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The short-term force-frequency characteristics of canine left ventricular myocardium were examined in both isolated and intact preparations by briefly pertubing the frequency of contraction with early extrasystoles. The maximum rate of rise of isometric tension (Fmas) of the isolated trabeculae carneae was potentiated by the introduction of extrasystoles. The ratio of Fmas of potentiated to control beats (force-frequency ratio) was not altered significantly by a change in muscle length. However, exposure of the trabeculae to isoproterenol (10(-7)M) significantly changed the force-frequency ratio obtained in response to a constant frequency perturbation. Similar experiments were performed on chronically instrumented conscious dogs. Left ventricular minor axis diameter was measured with implanted pulse-transit ultrasonic dimension transducers, and intracavitary pressure was measured with a high fidelity micromanometer. Atrial pacing was performed so that the end-diastolic diameters of the beats preceding and following the extrasystole could be made identical. Large increases in the maximum rate of rise of pressure (Pmas) were seen in the contraction after the extrasystole. The ratio of Pmax of the potentiated beat to that of the control beat was not changed by a 9% increase in the end-diastolic diameter, produced by saline infusion. Conversely, isoproterenol significantly altered this relationship in the same manner as in the isolated muscle. Thus, either in vitro or in situ, left ventricular myocardium exhibits large functional changes in response to brief perturbations in rate. The isoproterenol and length data indicate that the force-frequency ratio reflects frequency-dependent changes in the inotropic state, independent of changes in length.
Anderson et al. (Wed,) studied this question.