EFFECT OF HEART-RATE ON MYOCARDIAL RELAXATION IN ISOMETRIC TWITCHES

Objective: Increased heart rate increases the amount of calcium that triggers myofilament interaction and thereby the load of calcium to be sequestered to cause relaxation, while decreasing the time for calcium sequestration. By studying the effect of heart rate on myocardial relaxation the hypothesis was therefore tested that increased heart rate impairs myocardial relaxation. Methods: In situ blood-perfused right ventricular papillary muscles of seven isolated Suga-Sagawa cross circulated canine hearts were studied with a servo system that clamped muscle length to produce isometric force, F(t), at one isometric length in each muscle at paced heart rates within the range 100-150. Each curve of F(t) was fitted, by Marquardt's algorithm, with the relation F(t) = C(t/A)B e1-(t/A)B. It was previously shown that the parameter B reflects changes in relaxation, whereas the parameters A and C reflect chronotropic and inotropic states. Changes in B reflect changes in the time course of relaxation that are not simply due to changes in total twitch duration guaranteed to be produced by altered heart rate. Linear regression analysis was performed on data from each muscle to determine whether each of the parameters A, B, and C changed significantly with heart rate. Results: The mean coefficient of determination, a measure of goodness of fit of the model to observed data, was 0.996(SD 0.001). With increase in heart rate, A and C increased significantly in six of the seven muscles. On the other hand B did not show significant change with heart rate in six of the seven muscles; in the seventh muscle, B increased significantly with heart rate. Similar results were obtained when other indices of lusitropy were used. Conclusion: Increase in heart rate does not impair myocardial lusitropy as measured by the parameter B which reflects changes in the time course of relaxation independent of change in total time course of mechanical activity.