Short-term memory in the in situ canine myocardium.

We studied the effect of intracycle (short-term) mechanical history on canine myocardial performance. Intracycle muscle force and/or length history was varied, and the resultant changes in end-systolic force-length relationship were analyzed. Antecedent isotonic shortening impaired, whereas isometric force development enhanced end-systolic myocardial performance. A history of shortening concurrent with force development produced an intermediate effect. We conclude that decreasing force or length impairs whereas increasing length or force enhances performance in the same cycle. Different combinations of antecedent force and length changes affect end-systolic performance by algebraic summation (superposition) of their disparate effects. Time measurements established that 1) total systolic time varied little with altered history, 2) isotonic shortening took longer than isometric contraction in reaching a point P in the force-length plane, and 3) less time was therefore available for contraction after P with antecedent isotonic shortening than with antecedent isometric force development. This history-dependent time differential accounts for the corresponding differential in performance.