ET(B) receptor in renal medulla is enhanced by local sodium during low salt intake

New concepts in endothelin control of sodium balance

1. Endothelin (ET)-1, which was originally found to be secreted by the vascular endothelium, is highly expressed in the kidney, particularly in the renal medulla. 2. Recent studies using genetic models have provided significant breakthroughs in the role of ET-1 in the kidney. For example, ET-1 in the medullary collecting duct physiologically regulates water and salt reabsorption, thereby controlling blood pressure. Surprisingly, to explain the blood pressure regulation both ET(A) and ET(B) receptors are necessary in collecting duct. In fact, we recently revealed that ET(A) receptor stimulation in the renal medulla was natriuretic and diuretic. 3. The expression and secretion of ET-1 in the renal medulla are regulated by multiple mechanisms, such as changes in osmolality, exaggerated renin-angiotensin system activity and hypoxia. The changes in the renal medullary ET system are likely to work as compensatory 'protective' natriuretic factors in response to high sodium exposure in the kidney. 4. In the present review, we focus on recent publications that describe our current knowledge of the functional role of renal medullary ET-1, including the recently characterized actions of ET(A) receptors, the second messenger systems, mechanisms of stimulating ET-1 production and how the ET system is involved in the development of hypertension.

Low NaCl intake elevates renal medullary endothelin-1 and endothelin A (ETA) receptor mRNA but not the sensitivity of renal Na+ excretion to ETA receptor blockade in rats

AIMS

This study was performed to investigate the effects of NaCl intake on renal mRNA expression of pre-pro-endothelin-1 (ET-1), endothelin A (ET(A)) and endothelin B (ET(B)) receptors as well as on renal ET-1 content in rats. We further tested for NaCl intake-dependent differences in the contribution of the ET system to renal sodium handling.

METHODS

Male Sprague-Dawley rats with telemetric devices were randomized to 0.15%, 0.60% and 1.80% NaCl diets with or without losartan. Renal sodium balance and arterial pressure were monitored. Renal blood flow and fractional sodium excretion (FENa) were measured in response to acute infusion of ET(A) and ET(B) blockers into the inner stripe of the outer renal medulla.

RESULTS

Medullary pre-pro-ET-1, ET(A) and ET(B) receptor mRNA was 50%, 81% and 33% higher in rats on 0.15% vs. 1.80% NaCl. Losartan reduced medullary gene expression in rats on 0.15% NaCl. Medullary ET-1 content was 983 +/- 88 and 479 +/- 42 ng mg(-1) protein in rats on 0.15% and 1.80% NaCl (P < 0.001). Chronic ET(A) receptor blocker treatment reduced arterial pressure by 8-10 mmHg in rats on 0.15% vs. 1.80% NaCl without affecting renal sodium balances. Acute medullary ET(A) or ET(B) receptor blockade did not alter medullary blood flow and FENa in animals on either diet.

CONCLUSION

In rats renal medullary ET-1 content and mRNA expression of three ET system components are inversely related to NaCl intake. Higher expression levels on low NaCl intake are AT(1) receptor dependent but are not associated with increased sensitivity of renal sodium handling to ET(A) receptor blockade.

Role of Endothelin-1 in Exposure to High Altitude

Background— The degree of pulmonary hypertension in healthy subjects exposed to acute hypobaric hypoxia at high altitude was found to be related to increased plasma endothelin (ET)-1. The aim of the present study was to investigate the effects of ET-1 antagonism on pulmonary hypertension, renal water, and sodium balance under acute and prolonged exposure to high-altitude–associated hypoxia.

Methods and Results— In a double-blind fashion, healthy volunteers were randomly assigned to receive bosentan (62.5 mg for 1 day and 125 mg for the following 2 days; n=10) or placebo (n=10) at sea level and after rapid ascent to high altitude (4559 m). At sea level, bosentan did not induce any significant changes in hemodynamic or renal parameters. At altitude, bosentan induced a significant reduction of systolic pulmonary artery pressure (21±7 versus 31±7 mm Hg, P <0.03) and a mild increase in arterial oxygen saturation versus placebo after just 1 day of treatment. However, both urinary volume and free water clearance (H 2 OCl/glomerular filtration rate) were significantly reduced versus placebo after 2 days of ET-1 antagonism (1100±200 versus 1610±590 mL; −6.7±3.5 versus −1.8±4.8 mL/min, P <0.05 versus placebo for both). Sodium clearance and segmental tubular function were not significantly affected by bosentan administration.

