As discussed throughout this chapter, the cyclic nucleotide phosphodiesterases constitute a group of enzymes that catalyze the hydrolysis of various cyclic nucleotides including cAMP and cGMP. The contractility of all muscle types is regulated primarily by reversible phosphorylation of the protein myosin. The phosphorylation of myosin allows it to interact with actin, which results in muscle contraction. Through different mechanisms, cAMP and cGMP interfere with the ability of myosin to be phosporylated and/or interact with actin. This, in turn, results in muscle relaxation. Agents that interfere with the degradation of the cyclic nucleotide have usefulness in treating hypertension, erectile dysfunction, etc. Phosphodiesterase type 5 (PDE5) is responsible for the catalytic hydrolysis of cGMP in the smooth muscle of the arteries in the penis and lungs. Inhibitors of this particular phos-phodiesterase have been shown to relax arteries in the penis, thus allowing the corpus cavernosum to fill with blood and aiding in an erection.63
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