Relaxation cyclic nucleotide activity regulation

There are now as many as seven distinct families of PDE isozymes (Beavo, 1988 Beavo and Reifsnyder, 1990 Michaeli etal., 1993). These isozymes are distinct based on protein sequences and catalytic activity for the different cyclic nucleotides. In smooth muscle, cAMP hydrolysis is regulated by either PDE III, PDE IV, or both enzymes. Cyclic GMP hydrolysis is primarily regulated by PDE V, although PDE I (which is calmodulin regulated) may also play a role in relaxation. 

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