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Schematic of the biochemical pathway for erectile function and mode of action of phosphodiesterase-5 (PDE-5) inhibitors. NO, nitric oxide; eNOS, endothelial nitric oxide synthase; nNOS, neuronal nitric oxide synthase; GTP, guanosine triphosphate; GMP, guanosine monophosphate; cGMP, cyclic GMP.
