Figure 16

Figure 16 Cavernosal smooth muscle tone is the most important determinant of penile blood flow. This, in turn, is critically dependent on the level of intracellular calcium ([Ca ²⁺]), modulated by a number of mechanisms. The most important vasodilator transmitter is nitric oxide (NO), released from both nitrergic nerve endings and vascular endothelium. NO stimulates production of cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP) by the enzyme guanylate cyclase (GC). A second vasodilator mechanism involves the production of cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP) by adenylate cyclase (AC). Both vasoactive intestinal polypeptide (VIP) and prostaglandin E1 (PGE₁) activate AC. Both cGMP and cAMP lower intracellular calcium, thereby triggering smooth muscle relaxation. NO also activates sodium (Na₊)/potassium (K ₊)-channel ATPase, resulting in hyperpolarization of the smooth muscle cell membrane which, in turn, prevents the opening of voltage-dependent calcium channels, thereby reducing intracellular calcium