The balance between sarcoplasmic reticulum (SR) Ca2+ uptake and release is critical to determine the effects of Epac activation in the myocardium

LEZCANO N; LUCOTTI I; SAID M; VITTONE L; MUNDIÑA-WEILENMANN C

Santiago

Congreso; XX Reunión Anual de la Sección Latinoamericana de la ISHR.; 2012

International Society for Heart Research. Sección Latinoamericana

In cardiac muscle, the role of the exchange protein directly activated by cAMP (Epac) in the regulation of Ca2+ homeostasis and contractility, and the signalling pathways involved in these effects are controversial. To elucidate the reason for the discrepancies, the effects of a selective activator of Epac, 8-CPT, were studied in isolated adult rat myocytes loaded with Fura-2 AM at different extracellular Ca2+ ([Ca]o). 10 μM 8-CPT significantly increased fractional sarcomere shortening (134.6±15.5% of control, n=13), Ca2+ transient amplitude (116.6±6.4% of control, n=13) and SR Ca2+ content from 0.51±0.03 to 0.62±0.04 F340/380 (n=6) at 0.5 mM [Ca]o. At 1mM [Ca]o no changes were observed and at 1.8 mM [Ca]o, stimulation of the Epac reduced all three parameters (pb0.05). At all [Ca]o, activation of Epac increased CaMKII-dependent phosphorylation of phospholamban, which by enhancing SR Ca2+ uptake could be responsible for the increased SR Ca2+ content. At higher [Ca]o, 8-CPT increased CaMKII-dependent phosphorylation of the ryanodine receptor in association with an increased SR Ca2+ leak. The results suggest that Epac activation, recruits a CaMKII pathway, independently of PKA, which exerts both positive and negative inotropic effects depending on the balance between SR Ca2+ uptake and release.