Along with increasing blood flow in arteries, Viagra interferes with phosphodiesterases (PDEs), enzymes that break down the messenger molecule called cyclic guanosine monophosphate (cGMP), which would otherwise "put the brakes on" heart muscle cell growth.
Cyclic guanosine monophosphate (cGMP) does not stimulate pigment aggregation in Palaemonetes pugio ovarian chromatophores (Lambert and Fingerman, 1979), and the lipid-soluble analog, db-cGMP, does not cause pigment aggregation in M.
Viagra blocks an enzyme that normally breaks down cyclic guanosine monophosphate (cGMP), the substance that is produced in response to nitric oxide stimulation and is actually responsible for blood vessel dilation.
NO formation and cyclic guanosine monophosphate production within platelets was increased during the luteal phase of the cycle compared with the follicular phase in patients with menstrual migraine, suggesting that enhanced NO production could be involved in menstrual migraine pathogenesis if platelet abnormalities reflect events within the central nervous system or vascular endothelium.
Erections occur when a stimulus triggers the release of nitric oxide, which activates cyclic guanosine monophosphate (cyclic GMP), an enzyme that causes the smooth muscle of the penis to relax and blood flow to increase.