THE ROLE OF CALCIUM IN LUTEINIZING-HORMONE HUMAN CHORIONIC-GONADOTROPIN STIMULATION OF LEYDIG-CELL IMMUNOACTIVE INHIBIN SECRETION INVITRO

The mechanism by which luteinizing hormone (LH) stimulates Leydig cell immunoactive inhibin (I-inhibin) secretion was investigated using Percoll-purified adult rat Leydig cells. Using a maximally stimulating dose of LH (16 ng/ml), Leydig cell I-inhibin secretion was non-detectable at 1-2 h of incubation, but subsequently increased at all time points during a 25 h incubation period. LH stimulated both Leydig cell content and release of I-inhibin. Increasing concentrations of LH stimulated both inhibin and testosterone immunoactivity in the incubation media over a similar dose-response range, with a 2- to 4-fold rise in I-inhibin secretion at maximal doses of LH. Dibutyryl cAMP stimulated testosterone secretion in a manner similar to that of LH, but I-inhibin secretion was less sensitive than testosterone and a significant stimulation was observed only at the highest doses (200-1000-mu-g/ml). LH-stimulated I-inhibin secretion was significantly decreased when Leydig cells were incubated in calcium-depleted (0.15 mM Ca2+ + 1 mM EGTA) or low [Ca2+] media (0.15 mM) as compared to normal (1.15 mM) or high [Ca2+] (2-5 mM) media. In contrast, LH-stimulated testosterone secretion remained unchanged by altering extracellular [Ca2+], and although decreased in the presence of EGTA, testosterone secretion remained significantly greater than basal levels. Furthermore both diltiazem and verapamil completely blocked the LH and dibutyryl cAMP-stimulated increase in Leydig cell I-inhibition, but did not reduce either LH or dibutyryl cAMP-stimulated testosterone production to basal levels. We conclude that LH stimulates both I-inhibin synthesis and release by adult rat Leydig cells in culture by mechanisms involving calcium.