Acebutolol-induced changes in refractoriness and monophastic action potential of the right ventricle of the dog heart in situ

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Abstract
The effect of acebutolol, a β-adrenergic receptor blocking agent, on the electrophysiology of the right ventricle was studied in the dog heart in situ. Pentobarbital anaesthesia which is known to increase the sympathetic tone was used. Monophasic action potential recordings were obtained by the suction electrode technique and refractoriness was measured by means of programmed electrical stimulation. A stepwise increase in the frequency of stimulation from 170 to 200, 230, and 260 per min caused a progressive decrease in the refractoriness as well as the duration of the monophasic action potential. Intravenous injection of acebutolol 2.0 mg·kg−1 increased the times for 50 and 90% repolarisation of the monophasic action potential. This increase is probably due to β-adrenergic receptor blockade in the presence of α-adrenergic receptor stimulation. The effective and functional refractory periods, however, were increased to an even greater extent than the monophasic action potential duration. It is suggested that this is the result of a blockade of a catecholamine-induced increase in the velocity of the depolarisation.