New Insight Into the Role of Enhanced Adrenergic Receptor-Effector Coupling in the Heart

The ability of the heart to augment the perfusion of vital organs and skeletal muscles during stress is predicated on its ability to increase contractility in response to adrenergic neurohormones that are released from postsynaptic nerve terminals within the heart and/or from the adrenal gland. These neurohormonal signals must then be detected and processed by a complex of proteins located within the sarcolemmal membrane, resulting in the conversion of the adrenergic signal to an alteration in the biomechanical properties of individual myocytes. This sarcolemmal protein complex consists of adrenergic receptors (ARs), guanine nucleotide–binding regulatory proteins, and the effector enzyme adenylyl cyclase. Myocardial contractility is augmented by the interaction of adrenergic agonists with β1- and/or β2-adrenergic receptors (βARs) located on the surface of the sarcolemmal membrane (see review in Reference 11 ). These 2 βAR subtypes are coupled to adenylyl cyclase activation by the stimulatory guanine nucleotide–binding protein (Gs). Interaction of the agonist-βAR complex with the heterotrimeric G protein catalyzes the release of GDP from the α-subunit of the G protein (αs), allowing the binding of GTP and the subsequent activation of adenylyl cyclase by αs-GTP. This activation persists until intrinsic GTPase activity of αs hydrolyzes the nucleotide, resulting in an inactive αs-GDP moiety.1 G-protein–mediated activation of adenylyl cyclase effects the synthesis of the intracellular second messenger cAMP and the resulting phosphorylation of a cAMP-dependent protein kinase (PKA). Once phosphorylated, PKA is then able to effect positive inotropic and chronotropic responses by phosphorylating a group of intracellular proteins, including phospholamban, and the L-type voltage-dependent calcium channel while at the same time enhancing lusitropy via alterations in the sensitivity of troponin for Ca2+. α-Subunits of some G proteins also appear to effect myocyte ion channel activity independent …