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Nitric oxide signaling pathway in penile erection. NO: nitric oxide, nNOS: neuronal nitric oxide synthase, eNOS: endothelial nitric oxide synthase, iNOS: immunoactivated macrophage-derived nitric oxide synthase, c-GMP: cyclic guanosine monophosphate, 5′ GMP: 5′-guanosine monophosphate, GTP: guanosine triphosphate, PKG: protein kinase G, PDE5: phosphodiesterase type 5, Ca2+: calcium ions.
