A role for nitric oxide in hypoxia-induced activation of cardiac KATP channels in goldfish (Carassius auratus)

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Abstract
SUMMARY Hypoxia-induced shortening of cardiac action potential duration (APD) has been attributed in mammalian hearts to the activation of ATP-sensitive potassium (KATP) channels. Since KATP channels are also present at high densities in the hearts of vertebrate ectotherms, speculation arises as to their function during periods of reduced environmental oxygen. The purpose of the present study was to determine whether nitric oxide (NO) plays a role in cardiac sarcolemmal KATP channel activation during hypoxia in a species with a high degree of tolerance to low oxygen environments: the goldfish (Carassius auratus). Conventional intracellular and patch-clamp recording techniques were used to record responses from excised ventricles or isolated ventricular myocytes and inside-out patches, respectively, from fish acclimated at 21°C. During moderate, substrate-free hypoxia (6.1±0.2 kPa), ventricular APD was significantly shortened at 50% and 90% of full repolarization, a response that was reversible upon reoxygenation and blocked by the KATP channel antagonist BDM. Under normoxic conditions, APD was also reduced in the presence of the NO-donor SNAP (100 μmol l-1). In cell-attached membrane patches, sarcolemmal KATP channel activity was enhanced after 10 min hypoxia, an effect that was reduced or eliminated by simultaneous exposure to BDM, to the guanylate cyclase inhibitor ODQ or to the NO synthase inhibitor l-NAME. In cell-free patches, KATP channel activity was abolished by 2 mmol l-1 ATP but increased by SNAP; the cGMP analog 8-Br-cGMP (200 μmol l-1) also enhanced activity, an effect that was eliminated by BDM. Our data indicate that NO synthesized in cardiac myocytes could enhance sarcolemmal KATP channel activation during moderate hypoxia in goldfish. This response may serve a cardioprotective role by helping to conserve ATP or by reducing intracellular Ca2+ accumulation.