In vitro acute and prolonged effects of melatonin on purified rat Leydig cell steroidogenesis and adenosine 3',5'-monophosphate production.

The effects of melatonin (MLT; 4.3 pM to 4.3 microM) on rat Leydig cell steroidogenesis and cAMP production were investigated during 3-h LH (30 mIU/ml) stimulation. Having noted a dose-dependent inhibition of testosterone (T) release, we also tested MLT in the presence of the cAMP activator forskolin (1 microM), the phosphodiesterase inhibitor isobutylmethylxanthine (100 microM), a combination of these two, and LHRH (100 nM), a non-cAMP-mediated stimulus. Regardless of the stimulus, levels of T, androstenedione, and cAMP were reduced, whereas that of 17-hydroxyprogesterone was enhanced. Cells were also tested after prolonged exposure to MLT (215 nM for 16 h). When compared with data from cells not preincubated with MLT, cAMP and T levels were 30% higher during LH stimulation (30 mIU/ml); comparable during treatment with forskolin (1 microM), isobutylmethylxanthine (100 microM), or their combination; and reduced during LHRH (100 nM). Scatchard analysis did not reveal changes in LH receptors during prolonged MLT exposure. Our data show that MLT acutely reduces cAMP- and non-cAMP-stimulated T. This effect is linked in part to reduced cAMP production and in part to reduced 17-20-desmolase enzymatic activity, which, however, can occur even with non-cAMP-mediated stimulation. On the other hand, prolonged exposure to MLT results in sensitization of the LH-dependent adenylate cyclase activity.