Effect of pregnancy on vasopressin-mediated responses in guinea-pig uterine arteries with intact and denuded endothelium.

The effect of pregnancy on vasopressin-induced contraction of guinea-pig uterine arterial rings was investigated. Initially, vasopressin induced contraction (pD2 = 9.14) in pregnant guinea-pig uterine artery with greater potency than in non-pregnant guinea-pig uterine artery (pD2 = 8.77). Removal of the endothelium did not affect vasopressin-induced contractions, regardless of pregnancy status. In all types of preparations, [d(CH2)5Tyr(Me)2]vasopressin (10-100 nM) and [d(CH2)5,D-Ile2,Ile4]vasopressin (300 nM-3 microM) produced parallel rightward shifts of the curves for vasopressin. The Schild plots constrained to a slope of unity gave the following -log KB values: [d(CH2)5Tyr(Me)2]vasopressin vs. [d(CH2)5,D-Ile2,Ile4]vasopressin 8.74 vs. 6.82 and 8.50 vs. 6.72 for non-pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively; 8.38 vs. 6.49 and 8.36 vs. 6.75 for pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively. The pKA values for vasopressin itself also did not differs between preparations: 6.49 and 6.55 for non-pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively; 6.48 and 6.52 for pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively. The receptor reserve (KA/EC50) was significantly greater in preparations taken from pregnant than from non-pregnant animals. It is concluded that vasopressin-induced contractions of guinea-pig uterine artery are not modulated by the endothelium, regardless of pregnancy status. The receptor reserve for vasopressin in guinea-pig uterine artery is increased during pregnancy, that is not related to the changes of vasopressin receptor affinity for vasopressin. It is probable that vasopressin receptors involved in vasopressin-induced contraction of all types of vessels studied belong to the V1A-like subtype.