Endothelium-dependent blunted membrane potential responses to ATP-sensitive K + channel modulators in aortae from rats with cirrhosis

Abstract

Background/Aims: In vivo studies have shown that arterial vasodilation induced by synthetic openers of ATP-sensitive K + (K ATP) channels is decreased in rats with cirrhosis. Since vasodilation induced by these substances is mediated by membrane potential hyperpolarization in arterial smooth muscle cells, membrane potential hyperpolarization in response to K ATP channel openers may be altered in cirrhotic smooth muscle cells. The aim of the present study was to investigate the effects of K ATP channel modulators (i.e. openers and blockers of these channels) on the membrane potential in smooth muscle cells in isolated aortae from cirrhotic and normal rats. The influence of endothelin-1 production by endothelial cells on smooth muscle cells membrane potential responses to K ATP channel modulators was also studied. Methods: Cells were impaled in situ (in intact and endothelium-denuded aortae) with a microelectrode that was used to measure membrane potentials. K ATP channel openers were diazoxide or cromakalim; blockers were glibenclamide or tolbutamide. Bosentan (a mixed endothelin receptor antagonist) and exogenous endothelin-1 were also used. Preproendothelin-1 mRNA was assayed in aortae by RNase protection assay. Aortic wall endothelin-1 concentration was measured by double antibody radioimmunoassay technique. Results: As expected, in smooth muscle cells in intact normal aortae, K ATP channel openers induced membrane potential hyperpolarization and K ATP channel blockers membrane potential depolarization. In smooth muscle cells in intact cirrhotic aortae, K ATP channel openers and blockers did not significantly change the membrane potential. Endothelium removal or exposure of intact aortae to bosentan restored normal membrane potential responses to K ATP channel modulators in cirrhotic smooth muscle cells and did not alter the effects of these substances in normal smooth muscle cells. In endothelium-denuded aortae, exposure to exogenous endothelin-1 suppressed membrane potential responses to K ATP channel modulators. In intact aortae, the abundance of preproendothelin-1 mRNA and endothelin-1 did not significantly differ between normal and cirrhotic rats. Conclusions: K ATP channel opener-induced membrane hyperpolarization and K ATP channel blocker-elicited membrane depolarization are blunted in smooth muscle cells in intact cirrhotic aortae. This blunting is due to the activation of the endothelin-1 pathway in the aortic wall, downstream to the endothelial production of endothelin-1.