Studies on regulation of chorionic gonadotropin secretion in primates.

The regulation of secretion of chorionic gonadotropin in primates has been studied using both in vivo and in vitro models. In vivo studies using the pregnant bonnet monkey revealed that at the doses tested, the administration of progesterone or estradiol 17β in combination or alone did not result in any appreciable change in the duration or magnitude of serum chorionic gonadotropin levels. However, administration of lutropin-releasing hormone by intravenous route resulted in significant increase in chorionic gonadotropin levels within 30–60 min and the extent of stimulation seemed to depend on the state of pregnancy. For in vitro studies, explants or cells prepared from first trimester human placenta has been used. The functional integrity of these cells has been established by demonstrating the binding of [125I]- labelled human chorionic gonadotropin antibody to the cells as well as the synthesis of [3H]- labelled human chorionic gonadotropin. In vitro studies using the cells revealed that addition of lutropin-releasing hormone caused a significant increase in chorionic gonadotropin and estradiol 17 β secreted into the medium. Thus both in vivo and in vitro results suggest that lutropin-releasing hormone could be one of the factors involved in regulation of chorionic gonadotropin secretion in primates.

On the presence of sulphate in pituitary lutropin.

The presence of sulphate in the carbohydrate of pituitary lutropin from different species has been investigated using a biosynthetic approach. Pituitaries from rats, rabbits, goats, and buffaloes were incubated in the presence of 35SO and the 35SO 4 – -labelled proteins in the tissue immunoprecipitated with a well characterized anti-sheep lutropin serum. The incorporation into immunoreactive lutropin was low in the case of rat, rabbit and goat pituitaries while, it was considerable in the case of buffalo pituitaries. Hence further characterization studies were carried out on 35SO -labelled proteins of buffaloes. The physico-chemical, immunological and biological properties of radio-labelled buffalo pituitary material were shown to be similar to those of standard lutropin. In in vitro conditions of incubations, most of the incorporation of 35SO was observed into tissue lutropin while under similar conditions of incubation, [14C]-amino acids were found to get incorporated mostly into medium lutropin. The physiologically specific releasing hormone, lutropin-releasing hormone was found to stimulate the release of 35SO -labelled lutropin from the rabbit pituitaries into the medium. These results give indirect evidence that sulphate could be present in pituitary lutropin.

Modulation of testicular lutropin receptors in the developing male rat.

In the developing male rat around 40 days of age, the testis appears to contain the maximum amount of lutropin receptors per unit weight. During this period, circulating levels of testosterone markedly increase without the concomitant major surges in lutropin levels. The increased sensitivity and responsiveness of tests to basal levels of circulating lutropin during this period is accompanied by enhanced serum prolactin levels suggesting that this hormone may be involved in this process. The finding that prolactin treatment of pubertal rats for 3 days induced the formation of more testicular lutropin receptors supports the above premise. However, shortterm immunoneutralisation of endogenous prolactin did not significantly alter the specific binding of [ 125 I ]-labelled lutropin to testicular membranes. Interestingly, during development, a close correction exists between receptor occupancy and capacity of the tissue to bind labelled lutropin. The apparent dissociation between serum lutropin levels, on the one hand and tissue occupancy and free receptor contents on the other, suggests that factors other than lutropin (presumably prolactin) are involved in the modulation of the sensitivity and the responsiveness of the testis to lutropin during early development.

The role of follitropin and lutropin on the ovarian function in rats.

Adult cycling female rats were treated with antisera to highly purified human follitropin and lutropin for eight days. The effect of this treatment on the in vitro steroidogenic response of the ovarian cells isolated from these rats to follitropin and lutropin has been investigated. Neutralisation of follitropin did not have significant effect on steroid production in response to lutropin. However, neutralisation of lutropin resulted in a very significant inhibition of response to both follitropin and lutropin.