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Canine cardiac Purkinje fibers and atrial trabeculae and rat and cat papillary muscles superfused with a hyperkalemic, hypoxic, and acidotic Tyrode solution were depolarized to membrane potentials (-70 to -60 mV) at which action potential amplitude declined as the coupling intervals of pacing stimuli were prolonged from 500 to 4,500 ms. The rate-related decline of action potential amplitude appeared to be due to time-dependent recovery of the early outward current rather than to a decrease in inward calcium current, since it was prevented by 4-aminopyridine (1.0 mM), but not by isoproterenol (1.0 microM), caffeine (5.0 mM), or CsCl (5-20 mM) and it was accompanied by an exponential increase of developed tension. Experiments using Purkinje fibers mounted in a single sucrose gap chamber demonstrated that the rate-related decline of action potential amplitude was maximal at membrane potentials between -70 and -40 mV and was negligible at less negative or more negative membrane potentials. These results may pertain to the mechanism for deceleration-dependent bundle branch block.
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