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

• Published in: Clinical and experimental pharmacology & physiology 2016-01, Vol.43 (1), p.88-94
• Main Authors: Kaur, Sarbjot, Kong, Cherrie HT, Cannell, Mark B, Ward, Marie-Louise
• Format: Article
• Language: English
• Subjects: Ca2+/calmodulin-dependent protein kinase type II; cardiac myocytes; cardiac trabeculae; exchange protein directly activated by cAMP; excitation-contraction coupling; fura-2; guanine nucleotide exchange factors
• ISSN: 0305-1870; 1440-1681
• DOI: 10.1111/1440-1681.12504

Summary 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, this study aimed to examine whether Epac activation modulates excitation–contraction coupling in ventricular preparations from rat hearts. The study 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 about 7 to 32/100 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 mmol/L [Ca2+]o) revealed no effect of Epac activation on either the amplitude of Ca2+ transients (Control 0.7 ± 0.1 vs cpTOME 0.7 ± 0.1; 340/380 fura‐2 ratio, P = 0.35) or on peak stress (Control 24 ± 5 mN/mm2 vs cpTOME 23 ± 5 mN/mm2, 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 of contraction during low inotropic states.