MYOMETRIUM AND THE RELAXIN ENIGMA

Relaxin was found to inhibit spontaneous myometrial activity in vivo in rats, guinea pigs and sheep yet leave the uterus responsive to oxytocin. It was found to be possible to inhibit the PGF2α driven rat uterus with relaxin, but in very large doses the polypeptide had no effect on oxytocin stimulated activity. These findings suggest that oxytocin and relaxin stimulate the uterus through different mechanisms. Oestradiol was found to inhibit spontaneous uterine activity in the rat and the ewe. In both species, the oxytocin response was preserved and in the former detailed studies revealed a close similarity between oestrogen and relaxin inhibitions. The suggestion is made that the action of oestrogen may be mediated through the secretion of relaxin. These findings have led to the hypothesis that in the rat relaxin provides a protective mechanism, at least in late pregnancy. Thus the evolution of spontaneous activity is restrained while the conductivity and oxytocin sensitivity of the uterus increases to a peak. This peak of contractility is synchronized with the release of a centrally originating signal capable of overriding relaxin inhibition, which culminates in labour. © 1979.