The best understood intracellular messenger is cyclic AMP (cAMP). Which of the following statement(s) concerning this intracellular messenger is/are correct?

a. Intracellular cyclic AMP is constantly degraded by a specific enzyme, cAMP phosphodiesterase

b. Most of the actions of cAMP are mediated by activation of protein kinase A

The prototypic intracellular messenger is cAMP. To function as a mediator, the concentration of cAMP must change rapidly. In resting cells, cAMP is continuously being degraded by a specific enzyme, cAMP phosphodiesterase. cAMP levels can increase 10-fold or more within seconds of receptor binding through activation of adenylate cyclase. cAMP acts as an allosteric regulator, and most, if not all, of its actions are mediated by activation of cAMP-dependent protein kinase A. cAMP is not the only cyclic nucleotide active as an intracellular messenger. Most animal cells also produce cGMP. Intracellular calcium ions also serve as second messengers in a large number of cells.