Durations of Transmembrane Action Potentials and Functional Refractory Periods of Canine False Tendon and Ventricular Myocardium:

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Abstract
In vitro studies have shown that alterations in the ionic composition of the perfusion fluid, substitution of blood or plasma for Tyrode's solution, time lapse following removal of the cardiac preparation from the animal, and addition of varying amounts of acetylcholine and/or epinephrine, do not change differentially the functional refractory period and action potential duration of false tendon and papillary muscle fibers; false tendon values exceeded those of papillary muscle in all of the above conditions. Cycle length was the only variable found to influence differentially false tendon and papillary muscle functional refractory periods and action potential durations. At very short cycle lengths, produced by premature responses, the functional refractory period and action potential duration of false tendon and papillary muscle cells approached each other. In some experiments the functional refractory periods of premature false tendon responses were less than corresponding values for papillary muscle. The relationship between the action potential durations and functional refractory periods of canine epicardial and endocardial cells was investigated also. It was found that the epicardial cells had a shorter functional refractory period and action potential duration than endocardial units when studied at physiological rates.