Sustained β 1 -Adrenergic Stimulation Modulates Cardiac Contractility by Ca 2+ /Calmodulin Kinase Signaling Pathway

Abstract

A tenet of β₁-adrenergic receptor (β₁AR) signaling is that stimulation of the receptor activates the adenylate cyclase-cAMP-protein kinase A (PKA) pathway, resulting in positive inotropic and relaxant effects in the heart. However, recent studies have suggested the involvement of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) in β₁AR-stimulated cardiac apoptosis. In this study, we determined roles of CaMKII and PKA in sustained versus short-term β₁AR modulation of excitation-contraction (E-C) coupling in cardiac myocytes. Short-term (10-minute) and sustained (24-hour) β₁AR stimulation with norepinephrine similarly enhanced cell contraction and Ca²⁺ transients, in contrast to anticipated receptor desensitization. More importantly, the sustained responses were largely PKA-independent, and were sensitive to specific CaMKII inhibitors or adenoviral expression of a dominant-negative CaMKII mutant. Biochemical assays revealed that a progressive and persistent CaMKII activation was associated with a rapid desensitization of the cAMP/PKA signaling. Concomitantly, phosphorylation of phospholamban, an SR Ca²⁺ cycling regulatory protein, was shifted from its PKA site (¹⁶Ser) to CaMKII site (¹⁷Thr). Thus, β₁AR stimulation activates dual signaling pathways mediated by cAMP/PKA and CaMKII, the former undergoing desensitization and the latter exhibiting sensitization. This finding may bear important etiological and therapeutical ramifications in understanding β₁AR signaling in chronic heart failure.