Biphasic effect of nitric oxide on testosterone and cyclic GMP production by purified rat Leydig cells cultured in vitro

Nitric oxide (NO) biphasically modulates osteoclast function and sperm moti lity by exerting a positive effect at low concentrations and a negative eff ect at high concentrations. We therefore tested whether NO exerts a compara ble effect on testosterone secretion by cultured rat Leydig cells. Three NO -donors, S-nitroso-N-acetylpenicillamine (SNAP), diethylamine/nitric oxide complex sodium salt (DEA/NO) and diethylenetriamine nitric oxide adduct (DE TA/NO) were administered in a wide range of concentrations (10(-8)-10(-3) M for 3 h) to Percoll-purified Leydig cells from adult rats. These drugs rai sed testosterone and cGMP secretion when used at low concentrations (10(-8) -10(-5) M); however, they inhibited testosterone, but did not affect cGMP, secretion at concentrations higher than 10(-5) M. Administration of the NO scavenger haemoglobin (160 mu g/mL) prevented both the stimulatory and the inhibitory effect of these drugs. Nitrite accumulation was measured as a ma rker of NO released by the drugs in our in vitro system; it fell within the range of control media in the presence of NO-donor concentrations lower th an 10(-5) M, but was several-fold higher in the media of cells treated with . concentrations of the NO-donors greater than 10(-5) M. These data show th at (1) NO exerts a biphasic effect on testosterone secretion, which is stim ulatory at low and inhibitory at high concentrations; (2) the stimulatory e ffect of NO is mediated by cGMP, the classic second messenger for NO action .