The real estate of cardiac signaling: location, location, location.

D etermining how cells distinguish between adaptive and maladaptive signals when they appear to share the same molecular pathways has been a vexing biological problem. The ability to identify distinctive features of a pathophysiological response compared with a physiological response would allow for the rational design of approaches to eliminate or diminish undesirable consequences of pathophysiological responses while preserving the beneficial effects of physiological signals. These issues are especially pertinent in the heart and in understanding the development of heart failure, for which 500,000 new cases are diagnosed in the United States each year. Major factors contributing to worsening heart failure include a number of compensatory neurohormonal signals intended to counteract decreased cardiac output, such as hyperadrenergic stimulation (1). In a recent issue of PNAS, Balijepalli et al. (2) provide new insight into how adrenergic signaling pathways are organized in the heart. Adrenergic signaling in the myocardium contributes to the control of heart rate (chronotropy), strength of contraction (inotropy), and rate of relaxation (lusitropy) by changing the levels of intracellular Ca2 or by altering the sensitivity of critical regulatory proteins to Ca2 . Signaling is mediated predominantly by two distinct -adrenergic receptors, 1 and 2, which differ in their abundance, distribution, and downstream signal transducers (3). Approximately 75% of the cardiac -adrenergic receptors are 1, which appear to be distributed globally throughout the sarcolemma. 1 receptors couple to the Gs heterotrimeric G protein. The lessabundant 2 receptors reside predominantly in caveolae (4), specialized compartments of the plasma membrane organized by caveolins. Caveolae are flask-shaped membrane invaginations rich in cholesterol and glycosphingolipids that house and coordinate multiple signaling components, many of which appear to be dedicated to Ca2 signaling (5). Besides their distinct homes, 2 receptors also differ from 1 in that they couple to both Gs and Gi. Nevertheless, stimulation of either 1 or 2 activates adenylyl cyclase to increase intracellular cAMP. In turn, cAMP activates protein kinase A, resulting in the phosphorylation of key elements of the contractile apparatus and of proteins that control internal Ca2 levels. Prominent among the PKA targets are the L-type voltagegated Ca2 channels (CaV1.2), which open upon membrane depolarization, allowing Ca2 to enter the cell. The ‘‘receptors’’ for this Ca2 signal are the ryanodine receptors (RyR2), Ca2 release channels on the sarcoplasmic reticulum (SR) that flood the cytoplasm with additional Ca2 that then initiates contraction (Fig. 1). PKA phosphorylation of L-type Ca2 channels potentiates inward Ca2 current and thereby augments contraction. Electrophysiological studies of L-type Ca2 current after adrenergic stimulation revealed important consequences of the localization and G protein-coupling differences between 1 and 2 receptor subtypes. By isolating L-type Ca2 channels within a patch pipette, Chen-Izu et al. (6) determined that remote stimulation (outside of the pipette) of 1 increased Ca2 channel current within the pipette, suggesting that 1 signaling included a diffusive second messenger. In contrast, 2-specific agonists were effective only when included within the pipette. This membrane-delimited 2 signaling depended on Gi, because inactivation of Gi with pertussis toxin made 2 signaling diffusive. Several other studies have provided additional evidence for important functional consequences of differential signaling through the -adrenergic receptors. For example, sustained signaling through 1 receptors led to myocyte apoptosis; this 1-mediated proapoptotic signal depended on Ca2 influx through Ltype Ca2 channels and activation of Ca2 calmodulin-dependent kinase II (CaMKII) (7). On the other hand, 2 activation was protective against apoptotic signals (8–10). Like the membranedelimited activation of L-type Ca2 currents, coupling of 2 to Gi was also necessary for prosurvival signaling; Gi inactivation with pertussis toxin blocked protection (8). Balijepalli et al. (2) provide a new wrinkle to this compartmentation story. They demonstrate for the first time that L-type Ca2 channels can be found within caveolae in cardiac myocytes and that 2 activation of L-type Ca2 channels requires intact caveolae. Electron microscopy showed 1C, the L-type Ca2