Growth hormone directly stimulates testosterone and oestradiol secretion by rat leydig cells in vitro and modulates the effects of LH and T-3

The modulatory effect of GH on basal, LH and Tg mediated secretion of testo sterone and oestradiol by purified rat (60 day old) Leydig cells was studie d in vitro. Percoll gradient purified Leydig cells (1 x 10(3)) were culture d for 48 hours at 34 degrees C in a medium containing different concentrati ons of rat GH (5-400 ng/mL), after an initial culture for 24 hours at 37 de grees C. GH increased testosterone and oestradiol secretions in a dose depe ndent manner. While testosterone secretion reached the saturation point wit h 50 ng GH, oestradiol secretion reached the saturation point with 150 ng G H, followed by diminished secretions. Go-administration of minimum (10 ng) effective does of GH with minimum (25 ng) or maximum (100 ng) effective dos es of oLH significantly decreased the testosterone secretion. However, an i ncreased secretion of testosterone was observed when maximum effective dose s of rGH (50 ng) and oLH (100 ng) were co-administered. Minimum effective ( 25 ng) or maximum effective (50 ng) doses of T-3 inhibited GH mediated secr etion of testosterone in vitro. Oestradiol concentration in the culture med ium increased when either dose of rGH was co-administered with the minimum or maximum effective doses of oLH. T3 50 ng augmented the secretion of oest radiol by Leydig cells in the presence of GH. These results indicate that G H acts as a gonadotrophin to stimulate testosterone and oestradiol secretio ns by Leydig cells, and that it modulates LH or TS induced secretion of the se steroids, depending on the intensity of their stimulation.