Licorice-induced hypertension and common variants of genes regulating renal sodium reabsorption.

AIM: To study if gene alterations affecting renal sodium reabsorption associate with susceptibility to licorice-induced hypertension.METHODS: Finnish subjects (n = 30) with a previously documented incident of licorice-induced hypertension were recruited for the study using a newspaper announcement. Their previous clinical and family histories as well as serum electrolyte levels were examined. DNA samples from all individuals were screened for variants of the genes encoding 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2) and alpha-, beta-, and gamma-subunits of the epithelial sodium channel (ENaC).RESULTS: Upon licorice predisposition, the patients had a mean blood pressure of 201/118 mmHg. Circulating potassium, renin, and aldosterone levels were low. No significant DNA variations were identified in the 11betaHSD2 gene. Four subjects were heterozygous for beta- and gammaENaC variants previously shown to be associated with hypertension. Furthermore, a novel G insertion (2004-2005insG) in the SCNN1A gene encoding the alphaENaC was identified in two subjects. The frequency of these ENaC variants was significantly higher in subjects with licorice-induced hypertension (6/30 i.e. 20%) than in blood donors (11/301 i.e. 3.7%, P = 0.002).CONCLUSIONS: Defects of the 11betaHSD2 gene do not constitute a likely cause for licorice-induced hypertension. Variants of the ENaC subunits may render some individuals sensitive to licorice-induced metabolic alterations and hypertension.

Glomerular hyperfiltration and increased proximal sodium reabsorption in subjects with type 2 diabetes or impaired fasting glucose in a population of the African region.

BACKGROUND. Glomerular hyperfiltration (GHF) is a well-recognized early renal alteration in diabetic patients. As the prevalence of GHF is largely unknown in populations in the African region with respect to normal fasting glucose (NFG), impaired fasting glucose (IFG) and type 2 diabetes [diabetes mellitus (DM)], we conducted a cross-sectional study in the Seychelles islands among families including at least one member with hypertension. METHODS. The glomerular filtration rate (GFR), effective renal plasma flow (ERPF) and proximal tubular sodium reabsorption were measured using inulin, p-aminohippurate (PAH) and endogenous lithium clearance, respectively. Twenty-four-hour urine was collected on the preceding day. RESULTS. Of the 363 participants (mean age 44.7 years), 6.6% had IFG, 9.9% had DM and 63.3% had hypertension. The prevalence of GHF, defined as a GFR >140 ml/min, was 17.2%, 29.2% and 52.8% in NFG, IFG and DM, respectively (P trend <0.001). Compared to NFG, the adjusted odds ratio for GHF was 1.99 [95% confidence interval (CI) 0.73-5.44] for IFG and 5.88 (2.39-14.45) for DM. Lithium clearance and fractional excretion of lithium were lower in DM and IFG than NFG (P < 0.001). CONCLUSION. In this population of African descent, subjects with impaired fasting glucose or type 2 diabetes had a high prevalence of GHF and enhanced proximal sodium reabsorption. These findings provide further insight on the elevated incidence of nephropathy reported among African diabetic individuals.

Ethnic differences in proximal and distal tubular sodium reabsorption are heritable in black and white populations.

BACKGROUND: Segmental handling of sodium along the proximal and distal nephron might be heritable and different between black and white participants. METHODS: We randomly recruited 95 nuclear families of black South African ancestry and 103 nuclear families of white Belgian ancestry. We measured the (FENa) and estimated the fractional renal sodium reabsorption in the proximal (RNaprox) and distal (RNadist) tubules from the clearances of endogenous lithium and creatinine. In multivariable analyses, we studied the relation of RNaprox and RNadist with FENa and estimated the heritability (h) of RNaprox and RNadist. RESULTS: Independent of urinary sodium excretion, South Africans (n = 240) had higher RNaprox (unadjusted median, 93.9% vs. 81.0%; P < 0.001) than Belgians (n = 737), but lower RNadist (91.2% vs. 95.1%; P < 0.001). The slope of RNaprox on FENa was steeper in Belgians than in South Africans (-5.40 +/- 0.58 vs. -0.78 +/- 0.58 units; P < 0.001), whereas the opposite was true for the slope of RNadist on FENa (-3.84 +/- 0.19 vs. -13.71 +/- 1.30 units; P < 0.001). h of RNaprox and RNadist was high and significant (P < 0.001) in both countries. h was higher in South Africans than in Belgians for RNaprox (0.82 vs. 0.56; P < 0.001), but was similar for RNadist (0.68 vs. 0.50; P = 0.17). Of the filtered sodium load, black participants reabsorb more than white participants in the proximal nephron and less postproximally. CONCLUSION: Segmental sodium reabsorption along the nephron is highly heritable, but the capacity for regulation in the proximal and postproximal tubules differs between whites and blacks.

