The key enzyme in heart muscle signaling, PKA, is a member of a huge class of regulatory proteins called kinases. Each kinase is specialized to attach a phosphate molecule to a specific set of target proteins. These phosphorylation reactions switch targeted proteins from an inactive state to an active state. PKA actually activates other kinases, which in effect amplify the effect of PKA through a signaling cascade.

The activity of kinases is delicately balance by a group of enzymes called protein phosphatases, which simply remove phosphate groups from specific proteins, inactivating them. About 30 percent of all human proteins are regulated by kinases and phosphatases.

Activation of PKA is actually initiated at the exterior surface of heart cells where neurotransmitters and hormones bind to beta-adrenergic receptors. However, while drugs that boost PKA activity temporarily increased cardiac contractions, they also led to higher patient mortality in the long term.

A widely used class of drugs is called beta-blockers. Drugs in that class, including Atenolol, Bisoprolol, and Metoprolol are designed to take the opposite approach: they block the beta-adrenergic receptors, thereby reducing PKA activity and lowering cardiac output. Beta blockers are now taken daily by about 5 million U.S. patients suffering from heart failure, high blood pressure and other cardiovascular diseases. The effectiveness of beta blockers has highlighted the need to better understand the system of biochemical signaling within heart cells.

For example, the clinical observations and experimental findings of many scientists suggest that increasing the strength of heart cells contraction may be less beneficial to patients than restoring the normal PKA-dependent control system. During heart failure, the heart muscle contracts weakly, which causes the body to compensate by releasing more hormones and neurotrans
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Contact: Rex Graham
ragraham@ucsd.edu
858-822-3075
University of California - San Diego
7-Aug-2006