DECREASED RESPONSE TO VASOPRESSIN IN THE MESENTERIC RESISTANCE ARTERIES OF PREGNANT RATS - EFFECTS OF NIFEDIPINE AND BAY K 8644

Objective: The purpose of this study was to investigate the contributi on of potential-operated calcium (POC) channels in the mechanisms of t he blunted effects of vasoconstrictors on mesenteric resistance arteri es during normal pregnancy. Methods: Mesenteric resistance arteries of virgin and term pregnant rats were set up under optimum passive tensi on in wire myograph systems. Cumulative concentration-response curves of arginine8-vasopressin (AVP) were measured in the absence and presen ce of nifedipine or Bay K 8644, a blocker and an activator, respective ly, of POC channels. Binding studies were performed on membrane prepar ations of the mesenteric vascular bed of both groups of rats using sat uration with [H-3]nitrendipine. Results: The maximal response to AVP w as statistically similar in the two groups of arteries. Pregnancy shif ted the AVP concentration-response curves to the right. Nifedipine (1 mumol/L) similarly reduced the maximum response to AVP in arteries of both groups, but produced a larger increase in EC50, the concentration inducing 50% maximum response, in resistance arteries of virgin versu s pregnant rats. Bay K 8644 did not affect the maximum tension reached with AVP. However, it increased the effects of small concentrations o f AVP in arteries of both groups. This was more important in tissues o f virgin than pregnant rats. Binding of [H-3]nitrendipine to membrane preparations of mesenteric vessels was not modified by pregnancy. Conc lusion: Our results suggest a reduced functional influence of POC chan nels in the myotropic effects of AVP on mesenteric resistance arteries in pregnancy. This decreased influence of POC channels may contribute to resistance of the vasculature to vasopressor agents during pregnan cy.