Dissociation between cellular K+ loss, reduction in repolarization time, and tissue ATP levels during myocardial hypoxia and ischemia

Abstract

The mechanisms underlying the marked increase in [K+]o in response to ischemia are not fully understood. Accordingly, the present study was performed to assess the contribution of ATP-regulated K+ channels by using simultaneous measurements of cellular K+ efflux, [K+]o, transmembrane action potentials, and tissue ATP, ADP, phosphocreatine, and creatine content in a unique isolated, blood-perfused papillary muscle preparation during hypoxia compared with ischemia. During 15 minutes of hypoxic perfusion (PO2, 6.1±0.9 mm Hg) with normal [K+]o of 4.1±0.1 mM, action potential duration (APD) was not altered even though tissue ATP levels decreased markedly from 33.5±1.8 to 14.7±2.0 nmol · mg protein-1 (p<0.01). Net cellular K+ efflux, based on measured differences of [K+] between the venous effluent and the perfusate, was 13.23±0.79 μmol · g wet wt-1 during hypoxia. In contrast, after 15 minutes of zero-flow ischemia, APD at 80% of repolarization (APD80) decreased by 47% from 171±5 to 92±5 msec (p<0.01), but integrated net cellular K+ efflux over 15 minutes of ischemia was 8.4-fold less (1.57±0.13 μmol · g wet wt-1) than during hypoxia. Tissue ATP levels, however, decreased by only 35.2% to 21.7±2.1 nmol · mg protein-1, which was significantly less than that induced by 15 minutes of hypoxia. Perfusion with hypoxic blood containing high [K+]o of 10.3±0.3 mM resulted in APD shortening similar to that observed during ischemia. Cellular K+ loss, however, was inhibited markedly by high [K+]o perfusion (only 4.51±0.28 μmol · g wet wt-1). Pretreatment with glibenclamide (5 μM), a drug that has been reported to inhibit ATP-regulated K+ channels and accelerate glycolysis in normoxic tissue, partially inhibited cellular K+ efflux during hypoxic perfusion with normal [K+]o (7.35±0.71 versus 13.23±0.79 μmol · g wet wt-1, p<0.01) but had no significant influence on repolarization time or tissue ATP levels. Although glibenclamide partially prevented action potential shortening induced by hypoxic perfusion in the presence of elevated [K+]o, the proportion of cellular K+ efflux reduced by glibenclamide was less (23%) than that observed with glibenclamide in hypoxic perfusion with normal [K+]o (44%). These findings indicate that 1) APD remained relatively normal after 15 minutes of hypoxia despite the fact that cellular K+ loss was markedly greater and ATP content was significantly less than during ischemia, suggesting that neither ATP depletion nor cellular K+ loss per se is responsible for shortening of repolarization during ischemia, and 2) [K+]o accumulation is responsible for the shortening of repolarization that also inhibits cellular K+ loss.