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 V1A (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.