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Electric Currents Applied during the Refractory Period Enhance Contractility and Systolic Calcium in Ferret

S Mohri, J Shimizu, Y Mika, I Shemer, J Wang, S Ben Haim and D Burkhoff
Am J Physiol Heart Circ.Physiol 2002;284:1119-1123

We investigated the mechanism of positive inotropism of electric currents applied during the absolute refractory period. Ten Langendorff-perfused ferret hearts were instrumented to measure isovolumic left ventricular pressure (LVP) and the aequorin luminescence. Biphasic squarewave electric currents (~20 mA, total duration 30 ms) were delivered between pairs of electrodes. Six hearts were perfused at different extracellular Ca2 concentrations ([Ca2]o; 1, 2, 4, and 8 mM). These signals increased LVP from 50.0 + 9.4 to 70.1 + 14.7, from 67.5 + 11.0 to 79.0 + 15.6, from 79.3 + 21.0 to 87.1 + 22.8, and from 84.6 + 24.0 to 91.8 + 28.5 mmHg at the respective [Ca2]o (P < 0.05). Peak free intracellular [Ca2] ([Ca2]i) increased from 0.52 + 0.13 to 1.37 + 0.23, from 0.76 + 0.23 to 1.73 + 0.14, from 1.10 + 0.24 to 2.05 + 0.33, and from 1.41 + 0.36 to 2.24 + 0.36 uM/ml, respectively (P = 0.001). With the use of 1 mg/l propranolol with 1 mM [Ca2]o, LVP and [Ca2]i were increased significantly from 48.7 + 8.18 to 56.3 + 6.11 mmHg and from 0.61 + 0.11 to 1.17 + 0.20 uM, respectively (P < 0.05). In conclusion, positive inotropism of such electrical currents was due to increased peak [Ca2]i and Ca2 responsiveness of the myofilaments did not change significantly.

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