Effects of human relaxin on isolated rat and human myometrium and uteroplacental arteries

We investigated the effects of synthetic human relaxin (hRLX-2) on isolated rat and human myometrium and on uteroplacental arteries from term pregnant women. The preparations were mounted in organ baths and isometric tension was recorded. In isolated myometrium from nonpregnant rats, hRLX-2 (10(-10)-10(-7) mol/L) produced concentration-dependent inhibition of contractile activity induced by vasopressin (10(-8) mol/L). In isolated human myometrium from the fundus or isthmus, hRLX-2 (10(-10)-10(-7) mol/L) did not influence spontaneous activity or contractions induced by oxytocin (10(-9) mol/L) and prostaglandin (PG) F2 alpha (10(-5) mol/L). Nor did it influence the tension induced in small intramyometrial arteries by U46619 (10(-7) mol/L), noradrenaline (10(-5) mol/L), and endothelin (10(-9) mol/L); or the tension induced in fetal stem villus arteries by U46619 (10(-7) mol/L), endothelin (10(-9) mol/L), and PGF2 alpha (10(-5) mol/L). The inhibitory effects of hRLX-2 in preparations of rat myometrium were not influenced by the presence of human myometrium in the organ bath or by pre-incubation of hRLX-2 with human myometrium. These results suggest that direct inhibitory effects of relaxin may be of minor importance for the regulation of myometrial activity and uteroplacental circulation in term human pregnancy.