Sodium pump inhibition in sarcolemma from ischemic hearts

Ischemic myocardial cells lose K+ and accumulate Na+. The role of 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 μmol P1mg−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+1 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.