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Background: Proteases can increase the activity of the epithelial sodium channel (ENaC) by cleaving its α- or γ-subunit. However, evidence so far comes only from studies in vitro in either heterologous expression systems or isolated nephron segments. The present study has tested whether exposure to a luminal protease can alter sodium reabsorption along the rat collecting duct in vivo., Methods: Rats on normal laboratory chow were prepared for renal micropuncture. Late distal tubules of superficial nephrons were microinjected and perfused twice (3 nL min(-1) for 3-6 min) with a solution similar to native tubular fluid, but containing (14)[C]inulin and (22)Na. The first perfusion was either a control solution or solution containing amiloride 1 mM or hydrochlorothiazide (HCTZ ) 1 mM; the second perfusion was either a control solution (time control) or a solution containing chymotrypsin 2 µg mL(-1) ± aprotinin 100 µg mL(-1) or amiloride 1 mM or HCTZ 1 mM. Urinary recoveries of (14)[C]inulin and (22)Na were recorded., Results: In time controls, the Na/In ratio did not change significantly (32.2 ± 3.4% versus 34.5 ± 3.1%). In contrast, chymotrypsin reduced the ratio from 33.3 ± 3.8% to 25.5 ± 2.5% (P < 0.05), indicating an increase in sodium reabsorption. When co-injected with chymotrypsin, the protease inhibitor aprotinin abolished the stimulatory effect of chymotrypsin on sodium reabsorption (31.7 ± 3.4% versus 32.1 ± 2.1%), while aprotinin alone had no effect. When chymotrypsin was co-injected with HCTZ, the Na/In ratio decreased from 36.8 ± 2.3% to 28.0 ± 3.4% (P < 0.05), whereas when given with amiloride, there was no change in the ratio (45.8 ± 3.4% versus 45.5 ± 2.3%), indicating that stimulation of sodium reabsorption by chymotrypsin was ENaC-dependent., Conclusions: These findings demonstrate proteolytic activation of ENaC in vivo, and suggest that changes in protease activity of the glomerular filtrate and tubular fluid in health or disease could affect net renal sodium excretion.

Background: It was the aim to investigate the influence of gastric bypass on renal sodium and water handling. The relationship between sodium and water absorption along the gastrointestinal tract and their renal excretion is poorly understood. Beneficial effects on blood pressure have been seen after bariatric surgery before significant weight loss has occurred., Methods: Male Wistar rats (348 ± 19 g) underwent either gastric bypass (n = 14) or sham operation (n = 7) and were given a low-sodium diet with deionized water ad libitum. Before and after surgery, the rats received an oral sodium load (1.5 mmol/kg) as hyperosmolar saline (616 mM), and were then placed in individual metabolic cages so the urine volume, sodium content, and water intake for 8 hours could be recorded. The urine sodium concentration was also measured., Results: The rats that had undergone gastric bypass had a significantly lower body weight than the sham-operated controls throughout the follow-up period (346 ± 21 g versus 501.3 ± 8.0 g at day 60; P = .0004). An oral sodium load after gastric bypass led to an increase in water intake (.07 ± .01 mL/g versus .03 ± .01 mL/g; P = .023), urine output (.03 ± .01 mL/g versus .02 ± .002 mL/g; P = .027), and sodium excretion (65.99 ± 10.7 mol versus 31.71 ± 8.7 mol; P = .020). No change was seen in water intake, urine output, or sodium excretion after sham surgery., Conclusion: Urine output, water intake, and sodium excretion are all increased after gastric bypass surgery in rats given an oral sodium load compared with sham-operated controls. More rapid excretion, and less retention, of a dietary sodium load could be a part of the mechanism underlying the beneficial effect of bariatric surgery on blood pressure., (Copyright © 2011 American Society for Metabolic and Bariatric Surgery. Published by Elsevier Inc. All rights reserved.)

The maintenance of sodium (Na+) homeostasis is an essential function of the kidney. It is achieved by a variety of transport processes localized all along the highly specialised segments of the nephron. Impairment of these transport mechanisms, and thereby Na+ handling, is associated with disturbed Na+ and water balance, leading to hypertension and oedema. This review focuses on the novel regulation of sodium reabsorption by serine proteases acting along the entire nephron., (Copyright © 2011 S. Karger AG, Basel.)

