The opposing roles of nitric oxide and cGMP in the age-associated decline in rat testicular steroidogenesis.

The molecular mechanism of the aging-associated dysfunction of Leydig cells (LCs) is complex and poorly understood. In this study, we analyzed the contribution of nitric oxide (NO) and cGMP signaling to the age-dependent decline in LC function. Significant (>50%) decreases in serum, intratesticular, and LC androgens in aging rats (15-24 months) were accompanied by a proportional increase in NO production, an up-regulation of cGMP levels, and the expression of soluble guanylyl cyclase-1B and protein kinase G1 in LCs. In contrast, LC cAMP levels decreased with age, most likely reflecting the up-regulation of cAMP-specific phosphodiesterase expression. Moreover, the expression of genes encoding enzymes responsible for cholesterol transport and its conversion to T were reduced. Exposing LCs from aged animals to NO further increased cGMP levels and decreased cAMP and androgen production, whereas the addition of cell-permeable 8-bromoguanosine-cGMP alone had the opposite effect. In vivo inhibition of cGMP-specific phosphodiesterase-5 for 3 and 6 months in aged rats led to a partial restoration of androgens, NO, and cyclic nucleotide levels, as well as the expression of steroidogenic and NO/cGMP signaling genes. These results indicate that a progressive increase in NO production contributes to the age-dependent decrease in steroidogenesis in a cGMP-independent manner, whereas the sustained elevation in cGMP levels significantly slows the decline in LC function.