Adaptation to pregnancy leads to attenuated rat uterine artery smooth muscle sensitivity to oxytocin

Objective: We tested the hypothesis that oxytocin (OT) contracts blood vessels via vasopressin V_1A (VP) receptors, and that this depends on pregnancy. Study design: Concentration-contraction relationships (CCR) to OT and VP (10^-12-10^-6 mol/L) were obtained in different blood vessels. CCR were obtained in uterine arteries (UA) from nonpregnant, mid-pregnant, and late pregnant rats (n = 6-10 per group) in the absence and presence of selective antagonists (10^-7 mol/L). Results: Sensitivity to OT, but not to VP, is attenuated in pregnant rat UA. Antagonists shifted CCR of OT and VP to the right, and, to a lesser extent, of the counterpart, in all UA. VP antagonist depresses oxytocin CCR much more than OT antagonist in pregnant rat UA. Conclusion: OT and VP contract UA via their own receptors, although partial cross-activation is evident. Adaptation to pregnancy led to attenuated sensitivity of UA smooth muscle to OT and transformed OT receptors into VP-like ones.