Effects of cycle-length alteration on canine cardiac action potentials

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Abstract
Electro-physiologic studies in canine hearts demonstrate a characteristic change in the atrial action potential following abrupt rate changes. The action potential terminating a relatively prolonged cycle length exhibits shortening of phase 2 and lengthening of phase 3. In contrast, the action potential which concludes a relatively short interval generally displays lengthening of phase 2, with acceleration of terminal repolarization. These latter changes appear comparable to those previously described in ventricular cells. Purkinje cells, however, do not manifest this type of action potential change with abrupt rate alteration. Over a range of stimulus frequency, 30-200/min., the duration of Purkinje fiber action potential varies directly with the length of the preceding cycle length. This relation obtains irrespective of abrupt rate changes and is in accord with the more orthodox concept of phase 2 dependency on the duration of the preceding interval. A temporal relation between atrial action potential change and contractile potentiation, comparable to that observed in ventricular tissue, is described. This relation, observed in atrial and ventricular tissue, is contrasted with the behavior of the Purkinje fibers.