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

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 mumol.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 mumol.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 mumol.g wet wt-1). Pretreatment with glibenclamide (5 microM), 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 mumol.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%).(ABSTRACT TRUNCATED AT 400 WORDS)