Proximal sodium reabsorption: An independent determinant of blood pressure response to salt.

The purpose of this study was to evaluate the contribution of renal sodium handling by the proximal tubule as an independent determinant of blood pressure responsiveness to salt in hypertension. We measured blood pressure (BP), renal hemodynamics, and segmental renal sodium handling (with lithium used as a marker of proximal sodium reabsorption) in 38 hypertensive patients and 27 normotensive subjects (15 young and 12 age-matched) on a high and low sodium diet. In control subjects, changing the diet from a low to a high sodium content resulted in no change in BP and increases in glomerular filtration rate (P<0.05), renal plasma flow (P<0.05), and fractional excretion of lithium (FE(Li), P<0.01). In hypertensive patients, comparable variations of sodium intake induced an increase in BP with no change in renal hemodynamics and proximal sodium reabsorption. When analyzed by tertiles of their BP response to salt, salt-insensitive hypertensive patients of the first tertile disclosed a pattern of adaptation of proximal sodium reabsorption comparable to that of control subjects, whereas the most salt-sensitive patients of the third tertile had an inverse pattern with a high FE(Li) on low salt and a lower FE(Li) on high salt, suggesting an inappropriate modulation of proximal sodium reabsorption. The BP response to salt correlated positively with age (r=0.34, P=0.036) and negatively with the changes in FE(Li) (r=-0.37, P=0.029). In a multivariate analysis, the changes in FE(Li) were significantly and independently associated with the salt-induced changes in BP. These results suggest that proximal sodium reabsorption is an independent determinant of the BP response to salt in hypertension.

Calcium reabsorption in the distal tubule: regulation by sodium, pH, and flow.

We developed a mathematical model of Ca transport along the late distal convoluted tubule (DCT2) and the connecting tubule (CNT) to investigate the mechanisms that regulate Ca reabsorption in the DCT2-CNT. The model accounts for apical Ca influx across transient receptor potential vanilloid 5 (TRPV5) channels and basolateral Ca efflux via plasma membrane Ca-ATPase pumps and type 1 Na/Ca exchangers (NCX1). Model simulations reproduce experimentally observed variations in Ca uptake as a function of extracellular pH, Na, and Mg concentration. Our results indicate that amiloride enhances Ca reabsorption in the DCT2-CNT predominantly by increasing the driving force across NCX1, thereby stimulating Ca efflux. They also suggest that because aldosterone upregulates both apical and basolateral Na transport pathways, it has a lesser impact on Ca reabsorption than amiloride. Conversely, the model predicts that full NCX1 inhibition and parathyroidectomy each augment the Ca load delivered to the collecting duct severalfold. In addition, our results suggest that regulation of TRPV5 activity by luminal pH has a small impact, per se, on transepithelial Ca fluxes; the reduction in Ca reabsorption induced by metabolic acidosis likely stems from decreases in TRPV5 expression. In contrast, elevations in luminal Ca are predicted to significantly decrease TRPV5 activity via the Ca-sensing receptor. Nevertheless, following the administration of furosemide, the calcium-sensing receptor-mediated increase in Ca reabsorption in the DCT2-CNT is calculated to be insufficient to prevent hypercalciuria. Altogether, our model predicts complex interactions between calcium and sodium reabsorption in the DCT2-CNT.

Independent relations of left ventricular structure with the 24-hour urinary excretion of sodium and aldosterone.

