β-Adrenergic induced SR Ca2 + leak is mediated by an Epac-NOS pathway

- Beta-adrenergic receptors (B-AR) induce arrhythmogenic SR Ca leak in adult rabbit and mouse ventricular myocytes.
- Previously proposed parallel pathways involving Epac or NOS signals are actually in series.
- This SR Ca leak is via a B1AR-cAMP-Epac2-PI3K-Akt-NOS1-CaMKIIδ-RyR2 phosphorylation cascade.
- This parallels B-AR/cAMP inotropic and lusitropic PKA effects on Ca current and SR Ca uptake.

Cardiac β-adrenergic receptors (β-AR) and Ca2 +-Calmodulin dependent protein kinase (CaMKII) regulate both physiological and pathophysiological Ca2 + signaling. Elevated diastolic Ca2 + leak from the sarcoplasmic reticulum (SR) contributes to contractile dysfunction in heart failure and to arrhythmogenesis. β-AR activation is known to increase SR Ca2 + leak via CaMKII-dependent phosphorylation of the ryanodine receptor. Two independent and reportedly parallel pathways have been implicated in this β-AR-CaMKII cascade, one involving exchange protein directly activated by cAMP (Epac2) and another involving nitric oxide synthase 1 (NOS1). Here we tested whether Epac and NOS function in a single series pathway to increase β-AR induced and CaMKII-dependent SR Ca2 + leak. Leak was measured as both Ca2 + spark frequency and tetracaine-induced shifts in SR Ca2 +, in mouse and rabbit ventricular myocytes. Direct Epac activation by 8-CPT (8-(4-chlorophenylthio)-2′-O-methyl-cAMP) mimicked β-AR-induced SR Ca2 + leak, and both were blocked by NOS inhibition. The same was true for myocyte CaMKII activation (assessed via a FRET-based reporter) and ryanodine receptor phosphorylation. Inhibitor and phosphorylation studies also implicated phosphoinositide 3-kinase (PI3K) and protein kinase B (Akt) downstream of Epac and above NOS activation in this pathway. We conclude that these two independently characterized parallel pathways function mainly via a single series arrangement (β-AR-cAMP-Epac-PI3K-Akt-NOS1-CaMKII) to mediate increased SR Ca2 + leak. Thus, for β-AR activation the cAMP-PKA branch effects inotropy and lusitropy (by effects on Ca2 + current and SR Ca2 +-ATPase), this cAMP-Epac-NOS pathway increases pathological diastolic SR Ca2 + leak. This pathway distinction may allow novel SR Ca2 + leak therapeutic targeting in treatment of arrhythmias in heart failure that spare the inotropic and lusitropic effects of the PKA branch.

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