Coupling function of endogenous α₁- and β-adrenergic receptors in mouse cardiomyocytes

Genetically altered mouse models constitute unique systems to delineate the role of adrenergic receptor (AR) signaling mechanisms as modulators of cardiomyocyte function. The interpretation of results from these models depends on knowledge of the signaling properties of endogenous ARs in mouse cardiomyocytes. In the present study, we identify for the first time several defects in AR signaling in cardiomyocytes cultured from mouse ventricles. β₁-ARs induce robust increases in cAMP accumulation and the amplitude of the calcium and cell motion transients in mouse cardiomyocytes. Selective β₂-AR stimulation increases the amplitude of calcium and motion transients, with only a trivial rise in cAMP accumulation in comparison. β₂-AR responses are not influenced by pertussis toxin in cultured mouse cardiomyocytes, α₁-ARs fail to activate phospholipase C, the extracellular signal-regulated protein kinase, p38-MAPK, or stimulate hypertrophy in mouse cardiomyocytes. Control experiments establish that this is not due to a lesion in distal elements in the signaling machinery, because these responses are induced by protease-activated receptor-1 agonists and phospholipase C is activated by Pasteurella multocida toxin (a G(q) α-subunit agonist). Surprisingly, norepinephrine activates p38-MAPK via β-ARs in mouse cardiomyocytes, but β-AR activation of p38-MAPK alone is not sufficient to induce cardiomyocyte hypertrophy. Collectively, these results identify a generalized defect in α₁-AR signaling and a defect in β₂-AR linkage to cAMP (although not to an inotropic response) in cultured mouse cardiomyocytes. These naturally occurring vagaries in AR signaling in mouse cardiomyocytes provide informative insights into the requirements for hypertrophic signaling and impact on the value of mouse cardiomyocytes as a reconstitution system to investigate AR signaling in the heart.