Interaction between vasopressin and endothelin in renal papillary tubules: uncoupling following cell isolation and culture

Aldosterone does not alter endothelin B receptor signaling in the inner medullary collecting duct

Recent studies suggest that aldosterone‐mediated sulfenic acid modification of the endothelin B receptor (ETB) promotes renal injury in an ischemia/reperfusion model through reduced ETB‐stimulated nitric oxide production. Similarly, aldosterone inactivation of ETB signaling promotes pulmonary artery hypertension. Consequently, we asked whether aldosterone inhibits collecting duct ETB signaling; this could promote fluid retention since CD ETB exerts natriuretic and diuretic effects. A mouse inner medullary collecting duct cell line (IMCD3) was treated with aldosterone for 48 h followed by sarafotoxin‐6c, an ETB‐selective agonist, and extracellular signal‐related kinase 1/2 (ERK) phosphorylation assessed. S6c increased the phospho/total‐ERK ratio similarly in control and aldosterone‐treated cells (aldosterone alone increased phospho/total‐ERK). Since cultured IMCD cell lines lack ETB inhibited AVP signaling, the effect of S6c on AVP‐stimulated cAMP in acutely isolated IMCD was assessed. Rats (have much higher CD ETB expression than mice) were exposed to 3 days of a normal or low Na⁺ diet, or low Na⁺ diet + desoxycorticosterone acetate. S6c inhibited AVP‐stimulated cAMP in rat IMCD by the same degree in the high mineralocorticoid groups compared to controls. Finally, S6c‐stimulated cGMP accumulation in cultured IMCD, or S6c‐stimulated nitric oxide or cGMP in acutely isolated IMCD, was not affected by prior aldosterone exposure. These findings provide evidence that aldosterone does not modify ETB effects on ERK phosphorylation, AVP‐dependent cAMP inhibition, or NO/cGMP accumulation in the IMCD. Thus, while aldosterone can inhibit endothelial cell ETB activity to promote hypertension and injury, this response does not appear to occur in the IMCD.

Role of endothelin ETA receptors in sepsis-induced mortality, vascular leakage, and tissue injury in rats

The role of endothelin ETA receptors in sepsis-induced mortality and edema formation was evaluated with a selective antagonist ABT-627 [2-(4-methoxyphenyl)-4-(1,3-benzodioxol-5-yl)-1-(N,N-di(n-butyl)amino carbonylmethyl)-pyrrolidine-3-carboxylic acid]. Sprague-Dawley rats received saline (control group), Escherichia coli endotoxin (10 mg/kg, sepsis group) or infusion of ABT-627 prior and immediately after saline and endotoxin injection. Mortality, edema formation (wet/dry ratios), and multiple tissue injury (indicated by serum concentrations of creatinine, urea, bilirubin, creatine kinase, lactate dehydrogenase, and aspartate aminotransferase) were monitored within 5 h. Endotoxin injection elicited 64% mortality, significantly augmented edema formation in liver, heart, lung, and kidney, and raised serum levels of tissue injury markers. Pretreatment with ABT-627 completely reversed endotoxin-induced mortality, significantly attenuated wet/dry ratios of the heart, liver, and kidney, but not lungs, and reduced serum levels of creatine kinase, creatinine, aspartate aminotransferase, and lactate dehydrogenase, but not that of urea and bilirubin. These results suggest that endothelin ETA receptors play a significant role in promoting mortality, edema formation (except in the lungs), and tissue injury in animals with severe sepsis.

Ca2+ signalling by endothelin receptors in rat and human cultured airway smooth muscle cells

