Relaxant effect of oxytocin on isolated human oviduct.

Aim: Oxytocin produces concentration-dependent relaxation of isolated isthmus and ampulla of human oviduct precontracted by histamine. The aim of our study was to investigate whether this oxytocin effect was specific and by which receptors it was mediated.

Methods: We investigated effects of oxytocin and its antagonists on isolated isthmus and ampulla of the uterine tubes from 20 women who underwent surgery for uterine fibroids. Selective vasopressin and oxytocin antagonists were used to treat isolated preparations of the tubes.

Results: In a concentration-dependent manner, oxytocin enhanced spontaneous relaxation of both isthmus (EC50=1.23+/-0.03 x 10(-7) mol/L) and ampulla (EC50=1.04+/-0.26 x 10(-7) mol/L) precontracted by histamine. Neither predominantly selective vasopressin V1 receptor antagonist, [beta-mercapto-beta,beta-cyclopentamethylene-propionyl1,0-metyr2,arg8]-vasopressin (1.0 x 10(-9)-1.0 x 10(-7) mol/L), nor predominantly selective vasopressin V2 antagonist, [1-(beta-mercapto-beta,beta-cyclopentamethylene-propionic acid), 2-D-isoleucine, 4-D-isoleucine]-arginine-vasopressin (1.0 x 10(-9)-1.0 x 10(-7) mol/L) affected significantly the relaxation of isolated ampulla and isthmus produced by oxytocin. On the other hand, [Deamino-Cys1,D-Tyr (Et)2, Thr4, Orn8]-oxytocin, a selective blocker of oxytocin receptors, produced in a concentration-dependent manner (6.7 x 10(-9) mol/L, 2.0 x 10(-8) mol/L, and 6.7 x 10(-7) mol/L) significant shifts of the concentration-response curves of relaxation for oxytocin to the right in isolated preparations of both the ampulla and the isthmus. The values of pA2 for [Deamino-Cys1,D-Tyr (Et)2, Thr4, Orn8]-oxytocin calculated from constrained Schilds plot were 8.08+/-1.53 for ampulla and 7.94+/-0.67 for isthmus.

Conclusion: Oxytocin relaxes smooth muscles in human oviduct through a specific effect on oxytocin receptors.