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

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.

Cardiac plasticity in fishes: environmental influences and intraspecific differences

“…In mammals, numerous cellular pathways and end-effectors are involved in preconditioning (Okubo et al, 1999;Nakano et al, 2000;Yellon and Downey, 2003). No experiments have directly investigated the cellular mechanisms that mediate myocardial preconditioning in fishes, although recent studies suggest that sarcolemmal (Cameron et al, 2003) and mitochondrial (MacCormack and Driedzic, 2002) ATPsensitive K + channels, and MAPK signaling pathways (ERK, JNKs and p38-MAPK; Gaitanaki et al, 2003) may be involved.…”

Nitric oxide level and von Willebrand factor (vWF) secretion are not candidate markers of endothelial cell dysfunction in adenosine triphosphate (ATP) depleted endothelial cells

“…Intracellular alteration of Na and calcium concentrations following ATP depletion is associated with activation of inwardly rectifier K channels (KIR) of high conductance, to facilitate cell membrane re-polarization (Lien et al, 2003;Mintert et al, 2007;Harrell et al, 2007;Pouget, 2008). During hypoxia, ischemia or metabolic inhibition, ATP-dependent K channels (K ATP ) open, which facilitate K influx and shortening of action potential duration (Harrell, 2007;Pouget, 2008;Lebuffe et al, 2003;Cameron et al, 2003;Lascano et al, 2002). Therefore, reduced extracellular K concentration, following ATP depletion seems to be due to the activation of ATPdependent KIR channels, located in the membrane of endothelial cells (Cameron et al, 2003;Hu et al, 2003).…”

“…During hypoxia, ischemia or metabolic inhibition, ATP-dependent K channels (K ATP ) open, which facilitate K influx and shortening of action potential duration (Harrell, 2007;Pouget, 2008;Lebuffe et al, 2003;Cameron et al, 2003;Lascano et al, 2002). Therefore, reduced extracellular K concentration, following ATP depletion seems to be due to the activation of ATPdependent KIR channels, located in the membrane of endothelial cells (Cameron et al, 2003;Hu et al, 2003). The other explanation for reduced extracellular K concentration might be increased K uptake through Na-K-Cl cotransport, as seen in rat brain capillary endothelial cells (RBECs) under hypoxic conditions (Kawai et al, 1996).…”

Cardioprotective effects of KATP channel activation during hypoxia in goldfish Carassius auratus

“…Procedures have been previously described (Cameron et al, 2003); briefly, the entire heart was mounted in a tissue bath and superfused with normoxic saline solution at a rate of 15·ml·min -1 . This solution contained (in mmol·l -1 ) 150 NaCl, 5 KCl, 1.2 NaH 2 PO 4 , 1.2 MgSO 4 , 1.8 CaCl 2 , 10 Hepes and 10 glucose; pH was maintained at 7.6 and temperature held at 21±1°C.…”

“…In addition, nitric oxide synthase (NOS) has been localized in the hearts of goldfish (Bruning et al, 1996), and NO has been shown to play a critical role, via cGMP, in regulating the heart and peripheral vasculature in a variety of teleost species (Imbrogno et al, 2003;Jennings et al, 2004;Pellegrino et al, 2003). In goldfish, NO synthesized in cardiac myocytes plays a role in sarcK ATP channel activation during moderate hypoxia (Cameron et al, 2003). Although it has been proposed that hypoxia-induced K ATP channel activation, whether in the heart or in the brain, contributes to the enhanced tolerance of environmental oxygen depletion exhibited by many ectothermic vertebrates (Cameron and Baghdady, 1994;Pek-Scott and Lutz, 1998), this possibility has not been directly tested in fish.…”