Previous studies reported on the association of left ventricular mass index (LVMI) with urinary sodium or with circulating or urinary aldosterone. We investigated the independent associations of LVMI with the urinary excretion of both sodium and aldosterone. We randomly recruited 317 untreated subjects from a white population (45.1% women; mean age 48.2 years). Measurements included echocardiographic left ventricular (LV) properties, the 24-hour urinary excretion of sodium and aldosterone, plasma renin activity (PRA), and proximal (RNa(prox)) and distal (RNa(dist)) renal sodium reabsorption, assessed from the endogenous lithium clearance. In multivariable-adjusted models, we expressed changes in LVMI per 1-SD increase in the explanatory variables, while accounting for sex, age, systolic blood pressure, and the waist-to-hip ratio. LVMI increased independently with the urinary excretion of both sodium (+2.48 g/m(2); P=0.005) and aldosterone (+2.63 g/m(2); P=0.004). Higher sodium excretion was associated with increased mean wall thickness (MWT: +0.126 mm, P=0.054), but with no change in LV end-diastolic diameter (LVID: +0.12 mm, P=0.64). In contrast, higher aldosterone excretion was associated with higher LVID (+0.54 mm; P=0.017), but with no change in MWT (+0.070 mm; P=0.28). Higher RNa(dist) was associated with lower relative wall thickness (-0.81x10(-2), P=0.017), because of opposite trends in LVID (+0.33 mm; P=0.13) and MWT (-0.130 mm; P=0.040). LVMI was not associated with PRA or RNa(prox.) In conclusion, LVMI independently increased with both urinary sodium and aldosterone excretion. Increased MWT explained the association of LVMI with urinary sodium and increased LVID the association of LVMI with urinary aldosterone.

A novel neutrophil elastase inhibitor prevents elastase activation and surface cleavage of the epithelial sodium channel expressed in Xenopus laevis oocytes.

The amiloride-sensitive epithelial sodium channel (ENaC) constitutes a limiting step in sodium reabsorption across distal airway epithelium and controlling mucociliary clearance. ENaC is activated by serine proteases secreted in the extracellular milieu. In cystic fibrosis lungs, high concentrations of secreted neutrophil elastase (NE) are observed. hNE could activate ENaC and contribute to further decreased mucociliary clearance. The aims of this study were (i) to test the ability of an engineered human neutrophil elastase inhibitor (EPI-hNE4) to specifically inhibit the elastase activation of ENaC-mediated amiloride-sensitive currents (I(Na)) and (ii) to examine the effect of elastase on cell surface expression of ENaC and its cleavage pattern (exogenous proteolysis). Oocytes were exposed to hNE (10-100 microg/ml) and/or trypsin (10 microg/ml) for 2-5 min in the presence or absence of EPI-hNE4 (0.7 microm). hNE activated I(Na) 3.6-fold (p < 0.001) relative to non-treated hENaC-injected oocytes. EPI-hNE4 fully inhibited hNE-activated I(Na) but had no effect on trypsin- or prostasin-activated I(Na). The co-activation of I(Na) by hNE and trypsin was not additive. Biotinylation experiments revealed that cell surface gamma ENaC (but not alpha or beta ENaC) exposed to hNE for 2 min was cleaved (as a 67-kDa fragment) and correlated with increased I(Na). The elastase-induced exogenous proteolysis pattern is distinct from the endogenous proteolysis pattern induced upon preferential assembly, suggesting a causal relationship between gamma ENaC cleavage and ENaC activation, taking place at the plasma membrane.

Dysfunction of epithelial sodium transport: from human to mouse.

The highly amiloride-sensitive epithelial sodium channel (ENaC) is an apical membrane constituent of cells of many salt-absorbing epithelia. In the kidney, the functional relevance of ENaC expression has been well established. ENaC mediates the aldosterone-dependent sodium reabsorption in the distal nephron and is involved in the regulation of blood pressure. Mutations in genes encoding ENaC subunits are causative for two human inherited diseases: Liddle's syndrome, a severe form of hypertension associated with ENaC hyperfunction, and pseudohypoaldosteronism (PHA-1), a salt-wasting syndrome caused by decreased ENaC function. Transgenic mouse technologies provide a useful tool to study the role of ENaC in vivo. Different mouse lines have been established in which each of the ENaC subunits was affected. The phenotypes observed in these mice demonstrated that each subunit is essential for survival and for regulation of sodium transport in kidney and colon. Moreover, the alpha subunit plays a specific role in the control of fluid absorption in the airways at birth. Such mice can now be used to study the role of ENaC in various organs and can serve as models to understand the pathophysiology of these human diseases.

Vasopressin-dependent coupling between sodium transport and water flow in a mouse cortical collecting duct cell line.

