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 depend ent upon the stimulation of guanylyl cyclase activity by nitric oxide (NO), which is released from nerve terminals and endothelial cells within the ca vernous tissue, and the subsequent accumulation of cyclic guanosine-3', 5'- monophosphate (cGMP) in the intracellular space. The present study was unde rtaken to determine whether or not plasma levels of cGMP in the systemic an d cavernous blood of healthy male subjects change from penile flaccidity to tumescence, rigidity and detumescence. Fifteen adult healthy males were ex posed to visual and tactile erotic stimuli to elicit penile tumescence and rigidity. Whole blood was simultaneously aspirated from the corpus cavernos um and the cubital vein in the respective penile stages, and cGMP was deter mined in plasma aliquots by means of a radioimmunoassay. Mean systemic and cavernous plasma levels of cGMP in the blood samples obtained from the heal thy volunteers ranged from L-2-1.7 pmol/ml. cGMP levels in the systemic cir culation and in the cavernous blood did not change during developing erecti on, rigidity and detumescence. No significant differences were found betwee n cGMP plasma levels in the systemic and cavernous blood in the different p enile stages. Our results may reflect the fact that the stimulation of NO p roduction in healthy males during sexual arousal and developing penile erec tion 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 l evels of cGMP in the blood flushing the lacunar spaces of the cavernous bod y 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 t he quantification of cGMP is of no use in the evaluation of the physiologic mechanisms of penile erection in vivo.