitive alkaline phosphatase is found in the same sites, and an ethacrynic acid sensitive K+-stimulated p-NPPase is found on the apical membrane. The significance of these results with regard to the location of the pump responsible for net transepithelial sodium transport is discussed.

Inhibition of ion transport by bile salts in canine gastric mucosa.

Studies were conducted with in vivo and in vitro canine stomach preparations. Instillation of 5, 10, and 20 mM bile salts in TES bufer (pH 7.4) into the nonsecreting stomach in vivo caused a progressive decrease in electrical potential difference (PD) and an increase in electrical resitance (R). The rate of acid secretion, determined by the pH-stat method in the histamine-stimulated stomach, decreased with 5 and 20 mM bile salts. Mucosal adenosine triphosphate (ATP) content of the nonsecreting or secreting stomach was reduced by bile salts. In vitro flux studies demonstrated that within the first hour after 1 mM bile salts were added to the mucosal side of the chamber, PD decreased, R increased, and net sodium transport decreased. In the second hour, unidirectional fluxes of sodium increased, indicating an increase in permeability of the gastric mucosa to sodium. These results demonstrate that the initial action of bile salts is inhibition of ion transport, which is followed by an increase in permeability.

In vitro behavior of human intestinal mucosa. The influence of acetyl choline on ion transport.

The possibility that the autonomic nervous system may influence the function of intestinal mucosa was investigated by assessing the effect of acetyl choline on ion transport in human intestine. Isolated pieces of stripped ileal mucosa were mounted in Perspex flux-chambers and bathed in isotonic glucose Ringer's solution. Acetyl choline caused a rise in mean potential difference (8.8-12.3 mV, P less than 0.002) and short circuit current (287.7-417.2 muA-cm-2, P less than 0.01) (n = 12), observable at a concentration of 0.01 mM and maximal at 0.1 mM. This effect was enhanced by neostigmine and blocked by atropine. Isotopic flux determinations revealed a change from a small mean net Cl absorption (58) to a net Cl secretion (-4.3mueq-cm-2-h-1P less than 0.001) due predominantly to an increase in the serosal to mucosal unidirectional flux of Cl (10.63-14.35 mueq-cm-2-h-1P less than 0.05) and a smaller reduction in the mucosal to serosal flux (11.22 to 10.02 mueq-cm-2-h-1P less than 0.05). Unidirectional and net Na transport was unaffected. A similar electrical and ion transport response was observed in a single study of two pieces of jejunal mucosa. In the absence of glucose net chloride secretion was produced and again an insignificant effect on net sodium transport was noted. Acetyl choline did not provoke a sustained effect on mucosal cyclic adenine nucleotide levels although a short-lived cyclic adenine nucleotide response was seen in some tissues 20-30 s after drug addition. These studies demonstrate that acetyl choline does influence human intestinal ion transport by stimulating chloride secretion and suggest a possible mechanism by which the parasympathetic nervous system could be concerned in the control of ion transport.

On the influence of the natriuretic factor from patients with chronic uremia on the bioelectric properties and sodium transport of the isolated mammalian collecting tubule.

A gel filtration fraction of urine from patients with chronic renal disease (natriuretic factor) has been shown previously to cause natriuresis in rats and to inhibit sodium transport in the isolated toad bladder. The effect of this fraction on transtubular potential difference and sodium transport was examined on the isolated perfused cortical collecting tubule of the rabbit. A rapid inhibition of potential difference from -22.5 mV to -12 mV (P less than 0.001) was observed when the fraction was applied to the peritubular surface. This effect was accompanied by a decrease in net sodium flux from 6.29 to 3.21 pmol/cm per s (P less than 0.001). Unidirectional fluxes using isotopic sodium revealed that the inhibition of net sodium transport was due to a decrease in flux from the lumen to the peritubular surface, i.e., an inhibition of active sodium transport. There was no change in sodium flux in the reverse direction. These changes were all rapidly reversed by removal of the fraction from the peritubular surface. The addition of the fraction to the lumen had no effect on potential difference or net sodium flux. Control studies using the same fraction from the urine of normal subjects had no effect on any of the parameters studies. Where both a uremic and a normal fraction were sequentially applied to the peritubular surface of the same tubule, inhibition of potential difference was obtained only with the former. In the light of evidence implicating the collecting duct fraction from normal animals, the data are consistent with the view that the natriuretic factor may be biologically important in the regulation of sodium balance via it's regulatory role in active sodium transport in the collecting tubule.

