Epac-mediated activation of phospholipase C∈ plays a critical role in β-adrenergic receptor-dependent enhancement of Ca2+ mobilization in cardiac myocytes

Abstract Recently we demonstrated that PLCϵ plays an important role in β-adrenergic receptor (βAR) stimulation of Ca2+-induced Ca2+ release (CICR) in cardiac myocytes. Here we have reported for the first time that a pathway downstream of βAR involving the cAMP-dependent Rap GTP exchange factor, Epac, and PLCϵ regulates CICR in cardiac myocytes. To demonstrate a role for Epac in the stimulation of CICR, cardiac myocytes were treated with an Epac-selective cAMP analog, 8-4-(chlorophenylthio)-2′-O-methyladenosine-3′,5′-monophosphate (cpTOME). cpTOME treatment increased the amplitude of electrically evoked Ca2+ transients, implicating Epac for the first time in cardiac CICR. This response is abolished in PLCϵ-/- cardiac myocytes but rescued by transduction with PLCϵ, indicating that Epac is upstream of PLCϵ. Furthermore, transduction of PLCϵ+/+ cardiac myocytes with a Rap inhibitor, RapGAP1, significantly inhibited isoproterenol-dependent CICR. Using a combination of cpTOME and PKA-selective activators and inhibitors, we have shown that βAR-dependent increases in CICR consist of two independent components mediated by PKA and the novel Epac/PLCϵ pathway. We also show that Epac/PLCϵ-dependent effects on CICR are independent of sarcoplasmic reticulum loading and Ca2+ clearance mechanisms. These data define a novel endogenous PKA-independent βAR-signaling pathway through cAMP-dependent Epac activation, Rap, and PLCϵ that enhances intracellular Ca2+ release in cardiac myocytes.