Inhibition of Na/K ATPase From Rat Aorta by Two Na/K Pump Inhibitors, Ouabain and Marinobufagenin: Evidence of Interaction With Different α-Subunit Isoforms

Recently, we have shown that mammalian plasma cross-reacts with an antibody to a bufodienolide Na/K ATPase inhibitor, marinobufagenin. In rat aorta, marinobufagenin induced vasoconstriction via inhibition of the vascular smooth muscle Na/K pump, whereas ouabain had its predominant effect on pumps localized to nerve endings. Na/K ATPase inhibitory effects of ouabain and marinobufagenin were studied in two membrane fractions isolated from Fisher 344×BN rat thoracic aorta by sucrose density gradient centrifugation. One fraction contained predominantly the α-3 isoform of Na/K ATPase and represented membranes from the perivascular nerve endings (neuronal plasmalemma). The other membrane fraction, containing predominantly the α-1 isoform, was derived from the vascular smooth muscle sarcolemma. The IC50 for inhibition of the Na/K ATPase by ouabain and marinobufagenin were 2.6 nmol/L and 0.14 μmol/L in the neuronal plasmalemma, and 50 nmol/L and 2.1 nmol/L in sarcolemma, respectively. These results confirm the view that differential responsiveness to endogenous digitalis-like factors is a functional feature of α isoforms of Na/K ATPase