In vitro evidence suggests that intraluminal nucleotides, acting on apical P2 receptors, may influence amiloride-sensitive sodium reabsorption in collecting ducts. The present study has assessed this possibility directly in anesthetized rats, by determining the urinary recovery of 22Na relative to that of [14C]inulin (Na/inulin recovery ratio) during in vivo microperfusion of late distal tubules with artificial tubular fluid containing various P2 agonists (all at 1 mM). In animals maintained on a control diet, in which amiloride-sensitive 22Na reabsorption was modest, the poorly hydrolysable, broad-spectrum P2 agonist ATPgammaS had no significant effect on the Na/inulin recovery ratio. In contrast, in rats maintained on a low-sodium diet, in which amiloride-sensitive 22Na reabsorption was considerably enhanced, ATPgammaS caused a significant increase in the Na/inulin recovery ratio (control: 14 +/- 3%; ATPgammaS: 28 +/- 4%; n = 32 pairs; P < 0.001, paired t-test). No change in the Na/inulin recovery ratio was seen in time controls (13 +/- 3 vs. 14 +/- 4%; n = 15 pairs). In subsequent experiments in rats maintained on a low-sodium diet, we used more selective agonists in an attempt to identify the receptor subtype responsible for the effect of ATPgammaS. The P2Y1 agonist 2meSADP, the P2Y2/4 agonists Ap4A and Cp4U, and the P2X agonist BzATP were all without significant effect on the Na/inulin recovery ratio. These findings constitute the first in vivo evidence for a functional role for apical P2 receptors in collecting ducts, but the identity of the receptor subtype(s) involved remains elusive.

In order to test the proposal that the aldosterone specificity of mineralocorticoid receptors in the collecting duct depends on inactivation of glucocorticoids by the enzyme 11beta-hydroxysteroid dehydrogenase (11beta-HSD), we have assessed the effect of pharmacological inhibition of 11beta-HSD on collecting duct Na+ reabsorption in vivo. Adrenalectomized rats (n=14) were infused intravenously with high-dose corticosterone, and late-distal tubules were perfused orthogradely with artificial tubular fluid containing [14C]inulin and 22Na; urinary recoveries of the radioisotopes were monitored. Half of the rats received intravenous carbenoxolone to inhibit renal 11beta-HSD activity. The urinary recovery of [14C]inulin was complete in both groups of animals (101+/-2% versus 101+/-3%), but the recovery of 22Na was lower in carbenoxolone-treated rats (34+/-5%) than in the corticosterone-alone group (54+/-4%, P<0.01). These data, which provide the first demonstration of enhanced Na+ reabsorption in the distal nephron during inhibition of renal 11beta-HSD in vivo, strongly support the proposal that 11beta-HSD normally prevents endogenous glucocorticoid from exerting mineralocorticoid-like effects.

In vitro evidence suggests that the 'recycling' of K(+) ions through luminal K(+) channels in the thick ascending limb of the loop of Henle (TALH) is essential for the normal operation of the luminal Na(+)-K(+)-2Cl(-) co-transporter. In the present study these channels were investigated in vivo by perfusing superficial loops of Henle in anaesthetised rats with and without the K(+) channel blocker barium. Using a standard perfusate, intraluminal barium (5 mmol l(-1)) reduced sodium reabsorption (J(Na)) from 1887 +/- 50 to 1319 +/- 53 pmol min(-1) (P < 0.001). When the experiment was repeated using a low-Na(+) perfusate, designed to inhibit reabsorption in the pars recta (the initial segment of the loop of Henle), a similar reduction in J(Na) was observed (from 698 +/- 47 to 149 +/- 23 pmol min(-1), P < 0.001), strongly suggesting that the effect of barium is localised to the TALH. The magnitude of the reduction in J(Na) during blockade of K(+) channels confirms the importance of K(+) recycling in facilitating Na(+) reabsorption in the TALH in vivo. However, the reduction in J(Na) was not associated with a fall in the K(+) concentration of the fluid collected at the early distal tubule. When bumetanide, an inhibitor of the Na(+)-K(+)-2Cl(-) co-transporter, was included in the low-Na(+) perfusate, net K(+) secretion was observed. Addition of barium to this perfusate reduced, but did not abolish, the secretion, suggesting that bumetanide-induced K(+) secretion results partly from paracellular transport. Experimental Physiology (2001) 86.4, 469-474.

