EFFECT OF INHIBITION OF NA+/K+-ADENOSINE TRIPHOSPHATASE ON VASCULAR ACTION OF VASOPRESSIN

The present study was undertaken to examine the cellular interaction between a Na+/K+-ATPase inhibitor, ouabain, and arginine vasopressin (AVP) in rat vascular smooth muscle cells (VSMC) in culture. Preincubation with 10-5 M ouabain for 60 min increased basal cytosolic free Ca2+ ([Ca2+](i)) concentration and intracellular 45Ca2+ uptake. Ouabain, however, did not affect basal 45Ca2+ efflux or AVP-stimulated 45Ca2+ efflux. As assessed by cell shape change, preincubation with 10-5 M ouabain for 60 min also enhanced the sustained cellular contractile effect of a submaximal (10-8 M AVP, 21.5% vs. 30.5%, P < 0.01) but not maximal dose of 10-6 M AVP. Preincubation with 10-5 M ouabain for 60 min did not change AVP-induced V1-specific surface receptor binding or AVP-induced inositol phosphate production but did however potentiate the mobilization of [Ca2+](i) induced by a submaximal (10-8 MAVP, 301 vs. 385 nM, P < 0.01) but not a maximal dose of AVP. These effects of ouabain on the mobilization of [Ca2+](i) were abolished by incubation in Ca2+-free buffer or 5 x 10-5 M verapamil. Ouabain (10-5 M) also enhanced the sustained cellular contractile effect of a direct protein kinase C activator, phorbol 12-myristate 13-acetate. The present results therefore indicate that the inhibition of Na+/K+-ATPase may enhance the vascular action of AVP, and perhaps other vasoconstrictors, by increasing the AVP-induced mobilization of [Ca2+](i) and by potentiating the activity of protein kinase C stimulated by AVP through enhancing basal and AVP-stimulated Ca2+ uptake.