The effects of glucose and insulin on renal electrolyte transport.

The effects of hyperglycemia and hyperinsulinemia on renal handling of sodium, calcium, and phosphate were studied in dogs employing the recollection micropuncture technique. Subthreshold sustained hyperglycemia resulted in an isonatric inhibition of proximal tubular sodium, fluid, calcium, and phosphate reabsorption by 8-14%. Fractional excretion of sodium and phosphate, however, fell (P is less than 0.01) indicating that the increased delivery of these ions was reabsorbed in portions of the nephron distal to the site of puncture and in addition net sodium and phosphate transport was enhanced resulting in a significant antinatriuresis and antiphosphaturia. The creation of a steady state plateau of hyperinsulinemia while maintaining the blood glucose concentration of euglycemic levels mimicked the effects of hyperglycemia on proximal tubular transport and fractional excretion of sodium and calcium. Tubular fluid to plasma insulin ratio fell, similar to the hyperglycemic studies. These results suggest that the effects of hyperglycemia on renal handling of sodium and calcium may be mediated through changes in plasma insulin concentration. In contrast to hyperglycemia, however, hyperinsulinemia cuased a significant fall in tubular fluid to plasma phosphate ratio with enhanced proximal tubular phosphate reabsorption (P is less than 0.02). This occurred concomitantly with a significant inhibition of proximal tubular sodium transport. These data indicate that insulin has a direct effect on proximal tubular phosphate reabsorption, and this effect of insulin is masked by the presence of increased amounts of unreabsorbed glucose in the tubule that ensues when hyperinsulinemia occurs secondary to hyperglycemia. Fractional excretion of phosphate fell significantly during insulin infusion but unlike the hyperglycemic studies, the fall in phosphate excretion could be entirely accounted for by enhanced proximal reabsorption.

The effect of ethanol on ion transport in frog skin

Frog skin has been used as a model epithelial sodium-transporting system to study the effect of ethanol on ion transport. Treatment of the outside of frog skin with ethanol decreased the net sodium transport due to inhibition of 22Na + influx. Ethanol did not alter sodium outflux when bathing the outside of the skin. The inhibition was in proportion to the concentration of ethanol, 0.25 M resulting in 50% inhibition. The chloride permeability of the skin was increased several-fold when the skin was exposed to ethanol in either bathing solution. With 0.4 M ethanol in the inner bathing solution, all the unidirectional fluxes of Na + and Cl − were increased. The movement of Cl − was evaluated by comparison of Cl − flux with urea flux, since urea is thought to move passively across frog skin via an extracellular (shunt) pathway. Chloride flux was increased to a greater extent than urea flux. These experiments indicate that ethanol affects chloride permeability beyond an increase in extracellular ion flow and independent of its effect on Na + transport.

Pathway of sodium moving from blood to intestinal lumen under the influence of oxyphenisatin and deoxycholate.

The transfer of (51)CrEDTA and inulin--substances which are distributed only in the extracellular space--across the rat colonic mucosa in vivo is increased by oxyphenisatin O (3.5 times 10(-5)M) and deoxycholate D(3 times 10(-3)M). O and D do not change the size of the intra- and extracellular fluid compartments of the mucosa as measured with (51)CrEDTA from the blood side. The sodium and potassium content of the mucosal tissue is not altered. Therefore the calculated intracellular concentrations of sodium and potassium remain constant. The time course of the (22)Na uptake into the mucosal epithelium is not influenced by O and D up to 5 min after i.v. injection. The specific activity of sodium, however, in the luminal fluid increases under the influence of O(twofold) and D(fivefold). The uptake of (22)Na into the mucosal tissue after administration of (22)Nainto the intestinal lumen is not changed in presence of O and D. We conclude that the net transport of sodium and water from blood to lumen under the influence of O and D occurs mainly via the intercellular way.

