Ontogeny of oxytocin receptors and oxytocin-induced stimulation of prostaglandin synthesis in prepubertal heifers.

Abstract

Developmental aspects of oxytocin (OT) receptors (OTR) in uterine tissues before puberty are not known. Bovine ovaries secrete some estradiol, but no progesterone, before puberty; the circulating levels of estradiol are between 1 and 3 pg/ml until puberty. Cross-bred Angus-Brahman heifers, in which puberty occurs around 12 months of age, were used to determine the concentrations of OTR from the late fetal stage to adulthood. PGF2alpha release in response to OT was determined in 3-, 6-, and 9-month-old heifers (n = 4 each). Myometrium, endometrium, and cervical mucosa were obtained from 3-week-old, 3-month-old, 6-month-old, and 9-month-old heifers and from adult cows at estrus. Whole uterus and cervix were taken from third trimester fetuses and at birth. [3H]OT binding and specificity, localization of immunoreactive (ir) OTR, OTR messenger RNA, and OT-induced release of PGF2alpha were determined. The uterus from fetuses and the neonate expressed OTR messenger RNA and bound [3H]OT. At 3 weeks of age, OTR concentrations per mg protein were very low, but at 3 months of age they had increased markedly in all three tissues. At 6 and 9 months of age, levels of OTR had risen further and were similar to those in adult cows at estrus. Prepubertal uterus also possessed separate vasopressin VP1 subtype receptors. The ir-OTR was localized in luminal epithelial cells of endometrium and cervical mucosa, most of which were ir positive, whereas in myometrium, clusters of ir-OTR-positive cells were found among large numbers of ir-OTR-negative cells. The PGF2alpha response to OT was insignificant in heifers of all age groups, in contrast to that in cows at estrus. Endometrial cells from 4- to 5-month-old heifers did not respond to OT with PG release in the absence or presence of added arachidonic acid. Tumor promoters, lipopolysaccharide, and interleukin-2 also failed to elicit PG release in vitro, although they induced PG release in similar cell cultures from cyclic cows. In summary, uterine tissues of prepubertal heifers have high levels of OTR, which appear to be developmentally regulated. These receptors are not coupled to PG synthase, or alternatively, the PG synthase gene is not expressed before puberty, possibly because the tissues have had no previous exposure to progesterone.