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Electrophysiological effects of labetalol on canine atrial cardiac purkinje fibers and ventricular muscle

, : Electrophysiological effects of labetalol on canine atrial cardiac purkinje fibers and ventricular muscle. British Journal of Pharmacology 92(3): 627-634

1 Using conventional microelectrode techniques for the intracellular recordings of the membrane potential, the effects of labetalol were studied on cardiac Purkinje, atrial and ventricular muscle fibres of the dog. 2 Labetalol (1-10 .mu.M) reduced, in a concentration-dependent manner, the action potential amplitude (APA) and the maximum rate of rise of the action potential (Vmax) in Purkinje fibres. 3 The action potential duration (APD) was decreased in Purkinje fibres but significantly increased in ventricular fibres after small concentrations of labetalol (1-3 .mu.M). The atrial fibres were not very sensitive to labetalol. 4 Depolarization of the cardiac Purkinje fibres by increasing the external potassium concentration (8-12 mM), potentiated the labetalol effects on APA and Vmax but blocked its effects on the APD. 5 The effects of labetalol on Vmax of Purkinje fibres were dependent on the frequency of stimulation. 6 The ratio of the effective refractory period to the APD was increased both in normally polarized and depolarized Purkinje fibres after treatment with labetalol (10 .mu.M). 7 Labetalol (10 .mu.M) shifted the membrane responsiveness curve of Purkinje fibres by about 10 mV in the hyperpolarizing direction. 8 The slow response obtained in K-depolarized, Ba-treated Purkinje fibres was not significantly affected by labetalol (10-100 .mu.M). 9 It is suggested that labetalol can exert Class I and Class III antiarrhythmic actions in cardiac muscle of the dog in vitro.

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