1. The aim of the current study was to characterize the ET receptor subtypes in cultured airway smooth muscle cells derived from rat trachea and human bronchus using radioligand binding techniques and to investigate the coupling of ET receptors to intracellular calcium signalling mechanisms using endothelin receptor-selective agonists (sarafotoxin S6c) and antagonists (BQ-123, BQ-788) and digital image fluorescence microscopy. 2. Confluent rat airway smooth muscle cells in culture possessed a mixed ET receptor population (30% ETA : 70% ETB), with a density of approximately 3400+/-280 ETA and 8000+/-610 ETB receptors/cell (n = 3 experiments). The density of ETB, but not ETA receptors increased substantially in serum-containing medium. However, a 2-day period of serum deprivation, which inhibited cellular growth, substantially reduced ETB receptor density such that the ET receptor subtype proportions were approximately equal (55% ETA; 45% ETB) and similar to those previously observed in intact rat tracheal smooth muscle. 3. Challenge of rat airway smooth muscle cells in culture with endothelin- 1 elicited a concentration-dependent biphasic increase in [Ca2+]i (EC50: 16 nM), that comprised an initial transient peak [Ca2+]i increase (typically 350 nM) followed by a modest sustained component. The endothelin-1-induced biphasic [Ca2+]i increase was primarily due to ETA receptor activation, although a modest and inconsistent ETB response was observed. The ETA-mediated [Ca2+]i increase was due primarily to the mobilization of IP3-sensitive and to a lesser extent ryanodine-sensitive intracellular calcium stores. In contrast, ETB receptor activation was exclusively coupled to extracellular calcium influx. 4. Somewhat surprisingly, human airway smooth muscle cells in culture contained a homogeneous population of ETA receptors at a density of 6100+/-800 receptors cell(-1) (n = 3 experiments). Serum deprivation was without effect on either ET receptor subtype proportion or ETA receptor density. Challenge of human airway smooth muscle cells with endothelin-1 provoked a concentration-dependent increase in [Ca2+]i (EC50: 15 nM), with a peak [Ca2+]i increase to greater than 700 nM. Furthermore, the ETA-mediated calcium response in these human airway smooth muscle cells in culture was entirely dependent upon the mobilization of calcium from intracellular stores. 5. In summary, rat cultured tracheal airway smooth muscle cells contained both ETA and ETB receptors. ETA receptors, the numbers of which remained constant during cell growth, were linked to the release of Ca2+ from intracellular stores and a strong rise in [Ca2+]i in the majority of airway smooth muscle cells. In stark contrast, the numbers of ETB receptors increased significantly during cell growth, an effect that was diminished substantially by incubation in serum-free medium. Moreover, despite the greater number of ETB receptors, their activation in a small number of airway smooth muscle cells produced only a weak rise in [Ca2+]i, which appeared to be attributable to the influx of extracellular Ca2+. In contrast, the populations of ET receptors and their linkage to [Ca2+]i were markedly different in the human cultured airway smooth muscle cells used in the current study compared to that previously observed in intact human isolated bronchial smooth muscle.

Bosentan-improved cardiopulmonary vascular performance and increased plasma levels of endothelin-1 in porcine endotoxin shock

1. To evaluate the possible contribution of endothelin-1 (ET-1) to the pathophysiology of porcine septic shock, the non-peptide, mixed ET-receptor antagonist, bosentan (RO 47-0203) was administered (5 mg kg-1, i.v.) 30 min before infusion of lipopolysaccharide (LPS) (E. coli., serotype 0111:B4) (15 micrograms kg-1 h-1) and at 3.5 h of endotoxaemia in six anaesthetized and mechanically ventilated pigs. Six other pigs served as controls and received only LPS infusion. Pulmonary and systemic haemodynamics as well as splenic, renal and intestinal blood flows were measured continuously. Release and synthesis of ET-1 and Big ET-1 were also measured. 2. Only three of the six pigs in the control group survived 3 h of LPS infusion while in the bosentantreated group all six pigs were alive at that time. A biphasic increase in mean pulmonary arterial pressure (MPAP) and pulmonary vascular resistance (PVR) was seen in control pigs. Pretreatment with bosentan did not influence the first peak but markedly attenuated the second, more prolonged increase in MPAP and PVR. The second dose of bosentan completely restored these parameters to pre-LPS levels. The LPS-induced changes in mean arterial blood pressure, heart rate and systemic vascular resistance were similar in both groups, while cardiac output (CO) was significantly higher in the bosentan-treated group. The second bosentan dose increased CO and splenic and intestinal blood flow without further lowering of blood pressure. 3. Bosentan caused an increase of the basal arterial plasma levels of ET-1-like immunoreactivity (LI), from 16.8 +/- 1.3 pM to 49.6 +/- 10.0 pM (n = 6, P < 0.01). However, the rate of the increase of ET-1 levels during the LPS infusion was not affected by bosentan. Repeated administration of bosentan during LPS infusion caused an additional increase of ET-1-LI levels. Neither the basal levels of Big ET-LI nor the LPS induced 8 fold increase in Big ET-LI were changed by bosentan. The level of preproET-1 mRNA in the lung was increased about 3 fold after 4.5 h of LPS treatment. This elevation was not influenced by bosentan. 4. From these studies using bosentan, a non-peptide, selective and mixed ET-receptor antagonist, we conclude that during LPS-induced shock bosentan can abolish the late phase pulmonary hypertension and improve cardiac output as well as increase blood flow to the splenic and intestinal vascular beds without causing a further decrease in mean arterial blood pressure. Further investigations in the clinical setting are needed to evaluate the use of ET-receptor antagonists, such as bosentan, in treatment of septic shock.