Endothelin-1 inhibition of cardiac ATP-sensitive K+ channels via pertussis-toxin-sensitive G-proteins

Objective: Secretion of endothelin-1 (ET-1) and activation of cardiac ATP-sensitive K+ (K-ATP) channels are facilitated under myocardial metabolic stress. The aim of this study was to investigate the effects of ET-1 on K-ATP channels and to assess underlying mechanisms in ventricular myocytes. Methods: Single channel currents were measured with the voltage-clamp technique in cell-attached patches from enzymatically-isolated single guinea pig ventricular myocytes. In some experiments, the open-cell-attached mode was employed by permeating the membrane with streptolysin-O. Results: ET-1 concentration-dependently inhibited single K-ATP channel currents, which had been activated by metabolic poisoning, with an IC50 of 3.8 +/- 0.7 pM. BQ-123, an ET(A) receptor-selective antagonist, reduced the effects of ET-1. ET-1 effects were largely abolished in the myocytes pre-incubated with pertussis toxin. In the open-cell-attached mode, where the intracellular ATP concentration ([ATP]) could be virtually controlled, the effects of ET-1 were abolished. Muscarinic receptor stimulation inhibited the channels in a similar manner to ET-1, whereas beta-adrenoceptor stimulation accelerated channel activation. By analogy, ouabain also inhibited K-ATP channel activity under metabolic stress presumably because inhibition of the Na+/K+ pump spares subsarcolemmal ATP. ET-1 inhibited the K-ATP channels that had been reactivated in the continuous presence of ouabain. Conclusions: ET-1 reversibly inhibited K-ATP channels. This effect appears to be mediated by an increase in subsarcolemmal [ATP] which results from inhibition of adenylate cyclase activities through PTX-sensitive G-proteins coupled to ET receptors.