The Relation of Contractile Enhancement to Action Potential Change in Canine Myocardium

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Abstract
Simultaneous recordings of contractile tension and transmembrane potentials from canine ventricular tissue yielded a consistent correlation of action potential (AP) alteration with contractile change associated with abrupt rate change. The AP terminating a relative prolongation of the cycle-length manifested shortening of phase 2 with lengthening of phase 3 and was associated with potentiation of contractile force. Conversely, the AP terminating a relative abbreviation of cycle-length displayed a broader phase 2 with a more precipitous phase 3, while the associated contraction was less forceful than the control. In each circumstance, the relative magnitude of cycle-length change correlated with the extent of both AP change and contractile alteration. Changes in AP configuration may reflect changes in transmembrane flux of K+ during repolarization consistent with the findings of prior workers who have related K+ efflux to increased contractility. Mechanical alternans, in addition, was frequently observed in association with abrupt rate change and was consistently associated with an electrical alternans manifested by action potentials with alternately wide and narrow plateaus (phase 2). As above, the more forceful contractions were associated with action potentials which displayed a narrower phase 2. Mechanical alternans initiated by abrupt rate change may represent an adaptive phenomenon prior to the establishment of a stable contractile state, as reflected by a stable AP configuration.