CALCIUM SIGNALING IN SINGLE-RAT LEYDIG-CELLS

The steroidogenic activity of the Leydig cell is regulated by glycopro tein and peptide hormones with the potential to activate both adenylat e cyclase and phospholipase C. Although the control of androgen produc tion by LH is clearly mediated by cAMP, the extent to which Ca2+-mobil izing stimuli control Leydig cell function is less well defined. The b asal level of intracellular calcium ([Ca2+](i)) in adult rat Leydig ce lls was 70-160 nM and was unaffected by high K+ or the dihydropyridine calcium channel agonist, Bay K 8644. These findings are consistent wi th the absence of voltage-sensitive calcium channels in the Leydig cel l. In addition, no increase in [Ca2+]i was observed in cells treated w ith LH, CRF, and serotonin. However, both GnRH and endothelin-1 (ET-1) induced rapid and transient elevations of [Ca2+](i) that were not ass ociated with a sustained plateau phase and were unaffected by removal of Ca2+ from the incubation medium. The amplitude of the [Ca2+](i) res ponse was not altered by increasing concentrations of GnRH and ET-1, b ut the number of responsive cells increased progressively to a maximum of about 30% of the Leydig cell population. The calcium-mobilizing ac tions of GnRH and ET-1 were abolished by the GnRH and ET(A) receptor a ntagonists, [Dp-Glu(1),D-Phe(2),D-Trp(3,6)]GnRH and BQ-123, respective ly. The majority of the cells expressed solely GnRH or ET(A) receptors , and about 10% expressed both receptors. GnRH-induced Ca2+ responses were observed almost exclusively in medium-sized Leydig cells, whereas ET-induced responses were most frequent in large Leydig cells. These data demonstrate that single Leydig cells expressing GnRH and ET(A) re ceptors exhibit monophasic [Ca2+](i) responses that are activated in a n all-or-none fashion. Such transient Ca2+ signaling may trigger short term cellular responses or could modulate the actions of gonadotropin s acting through the cAMP signaling pathway.