Two functional Na+/K+‐ATPase isoforms in the left ventricle of guinea pig heart

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Abstract
Guinea pig left ventricular muscle contains two distinct molecular forms of the Na+/K+‐ATPase catalytic α subunit. Sarcolemmal vesicles highly enriched in Na+/K+‐ATPase were isolated by a new procedure that yielded specific activities of 60–100 μmol Pi· h−1· mg−1. SDS/PAGE of isolated sarcolemma after reduction and alkylation of the sulfhydryl groups and identification on immunoblots with specific anti‐(α subunit) antibodies indicated the presence of two major polypeptides of 100 kDa and 103 kDa, respectively. The two α subunits were functional: the dose/response curves of Na+/K+‐ATPase activity with ouabain, dihydrooubain and digitoxigenin were biphasic, revealing the presence of high‐affinity [concentration of drug causing 50% inhibition (IC50) = 10nM] and low‐affinity (IC50= 2 μM) forms with proportional contributions of 55% and 45%, respectively. The involvement of the high‐affinity form in the positive inotropic effect of digitalis and of the low‐affinity sites in both inotropy and toxicity are consistent with the literature data on rodents.