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The aim of this study was to investigate cardiomyocyte Ca2+ handling and contractile function in freshly excised human atrial tissue from diabetic and non‐diabetic patients undergoing routine surgery. Multicellular trabeculae (283 ± 20 μm in diameter) were dissected from the endocardial surface of freshly obtained right atrial appendage samples from consenting surgical patients. Trabeculae were mounted in a force transducer at optimal length, electrically stimulated to contract, and loaded with fura‐2/AM for intracellular Ca2+ measurements. The response to stimulation frequencies encompassing the physiological range was recorded at 37°C. Myofilament Ca2+ sensitivity was assessed from phase plots and high potassium contractures of force against [Ca2+]i. Trabeculae from diabetic patients (n = 12) had increased diastolic (resting) [Ca2+]i (p = 0.03) and reduced Ca2+ transient amplitude (p = 0.04) when compared to non‐diabetic patients (n = 11), with no difference in the Ca2+ transient time course. Diastolic stress was increased (p = 0.008) in trabeculae from diabetic patients, and peak developed stress decreased (p ≤ 0.001), which were not accounted for by reduction in the cardiomyocyte, or contractile protein, content of trabeculae. Trabeculae from diabetic patients also displayed diminished myofilament Ca2+ sensitivity (p = 0.018) compared to non‐diabetic patients. Our data provides evidence of impaired calcium handling during excitation‐contraction coupling with resulting contractile dysfunction in atrial tissue from patients with type 2 diabetes in comparison to the non‐diabetic. This highlights the importance of targeting cardiomyocyte Ca2+ homeostasis in developing more effective treatment options for diabetic heart disease in the future. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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., (© 2015 Wiley Publishing Asia Pty Ltd.)