Growth hormone directly stimulates testosterone and oestradiol secretion by rat Leydig cells in vitro and modulates the effects of LH and T3.

The modulatory effect of GH on basal, LH and T3 mediated secretion of testosterone and oestradiol by purified rat (60 day old) Leydig cells was studied in vitro. Percoll gradient purified Leydig cells (1×103) were cultured for 48 hours at 34°C in a medium containing different concentrations of rat GH (5-400ng/mL), after an initial culture for 24 hours at 37°C. GH increased testosterone and oestradiol secretions in a dose dependent manner. While testosterone secretion reached the saturation point with 50ng GH, oestradiol secretion reached the saturation point with 150ng GH, followed by diminished secretions. Co-administration of minimum (10ng) effective does of GH with minimum (25ng) or maximum (100ng) effective doses of oLH significantly decreased the testosterone secretion. However, an increased secretion of testosterone was observed when maximum effective doses of rGH (50ng) and oLH (100ng) were co-administered. Minimum effective (25ng) or maximum effective (50ng) doses of T3 inhibited GH mediated secretion of testosterone in vitro. Oestradiol concentration in the culture medium increased when either dose of rGH was co-administered with the minimum or maximum effective doses of oLH. T3 50ng augmented the secretion of oestradiol by Leydig cells in the presence of GH. These results indicate that GH acts as a gonadotrophin to stimulate testosterone and oestradiol secretions by Leydig cells, and that it modulates LH or T3 induced secretion of these steroids, depending on the intensity of their stimulation.