Effects of bumetanide on sodium transport of the isolated frog skin and on renal Na-K-ATPase.

The effects of bumetanide, a new potent diuretic, on net sodium transport of the isolated frog skin and on rat renal Na-K-ATPase were studied. A dose-related decrease in short-circuit current and potential difference with increased electrical resistance was observed when bumetanide was added to the corial side of the skin. Addition to the epithelial side resulted in enhanced net sodium transport with decreased electrical resistance. When applied to the corial side it abolished vasopressin- and aldosterone-stimulated transport. Present in the epithelial bath ouabain-inhibited transport was unaffected by this drug, while triamterene-induced inhibition of sodium transport was completely abolished. In vitro, no significant effects on Na-K-ATPase were noted. It is concluded that bumetanide shares properties of both furosemide and ethacrynic acid and excerts its effects on epithelial sodium transport by altering membrane permeability and possibly by inhibition of some step in the active transport mechanism for sodium.

The O2 supply of the isolated cell-free perfused rat kidney.

Compared to the organ in situ in all isolated cell-free perfused kidney preparations so far described tubular net sodium transport is considerably reduced (1, 2, 3, 4, 5). Since the active tubular transport of sodium is a basic function of the mammalian kidney a number of other dysfunctions of isolated kidneys may be caused by the reduced rate of sodium transport.

Effects of changes in the composition of the mucosal solution on the electrical properties of the toad urinary bladder epithelium.

By the use of microelectrode techniques, the potential profile and the electrical resistances of the cellular and shunt pathways across the toad urinary bladder epithelium were measured under control conditions and after exposing the mucosal side to solutions of low and high NaCl concentrations and osmolalities. The resistance of the shunt pathway increases at low NaCl concentration (even if the osmolality is kept constant), and decreases at high NaCl concentration (by a nonspecific osmotic mechanism). The inverse relationship between mucosal NaCl concentration and shunt resistance suggests a regulatory mechanism of net sodium transport by reduction of the passive blood-to-urine sodium flux at low urinary sodium concentrations. In addition, the transepithelial potential and the potentials at both cell borders fall in both low and high mucosal NaCl, and the magnitude of these changes is such that they cannot be explained by changes in the shunt pathway alone.

Ionic requirements of proximal tubular sodium transport. II. Hydrogen ion.

Simultaneous perfusion to proximal convoluted tubules and peritubular capillaries was used to study the effects of different perfusion fluids on sodium reabsorption and hydrogen secretion, which was calculated as bicarbonate reabsorption and titratable acid. Results show that sodium reabsorption was not tightly coupled to hydrogen secretion. Bicarbonate stimulates both sodium reabsorption and hydrogen secretion, but Tris stimulates only sodium reabsorption. Imposing an adverse chloride gradient across the proximal tubule (C1- peritubular greater than C1- luminal) decreased sodium reabsorption but did not diminish hydrogen secretion. Diamox inhibited both net sodium and hydrogen transport. It is concluded that there is not firm linkage between sodium reabsorption and hydrogen secretion and that bicarbonate probably stimulates sodium transport by a number of mechanisms, including an effect on the sodium transport unrelated to its ability to increase hydrogen ion secretion.

The effect of dodecylsulphate upon net sodium and water transport from tied jejunal loops in anaesthetized rats.

Abstract: Into tied jejunal loops in rats solutions of sodium chloride or sulphate were instilled, with or without addition of sodium dodecylsulphate. The sodium salt concentration was varied, so that there was either a sodium concentration gradient from the plasma to the lumen, or a gradient from the lumen to the plasma. In the presence of surfactant, net secretion took place when there was a significant gradient from the plasma to the lumen, and net absorption when the gradient was reversed. With only a small gradient from the lumen to the plasma there was complete inhibition of net sodium transport. In principle, sodium in the presence of dodecylsulphate was transported along its concentration gradient. In control experiments, sodium absorption essentially independent of sodium concentration occurred. The results indicate that with dodecylsulphate, normal carrier‐mediated net sodium absorption is successively substituted by passive diffusion, as the surfactant dose‐dependently causes increased epithelial permeability to sodium. Less than 1.7 mM dodecylsulphate seemed to cause this effect.

