Relaxin Receptors in the Myometrium and Cervix of the Pig

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Abstract
The binding of 125I-relaxin to particulate membrane preparations of myometrium and cervix from the pig was characterized and investigated in different physiological and experimental states. Binding properties were very similar for the 2 tissues: binding was rapid, exhibited a limited steady-state and had comparable rates of dissociation at 27.degree. C; divalent cations in the order Mn2+ > Ca2+ enhanced 125I-relaxin binding. Scatchard analysis yielded curvilinear plots compatible with the presence of 2 classes of binding sites. The high affinity binding sites in fractions from both tissues exhibited similar affinity constants Ka (in the order of 1010 M-1) although with differing binding capacities. Unlabeled relaxin had the greatest inhibitory action on the binding of labeled relaxin to both tissues while NGF [nerve growth factor] > proinsulin > insulin were less effective in this respect. Myometrial and cervical fractions from PMSG [pregnant mare''s serum gonadotropin]-hCG[human chorionic gonadotropin]-treated cyclic and prepubertal pigs exhibited high affinity binding sites. PMSG-hCG-treated prepubertal pigs exhibited slightly more binding in the myometrium than cervix which was unchanged 24 h after ovariectomy. Similar animals examined 10 days after ovariectomy had a significantly decreased number of myometrial binding sites (P < 0.01) which decreased to undetectable levels at 16 days postovariectomy. Cervical fractions showed complete lack of binding by 10 days postovariectomy. Similar ovariectomized animals, treated with estrogen daily for 7 days, showed recovery of relaxin receptors in the myometrial fractions comparable in number to those of control animals; cervical receptors remained undetectable. The data suggest a different hormonal control of the myometrium and cervix with regard to their sensitivity to relaxin as expressed by receptor concentraion. Apparently, this may have biological significance in view of the differential time scale for the need of relaxin action on uterine quiescence and cervical dilatation during pregnancy.