Anchored PKA synchronizes adrenergic phosphoregulation of cardiac Ca_v1.2 channels

Adrenergic modulation of voltage gated Ca²⁺ currents is a context specific process. In the heart Ca_v1.2 channels initiate excitation-contraction coupling. This requires PKA phosphorylation of the small GTPase Rad (Ras associated with diabetes) and involves direct phosphorylation of the Ca_v1.2 α₁ subunit at Ser1700. A contributing factor is the proximity of PKA to the channel through association with A-kinase anchoring proteins (AKAPs). Disruption of PKA anchoring by the disruptor peptide AKAP-IS prevents upregulation of Ca_v1.2 currents in tsA-201 cells. Biochemical analyses demonstrate that Rad does not function as an AKAP. Electrophysiological recording shows that channel mutants lacking phosphorylation sites (Ca_v1.2 STAA) lose responsivity to the second messenger cAMP. Measurements in cardiomyocytes isolated from Rad^-/- mice show that adrenergic activation of Ca_v1.2 is attenuated but not completely abolished. Whole animal electrocardiography studies reveal that cardiac selective Rad KO mice exhibited higher baseline left ventricular ejection fraction, greater fractional shortening, and increased heart rate as compared to control animals. Yet, each parameter of cardiac function was slightly elevated when Rad^-/- mice were treated with the adrenergic agonist isoproterenol. Thus, phosphorylation of Ca_v1.2 and dissociation of phospho-Rad from the channel are local cAMP responsive events that act in concert to enhance L-type calcium currents. This convergence of local PKA regulatory events at the cardiac L-type calcium channel may permit maximal β-adrenergic influence on the fight-or-flight response.