Water balance is achieved through the ability of the kidney to control water reabsorption in the connecting tubule and the collecting duct. In a mouse cortical collecting duct cell line (mCCD(c11)), physiological concentrations of arginine vasopressin increased both electrogenic, amiloride-sensitive, epithelial sodium channel (ENaC)-mediated sodium transport measured by the short-circuit current (Isc) method and water flow (Jv apical to basal) measured by gravimetry with similar activation coefficient K(1/2) (6 and 12 pM, respectively). Jv increased linearly according to the osmotic gradient across the monolayer. A small but highly significant Jv was also measured under isoosmotic conditions. To test the coupling between sodium reabsorption and water flow, mCCD(c11) cells were treated for 24 h under isoosmotic condition with either diluent, amiloride, vasopressin or vasopressin and amiloride. Isc, Jv, and net chemical sodium fluxes were measured across the same monolayers. Around 30% of baseline and 50% of vasopressin-induced water flow is coupled to an amiloride-sensitive, ENaC-mediated, electrogenic sodium transport, whereas the remaining flow is coupled to an amiloride-insensitive, nonelectrogenic sodium transport mediated by an unknown electroneutral transporter. The mCCD(c11) cell line is a first example of a mammalian tight epithelium allowing quantitative study of the coupling between sodium and water transport. Our data are consistent with the 'near isoosmotic' fluid transport model.

Renal sodium handling in acute and chronic salt loading/depletion protocols: the confounding influence of acute water loading.

OBJECTIVES: Renal tubular sodium handling was measured in healthy subjects submitted to acute and chronic salt-repletion/salt-depletion protocols. The goal was to compare the changes in proximal and distal sodium handling induced by the two procedures using the lithium clearance technique. METHODS: In nine subjects, acute salt loading was obtained with a 2 h infusion of isotonic saline, and salt depletion was induced with a low-salt diet and furosemide. In the chronic protocol, 15 subjects randomly received a low-, a regular- and a high-sodium diet for 1 week. In both protocols, renal and systemic haemodynamics and urinary electrolyte excretion were measured after an acute water load. In the chronic study, sodium handling was also determined, based on 12 h day- and night-time urine collections. RESULTS: The acute and chronic protocols induced comparable changes in sodium excretion, renal haemodynamics and hormonal responses. Yet, the relative contribution of the proximal and distal nephrons to sodium excretion in response to salt loading and depletion differed in the two protocols. Acutely, subjects appeared to regulate sodium balance mainly by the distal nephron, with little contribution of the proximal tubule. In contrast, in the chronic protocol, changes in sodium reabsorption could be measured both in the proximal and distal nephrons. Acute water loading was an important confounding factor which increased sodium excretion by reducing proximal sodium reabsorption. This interference of water was particularly marked in salt-depleted subjects. CONCLUSION: Acute and chronic salt loading/salt depletion protocols investigate different renal mechanisms of control of sodium balance. The endogenous lithium clearance technique is a reliable method to assess proximal sodium reabsorption in humans. However, to investigate sodium handling in diseases such as hypertension, lithium should be measured preferably on 24 h or overnight urine collections to avoid the confounding influence of water.

Colon-specific deletion of epithelial sodium channel causes sodium loss and aldosterone resistance.

Aldosterone promotes electrogenic sodium reabsorption through the amiloride-sensitive epithelial sodium channel (ENaC). Here, we investigated the importance of ENaC and its positive regulator channel-activating protease 1 (CAP1/Prss8) in colon. Mice lacking the αENaC subunit in colonic superficial cells (Scnn1a(KO)) were viable, without fetal or perinatal lethality. Control mice fed a regular or low-salt diet had a significantly higher amiloride-sensitive rectal potential difference (∆PDamil) than control mice fed a high-salt diet. In Scnn1a(KO) mice, however, this salt restriction-induced increase in ∆PDamil did not occur, and the circadian rhythm of ∆PDamil was blunted. Plasma and urinary sodium and potassium did not change with regular or high-salt diets or potassium loading in control or Scnn1a(KO) mice. However, Scnn1a(KO) mice fed a low-salt diet lost significant amounts of sodium in their feces and exhibited high plasma aldosterone and increased urinary sodium retention. Mice lacking the CAP1/Prss8 in colonic superficial cells (Prss8(KO)) were viable, without fetal or perinatal lethality. Compared with controls, Prss8(KO) mice fed regular or low-salt diets exhibited significantly reduced ∆PDamil in the afternoon, but the circadian rhythm was maintained. Prss8(KO) mice fed a low-salt diet also exhibited sodium loss through feces and higher plasma aldosterone levels. Thus, we identified CAP1/Prss8 as an in vivo regulator of ENaC in colon. We conclude that, under salt restriction, activation of the renin-angiotensin-aldosterone system in the kidney compensated for the absence of ENaC in colonic surface epithelium, leading to colon-specific pseudohypoaldosteronism type 1 with mineralocorticoid resistance without evidence of impaired potassium balance.