During the classic "fight-or-flight" stress response, sympathetic nervous system activation leads to catecholamine release, which increases heart rate and contractility, resulting in enhanced cardiac output. Catecholamines bind to β-adrenergic receptors, causing cAMP generation and activation of PKA, which phosphorylates multiple targets in cardiac muscle, including the cardiac ryanodine receptor/calcium release channel (RyR2) required for muscle contraction. PKA phosphorylation of RyR2 enhances channel activity by sensitizing the channel to cytosolic calcium (Ca 2+ ). Here, we found that mice harboring RyR2 channels that cannot be PKA phosphorylated (referred to herein as RyR2-S2808A +/+ mice) exhibited blunted heart rate and cardiac contractile responses to catecholamines (isoproterenol). The isoproterenol-induced enhancement of ventricular myocyte Ca 2+ transients and fractional shortening (contraction) and the spontaneous beating rate of sinoatrial nodal cells were all blunted in RyR2-S2808A +/+ mice. The blunted cardiac response to catecholamines in RyR2-S2808A +/+ mice resulted in impaired exercise capacity. RyR2-S2808A +/+ mice were protected against chronic catecholaminergic-induced cardiac dysfunction. These studies identify what we believe to be new roles for PKA phosphorylation of RyR2 in both the heart rate and contractile responses to acute catecholaminergic stimulation.
IntroductionDuring exercise, heart rate (chronotropy) and cardiac contractility (inotropy) increase to meet the metabolic demands of the organs. Stress-induced activation of the sympathetic nervous system (SNS) results in catecholamine release, stimulation of β-adrenergic receptors (β-ARs), generation of cAMP, and activation of cAMP-dependent protein kinase (PKA) in cardiac myocytes. Catecholaminergic stimulation of the heart increases both heart rate and contractility (1). The essential role of β-ARs in the stress-induced enhancement of cardiac function has been demonstrated using β 1 -AR knockout mice that are unable to develop normal responses to stress (2). However, the complexity of the β-AR signaling cascade has made it difficult to elucidate specific contributions of downstream targets to the physiologic responses to stress.β-AR stimulation and downstream activation of PKA enhances calcium (Ca 2+ ) signaling in myocytes (3). During excitation-contraction (EC) coupling in the heart, depolarization of the sarcolemmal membrane activates the voltage-gated calcium channel (Ca V 1.2), causing a small Ca 2+ influx into the cell. This in turn triggers the opening of ryanodine receptor/calcium release channel (RyR2) and the release of Ca 2+ from the sarcoplasmic reticu-