title: Depotentiation of intact rat cardiac muscle unmasks an Epac-dependent increase in myofilament Ca(2+) sensitivity.
abstract: 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 Ca(2+) spark frequency from about 7 to 32/100 μm(3)/s (n = 10), P = 0.05 with a reduction in the peak amplitude of the sparks. Simultaneous measurements of intracellular Ca(2+) and isometric force in multicellular trabeculae (n = 7, 1.5 mmol/L [Ca(2+)]o) revealed no effect of Epac activation on either the amplitude of Ca(2+) 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/mm(2) vs cpTOME 23 ± 5 mN/mm(2), P = 0.20). However, an effect of Epac in trabeculae was unmasked by lowering extracellular [Ca(2+)]o. In these depotentiated trabeculae, activation of the Epac pathway increased myofilament Ca(2+) sensitivity, an effect that was blocked by addition of KN-93, a Ca(2+)/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.
authors.affiliation: Department of Physiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand.
authors.forename: Sarbjot
authors.initials: S
authors.lastname: Kaur
authors.affiliation: Department of Physiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand.
authors.forename: Cherrie H T
authors.initials: CH
authors.lastname: Kong
authors.affiliation: Department of Physiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand.
authors.forename: Mark B
authors.initials: MB
authors.lastname: Cannell
authors.affiliation: Department of Physiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand.
authors.forename: Marie-Louise
authors.initials: ML
authors.lastname: Ward
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D056966:Excitation Contraction Coupling; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D006352:Heart Ventricles / Q000166:cytology / Q000378:metabolism; D036881:Long-Term Synaptic Depression; D032383:Myocytes, Cardiac / Q000166:cytology* / Q000378:metabolism* / Q000502:physiology; D009210:Myofibrils / Q000378:metabolism*; D051381:Rats
chemical_list: D020662:Guanine Nucleotide Exchange Factors; C526973:Rapgef3 protein, rat; D002118:Calcium
keywords: 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