SODIUM-PUMP INHIBITION IN SARCOLEMMA FROM ISCHEMIC HEARTS

Ischemic myocardial cells lose K+ and accumulate Na+. The role of the Na+/K+-pump in these changes was investigated by measuring both Na+/K+-ATPase activity and Na+ pumping in highly purified sarcolemmal vesicles from rabbit hearts made globally ischaemic for 1 h compared to non-ischemic controls, Purification of the sarcolemma was similar for control, 31 +/- 8-fold, and ischemia, 38 +/- 10-fold. The fraction of intact inside-out vesicles, in which Na+ pumping could be measured, was also the same for control, 60 +/- 16%, and ischemic, 56 +/- 8% as measured by H-3-ouabain binding in the presence and absence of detergent, Scatchard analysis of ouabain binding revealed a 26% increase in binding sites in ischemia compared to control. The Na+/K+-ATPase in the inside-out vesicles, measured as monensin-stimulated activity, was not affected by ischemia: 22 +/- 9 v 21 +/- 9 mu mol P-1 mg(-1) h(-1) for control and ischemic respectively, However, the initial velocity of ATP-dependent Na+ pumping into inside-out vesicles, assayed by subsequent exchange of Na-i(+) for Ca-45(2+) by the Na+-Ca2+ exchanger present in the vesicles, was inhibited in ischemia. At 18 mM Na-0(+) the velocity for control vesicles was 2.4 +/- 0.2 nmol mg(-1) s(-1) compared to 1.1 +/- 0.1 for ischemia vesicles, Passive sarcolemmal Na+ permeability was unchanged after 1 h of ischemia. The large reduction in Na+ pumping with unchanged Na+/K+-ATPase suggests uncoupling of the Na+/K+-pump in ischemia and a decreased ability to extrude Na+ despite the increase in number of pump sites in the sarcolemma. (C) 1995 academic Press Limited