What happens when phosphodiesterase III is inhibited?

When phosphodiesterase III is inhibited, the levels of cyclic AMP (cAMP) within cells increase. This occurs because phosphodiesterase enzymes are responsible for breaking down cAMP into AMP. By inhibiting phosphodiesterase III, the breakdown of cAMP is reduced, leading to an accumulation of cAMP.

The increase in cAMP has several physiological effects, particularly in the context of cardiac and smooth muscle function. In the heart, elevated cAMP enhances inotropic effects, leading to stronger and more forceful contractions, and can also improve heart rate under certain circumstances. In vascular smooth muscle, increased cAMP promotes relaxation, which results in vasodilation.

The other choices do not accurately reflect the physiological changes that occur upon the inhibition of this enzyme. Decreased heart rate is not directly linked to cAMP levels but is more affected by other factors such as autonomic nervous system activity. Enhanced smooth muscle contraction generally requires calcium influx rather than cAMP increase, and lowered calcium levels are not a result of phosphodiesterase inhibition; in fact, increased cAMP can lead to calcium mobilization in certain environments.