Conclusions— The present results indicate that the early beneficial effect of ET-1 antagonism on pulmonary blood pressure is followed by an impairment in volume adaptation. These findings must be considered for the prevention and treatment of acute mountain sickness.

Differential effects of endothelin on activation of renal mechanosensory nerves: stimulatory in high-sodium diet and inhibitory in low-sodium diet

Activation of renal mechanosensory nerves is enhanced by high and suppressed by low sodium dietary intake. Afferent renal denervation results in salt-sensitive hypertension, suggesting that activation of the afferent renal nerves contributes to water and sodium balance. Another model of salt-sensitive hypertension is the endothelin B receptor (ETBR)-deficient rat. ET and its receptors are present in sensory nerves. Therefore, we examined whether ET receptor blockade altered the responsiveness of the renal sensory nerves. In anesthetized rats fed high-sodium diet, renal pelvic administration of the ETBR antagonist BQ-788 reduced the afferent renal nerve activity (ARNA) response to increasing renal pelvic pressure 7.5 mmHg from 26+/-3 to 9+/-3% and the PGE2-mediated renal pelvic release of substance P from 9+/-1 to 3+/-1 pg/min. Conversely, in rats fed low-sodium diet, renal pelvic administration of the ETAR antagonist BQ-123 enhanced the ARNA response to increased renal pelvic pressure from 9+/-2 to 23+/-6% and the PGE2-mediated renal pelvic release of substance P from 0+/-0 to 6+/-1 pg/min. Adding the ETAR antagonist to ETBR-blocked renal pelvises restored the responsiveness of renal sensory nerves in rats fed a high-sodium diet. Adding the ETBR antagonist to ETAR-blocked pelvises suppressed the responsiveness of the renal sensory nerves in rats fed a low-sodium diet. In conclusion, activation of ETBR and ETAR contributes to the enhanced and suppressed responsiveness of renal sensory nerves in conditions of high- and low-sodium dietary intake, respectively. Impaired renorenal reflexes may contribute to the salt-sensitive hypertension in the ETBR-deficient rat.

Renocortical mRNA expression of vasoactive factors during spironolactone protective effect in chronic cyclosporine nephrotoxicity

We showed that spironolactone reduced structural damage and prevented renal dysfunction in chronic cyclosporine (CsA) nephrotoxicity. These findings evidenced an aldosterone renal vascular effect under this condition. To investigate aldosterone’s role in modulating renal vascular tone, renocortical vasoactive pathways mRNA levels in chronic CsA nephrotoxicity as well as spironolactone’s effect on renal function in acute CsA nephrotoxicity were evaluated. Two experimental sets were designed. For chronic nephrotoxicity, rats fed with low-sodium diet were divided into groups receiving vehicle, spironolactone (Sp), CsA, and CsA+Sp, for 21 days. Creatinine clearance, survival percentage, and renocortical mRNA levels of pro-renin, angiotensinogen (Ang), angiotensin receptors (AT1A, AT1B, and AT2), preproendothelin, endothelin receptors (ETA, ETB), cyclooxygenase-2 (COX-2), and adenosine receptors (Ad1, Ad2A, Ad2B, and Ad3) were analyzed. For acute nephrotoxicity, similar groups fed with a standard chow diet for 7 days were included. Serum potassium and sodium, glomerular filtration rate (GFR), and renal blood flow (RBF) were determined. In chronic model, CsA produced pro-renin and ET upregulation, altered adenosine receptors expression, and reduced Ang, AT1A, AT1B, ETB, and COX-2 mRNA levels. Spironolactone protective effect in chronic nephrotoxicity was associated with prevention of pro-renin upregulation and increased AT2, together with ETBreduction. In acute nephrotoxicity, spironolactone completely prevented GFR and RBF reduction induced by CsA. Our results suggest that aldosterone contributes to renal vasoconstriction observed in CsA nephrotoxicity and that renoprotection conferred by spironolactone was related to modification of renocortical vasoactive pathways expression, in which pro-renin normalization was the most evident change in chronic nephropathy. Finally, our data point to spironolactone as a potential treatment to reduce CsA nephrotoxicity in transplant patients.