Depotentiation of intact rat cardiac muscle unmasks an Epac-dependent increase in myofilament Ca2+ sensitivity

Recently, a family of guanine nucleotide exchange factors have been identified in many cell types as important effectors of cyclic adenosine 3',5'-monophospahte (cAMP) signalling that is independent of protein kinase A (PKA). In the heart, investigation of exchange protein directly activated by cAMP (Epac) has yielded conflicting results. Since cAMP is an important regulator of cardiac contractility, our aim was to examine whether Epac activation modulates excitation-contraction coupling in ventricular preparations from rat hearts. We used 8-(4-chlorophenylthio)-2’-O-methyladenosine-3’, 5’-cyclic monophosphate (cpTOME), an analogue of cAMP that activates Epac, but not PKA. In isolated myocytes, cpTOME increased Ca2+ spark frequency from ~ 7 to 32 per 100 μm3 s (n = 10), P = 0.05 with a reduction in the peak amplitude of the sparks. Simultaneous measurements of intracellular Ca2+ and isometric force in multicellular trabeculae (n = 7, 1.5 mM [Ca2+]o) revealed no effect of Epac activation on either the amplitude of Ca2+ transients (Control: 0.7 ± 0.1 versus cpTOME: 0.7 ± 0.1 340/380 fura-2 ratio, P = 0.35) or on peak stress (Control: 24 ± 5 versus cpTOME: 23 ± 5 mN mm-2, P = 0.20). However, an effect of Epac in trabeculae was unmasked by lowering extracellular [Ca2+]o. In these depotentiated trabeculae, activation of the Epac pathway increased myofilament Ca2+ sensitivity, an effect that was blocked by addition of KN-93, a Ca2+/calmodulin-dependent protein kinase II (CaMK-II) inhibitor. This study suggests that Epac activation may be a useful therapeutic target to increase the strength