Relaxant effect of oxytocin on isolated human oviduct

Aim. Oxytocin produces concentration-dependent relaxation of isolated isthm us and ampulla of human oviduct precontracted by histamine. The aim of our study was to investigate whether this oxytocin effect was specific and by w hich receptors it was mediated. Methods. We investigated effects of oxytocin and its antagonists on isolate d isthmus and ampulla of the uterine tubes from 20 women who underwent surg ery for uterine fibroids. Selective vasopressin and oxytocin antagonists we re used to treat isolated preparations of the tubes. Results. In a concentration-dependent manner, oxytocin enhanced spontaneous relaxation of both isthmus (EC50=1.23+/-0.03x10(-7) mol/L) and ampulla (EC 50=1.04+/-0.26x10(-7) mol/L) precontracted by histamine. Neither predominan tly selective vasopressin V-1 receptor antagonist, [beta -mercapto-beta,bet a -cyclopentamethylene-propionyl[1,0-metyr2,arg8]-vasopressin (1.0x10(-9)-1 .0x-10(-7) mol/L), nor predominantly selective vasopressin V-2 antagonist, [1-(beta -mercapto-beta, beta -cyclopentamethylene-propionic acid), 2-D-iso leucine, 4-D-isoleucine]-arginine-vasopressin (1.0x 10(-9)-1.0x10(-7) mol/L ) affected significantly the relaxation of isolated ampulla and isthmus pro duced by oxytocin. On the other hand, [Deamino-Cys(1),D-Tyr (Et)(2), Thr(4) , Orn(8)]-oxytocin, a selective blocker of oxytocin receptors, produced in a concentration-dependent manner (6.7x10(-9) mol/L, 2.0x10(-8) mol/L, and 6 .7x10(-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 pA(2) for [Deamino-Cys(1),D-Tyr (Et) (2), Thr(4), Orn(8)]-oxytocin calculated from constrained Schild's plot wer e 8.08+/-1.53 for ampulla and 7.94+/-0.67 for isthmus. Conclusion. Oxytocin relaxes smooth muscles in human oviduct through a spec ific effect on oxytocin receptors.