Plasma levels of cyclic guanosine-3',5'-monophosphate in the cavernous and systemic blood of healthy males during different functional conditions of the penis.

The relaxation of cavernous arterial and trabecular smooth muscle is dependent upon the stimulation of guanylyl cyclase activity by nitric oxide (NO), which is released from nerve terminals and endothelial cells within the cavernous tissue, and the subsequent accumulation of cyclic guanosine-3',5'-monophosphate (cGMP) in the intracellular space. The present study was undertaken to determine whether or not plasma levels of cGMP in the systemic and cavernous blood of healthy male subjects change from penile flaccidity to tumescence, rigidity and detumescence. Fifteen adult healthy males were exposed to visual and tactile erotic stimuli to elicit penile tumescence and rigidity. Whole blood was simultaneously aspirated from the corpus cavernosum and the cubital vein in the respective penile stages, and cGMP was determined in plasma aliquots by means of a radioimmunoassay. Mean systemic and cavernous plasma levels of cGMP in the blood samples obtained from the healthy volunteers ranged from 1.2-1.7 pmol/ ml. cGMP levels in the systemic circulation and in the cavernous blood did not change during developing erection, rigidity and detumescence. No significant differences were found between cGMP plasma levels in the systemic and cavernous blood in the different penile stages. Our results may reflect the fact that the stimulation of NO production in healthy males during sexual arousal and developing penile erection either does not yield substantial quantities of cGMP or that the rate of cGMP-extrusion from cavernous smooth muscle cells into the extracellular space accounts only for a minor fraction of plasma cGMP. Moreover, basal levels of cGMP in the blood flushing the lacunar spaces of the cavernous body in the state of developing erection may conceal any local release of cGMP that may occur within the penile erectile tissue. Thus, we conclude that the quantification of cGMP is of no use in the evaluation of the physiologic mechanisms of penile erection in vivo.