The response of the dog colon mucosa one hour's ischaemia.

Segments of dog colon were subjected to one hour's ischaemia, and their morphology and function were studied either immediately after the ischaemia or 24 hours later. As functional tests, net sodium transport across the mucosa in vitro, the transport capacity in vitro for sugars and amino-acids, tissue respiration, lactate production, and the liberation of acid phosphatase were applied. Immediately after the ischaemia, all parameters differed significantly from the control mucosa: net sodium transport was abolished; amino-acid and sugar uptake, oxygen consumption and lactate production were reduced; and lysosomal enzyme release was increased. All parameters, except sugar uptake, had normalised 24 hours after the trauma. Histological examinations revealed much variation in the lesions occurring after the ischaemia, varying from slight epithelial desquamation to considerable mucosal destruction. In most cases, the mucosa had almost normalised 24 hours later. Morphometric analysis revealed decreased mean mucosal thickness immediately after the trauma, though 24 hours later, there was again no significant difference with the control group.

Effects of graded solute diuresis on renal tubular sodium transport in the rat.

Sodium transport was studied across proximal and distal tubules of rats undergoing progressive intravenous loading with either isomotic saline or urea (200 mosmol)-saline (100 mosmol) solutions. Free-flow as well as recollection micropuncture techniques were used, and tubular fluid (TF) samples were analyzed for inulin-14C and sodium (Na). With administration of progressively larger intravenous saline loads, the delivery of fluid and sodium into the distal tubule rose. Concomitantly, the normally observed decline of tubular sodium concentrations along the distal tubule became attenuated until it was abolished at the highest infusion rates of saline solutions. Absolute reabsorption rates of Na across the distal tubule increased in proportion to tubular flow rate, and no tubular maximum (Tm) was observed. It is suggested that the delivery of increased amounts of sodium to the normally unsaturated later parts of the distal tubule and the elevated tubular sodium concentration after saline loading account for the observed stimulation of distal tubular net sodium transport. The extent of transport stimulation is also subject to control by the amount of urea accumulation along the distal tubule. As the latter declines, sodium reabsorption is proportionately enhanced.

Analysis of unidirectional fluxes of sodium during diarrhea induced by Clostridium perfringens enterotoxin in the rat terminal ileum

Net intestinal transport of sodium in vivo, in control and enterotoxin (Clostridium perfringens)-treated rats, was resolved into two unidirectional fluxes, influx from and efflux into the lumen of the terminal ileum. In rats treated with the toxin, sodium influx remained similar to control values even during fluid and electrolyte loss to the lumen. Net loss of sodium was shown to be due to nearly a twofold increase in sodium efflux to the lumen in toxin-treated animals. There was only slight histopathological damage to the mucosa, especially noticeable at the tips of villi.

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.

Effect of arterial perfusion on net water flux and active sodium transport across the isolated skin ofBufo bufo bufo (L.)

A method has been developed which allows perfusion of the blood vessels in isolated pelvic skins ofB. bufo. The effect of various doses of vasotocin (AVT) on net water flux (inside medium 220 mOsM, outside medium 11 mOsM) and active sodium transport were compared in perfused and unperfused skins. The unstimulated water flux (Fig. 3) and the active sodium transport (Fig. 6) were unaffected by perfusion. The threshold for stimulation of water flux was between 0.01 and 0.1 nM vasotocin in perfused skins and between 0.1 and 1 nM in unperfused skins. The threshold for stimulation of active sodium transport was between 0.005 and 0.05 nM vasotocin in perfused skins and between 0.05 and 0.5 nM in unperfused skins. The maximal water flux through perfused skins, 4 μl/cm2min, was obtained at 1 nM vasotocin. At 10 nM vasotocin the water flux was only 0.7 μl/cm2min in unperfused skins. The maximal active sodium transport was approximately of the same magnitude in perfused and unperfused skins, at 0.5 mM and at 50 nM, respectively.