1. The contribution of apical Na+-H+ exchange to sodium reabsorption in the thick ascending limb of the loop of Henle (TALH) in vivo was examined in anaesthetized rats by perfusing loops of Henle of superficial nephrons with solutions containing the Na+-H+ exchange inhibitor, ethyl isopropyl amiloride (EIPA). 2. Using a standard perfusate, no statistically significant effect of EIPA on net sodium reabsorption (JNa) was detected. However, when sodium reabsorption in the pars recta of the proximal tubule was minimized by using a low-sodium perfusate, EIPA reduced JNa from 828 +/- 41 to 726 +/- 37 pmol min-1 (P < 0.05), indicating that apical Na+-H+ exchange can make a small contribution to net sodium reabsorption in the TALH in vivo. This contribution appears to be dependent on the bicarbonate load, since an increase in the latter led to an enhancement of EIPA-sensitive sodium transport. 3. Addition of the Na+-K+-2Cl- cotransport inhibitor, bumetanide, to the low-sodium perfusate reduced baseline JNa to 86 +/- 27 pmol min-1. In this setting, EIPA reduced JNa further, to -24 +/- 18 pmol min-1 (P < 0.05), an effect similar to that seen in the absence of bumetanide. This finding argues against previous suggestions (based on in vitro evidence) that inhibition of the Na+-K+-2Cl- cotransporter leads to an increase in apical Na+-H+ exchange in the TALH.

1. The type 2 isoform of 11beta-hydroxysteroid dehydrogenase, an enzyme which converts cortisol or corticosterone to inactive 11-ketosteroid metabolites, is thought to be responsible for preventing access of endogenous glucocorticoids to mineralocorticoid receptors in the distal nephron; although direct in vivo evidence for this is still lacking. We have examined whether graded inhibition of renal 11beta-hydroxysteroid dehydrogenase activities in vivo results in corresponding changes in urinary electrolyte excretion due to exposure of mineralocorticoid receptors to circulating endogenous glucocorticoids.2. Anaesthetized rats were infused intravenously with vehicle alone or with one of three doses of carbenoxolone: 0.06, 0.6 or 6 mg/h. After measurement of renal electrolyte excretion, the kidneys were snap-frozen in liquid nitrogen and 11beta-hydroxysteroid dehydrogenase activities were measured directly by enzyme assay in the presence of NAD+ or NADP+.3. A dose-dependent inhibition of renal 11beta-hydroxysteroid dehydrogenase activities was observed: the low, intermediate and high doses of carbenoxolone causing approximately 50%, 80% and >90% inhibition respectively. Only with the high dose was an effect on renal function observed (decreased fractional Na+ excretion and urinary Na+/K+ ratio).4. The poor correlation between the extent of inhibition of renal 11beta-hydroxysteroid dehydrogenase and altered urinary Na+ excretion, apparent at the lower doses of carbenoxolone, suggests either that 11beta-hydroxysteroid dehydrogenase has considerable functional reserve, or that it may not be the only mechanism determining mineralocorticoid receptor specificity in the distal nephron.

