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 3H-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 mumol Pi 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 45Ca2+ by the Na(+)-Ca2+ exchanger present in the vesicles was inhibited in ischemia. At 18 mM Na+o 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.