Two functional Na+/K+-ATPase isoforms in the left ventricle of guinea pig heart (original) (raw)

Guinea pig left ventricular muscle contains two distinct molecular forms of the N a + /K+-ATPase catalytic a subunit. Sarcolemmal vesicles highly enriched in Na+/K+-ATPase were isolated by a new procedure that yielded specific activities of 60 -100 pmol Pi . h-' . mg-'. SDSjPAGE of isolated sarcolemma after reduction and alkylation of the sulfhydryl groups and identification on immunoblots with specific anti-(a subunit) antibodies indicated the presence of two major polypeptides of 100 kDa and 103 kDa, respectively. The two a subunits were functional: the dose/response curves of Na+/K+-ATPase activity with ouabain, dihydroouabain and digitoxigenin were biphasic, revealing the presence of high-affinity [concentration of drug causing 50% inhibition (IC50) = 10 nM] and low-affinity (ICs0 = 2 pM) 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.