Influence of estrogen and progesterone on the uterine sensitivity in vitro to neuropituitary hormones in the Brazilian marsupial Didelphis albiventris: comparison with lactating animals.

The influence of different neuropituitary hormones on the contraction of the Brazilian opossum uterus in vitro was studied in spayed adults injected with (i) peanut oil (ii) estrogen or (iii) estrogen plus progesterone, and in a fourth group of lactating animals. Two parameters were analyzed from the dose-response curve: pD2 and the relative contractile response compared to the maximal one induced by oxytocin. When oxytocin was administered to the bath, neither pD2 nor the contractile force was affected by any hormonal treatment or lactation. Oxytocin, however, remained in any hormonal status, the most powerful agonist to induce uterine contraction. Lysine vasopressin was the weaker agonist in any hormonal status. It binds slightly less to the isolated uterus than to the hormone-treated one. The maximal contractile force remains unchanged when the uterus is from ovariectomized or steroid-treated animals. However, after lysine vasopressin, uterus from lactating opossum develops a less intense contractility than that observed in other groups. Arginine vasopressin induces a contractile force comparable to that induced by oxytocin in any hormonal status. The affinity of this peptide for the uterine receptor is significantly lower in ovariectomized and estrogen-treated animals; after progesterone injection or in lactating animals the receptor affinity for this hormone is increased to the level of the affinity for oxytocin. Receptor affinity for arginine vasotocin is reduced in any hormonal state and brought to a level comparable to that for oxytocin only in lactating animals. On the other hand, progesterone reduced the maximal contractile force induced by this neuropeptide.(ABSTRACT TRUNCATED AT 250 WORDS)