1. The nephron sites involved in the blunted natriuretic response to frusemide during sodium depletion were investigated using micropuncture techniques in anaesthetized rats. 2. Glomerular filtration rate was lower, and fractional sodium reabsorption in the proximal convoluted tubule higher, in sodium-depleted than in sodium-replete rats. Consequently, sodium delivery to the loop of Henle was reduced (by approximately 35%) in the sodium-depleted animals. Intravenous frusemide (2.5 mg h-1 kg-1; urinary water and electrolyte losses replaced) had no effect on glomerular filtration rate or proximal tubular sodium reabsorption in either group. 3. The inhibitory effect of intravenous frusemide on fractional sodium reabsorption in the nephron segments constituting the loop of Henle (measured by free-flow micropuncture) was attenuated during sodium depletion. However, when loops of Henle were microperfused at identical rates with artificial late proximal tubular fluid, no difference in the responses of sodium-depleted and sodium-replete rats to intraluminal frusemide (10(-5) mol/l) could be detected. 4. In sodium-replete animals, the increased load of sodium delivered from the loop of Henle during frusemide administration resulted in a lowering of fractional sodium reabsorption in the distal tubule. In contrast, in sodium-depleted rats given frusemide, fractional distal sodium reabsorption tended to increase, so that values in the two groups of frusemide-treated animals were markedly different (0.30 +/- 0.04 versus 0.51 +/- 0.03). 5. It is concluded that the blunted natriuretic response to frusemide during sodium depletion results from at least three factors: a reduced sodium delivery to the loop of Henle; a reduced inhibitory effect of frusemide on fractional sodium reabsorption in the loop of Henle, which may be a consequence of the reduced sodium load; and enhanced fractional reabsorption of sodium in the distal tubule, which partially buffers the diuretic-induced increase in sodium delivery from the loop.

Patients with ileostomies show an early diuresis when sodium restricted; this, together with an obligatory ileal sodium loss, predisposes them to severe salt and water depletion. The role of arginine vasopressin in this circumstance and whether it is natriuretic, or antinatriuretic, is unclear. There is also controversy over its likely effect on small bowel fluid reabsorption. We have examined the effect of the non-pressor (V2) synthetic vasopressin analogue 1-deamino-8-D-arginine (desmopressin) on renal and ileal sodium and water excretion in ileostomy patients during acute adaptation to a low sodium diet. Patients were studied on two separate occasions (nonrandomised) with and without the administration of desmopressin (0.75 micrograms intramuscular, three times a day). In eight subjects without desmopressin there was pronounced diuresis on the first low sodium day, associated with a fall in renal sodium excretion and no change in ileal output or composition. In five (of the original) subjects with desmopressin there was pronounced antidiuresis, no change in renal sodium excretion, and no change in ileal output or composition. In both studies rises in plasma renin activity and salivary aldosterone concentration lagged behind the early decline in renal sodium excretion. We have confirmed the phenomenon of 'low sodium' diuresis after sodium restriction in ileostomy patients and shown that it can be prevented by desmopressin. Desmopressin has no direct or indirect effect on renal sodium excretion or ileal fluid and electrolyte loss in humans.

1. The fall in renal sodium excretion after dietary sodium restriction is prompt and reproducible. The importance of increased aldosterone secretion during the early phase (within 48 h) of this response is unclear. Using two indirect measures of aldosterone secretion (in urine and saliva), we have tried to relate changes in excretion and concentration of this hormone to renal sodium excretion during the abrupt transition from a normal (approximately 150 mmol/day) or high (260 mmol/day) to a low (5-25 mmol/day) sodium intake in 11 and seven male volunteers, respectively. 2. All subjects showed reduced renal sodium excretion within 36 h of dietary restriction, but the times at which increases in renal aldosterone excretion, saliva aldosterone concentration and plasma renin activity became statistically significant varied widely (8-72 h, 2.5- greater than 62.5 h and less than 4- greater than 38 h for renal aldosterone secretion, saliva aldosterone concentration and plasma renin activity, respectively). Circadian fluctuations in saliva aldosterone concentration were apparent and increased in amplitude during sodium restriction. 3. Urine flow rate tended to increase on the first day of sodium restriction and this reached statistical significance in the group initially on a high sodium intake (64.0 +/- 8.8 to 84.3 +/- 11.2 ml/h, P less than 0.01); although the pattern of urine flow did correlate with plasma arginine vasopressin concentration (r = -0.49, P less than 0.01), there was no significant decrease in mean plasma arginine vasopressin concentration [1.15 (0.92-1.44) to 0.90 (0.72-1.12) pmol/l, P = 0.08; geometric mean and 95% confidence limits].(ABSTRACT TRUNCATED AT 250 WORDS)