The effect of calcium on the potentiation of LH-stimulated steroidogenesis and inhibition of LH-stimulated cyclic AMP production by LHRH agonist (ICI 118630) in rat Leydig cells.

A luteinizing hormone-releasing hormone (LHRH) agonist (ICI 118630) potentiated the effects of luteinizing hormone (LH) and dibutyryl cyclic AMP on steroidogenesis during 4 h incubations with rat Leydig cells. LH-stimulated cyclic AMP levels were decreased by the addition of the LHRH agonist. The potentiation of the LH-increased steroidogenesis was dependent on Ca2+; maximum effects required at least 2.5 mM Ca2+ in the incubation medium. The calcium ionophore A23187 negated the potentiation in a dose-dependent manner (ED50 = 0.2-0.3 microM), but had no effect on LH-induced steroidogenesis, despite a 90% decrease in cyclic AMP production. The latter decrease was found to be dependent on the Ca2+ concentration. In the presence of the phosphodiesterase inhibitor methylisobutylxanthine (MIX), the ionophore A23187 induced a dose-dependent decrease in both LH and LH plus LHRH agonist-stimulated steroidogenesis and cyclic AMP production. The results obtained indicate that calcium, rather than cyclic AMP, is the mediator of the potentiating effects of LHRH agonist on LH-increased steroidogenesis in rat Leydig cells. The marked inhibition of the synergism in the presence of calcium ionophore A23187 suggests that Leydig cell calcium homeostasis must be intact for LHRH agonist action to occur. LHRH agonist causes a Ca2+-dependent decrease in LH-stimulated cyclic AMP production.