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Updated: Aug 4, 2026
Contractility Measurements on Isolated Papillary Muscles for the Investigation of Cardiac Inotropy in Mice
Published on: September 17, 2015
Effect of heart rate on myocardial relaxation in isometric twitches
1Division of Cardiology, Harris Chasanoff Heart Institute, Long Island Jewish Medical Center, New Hyde Park, NY 11042.
Insights
Increased heart rate does not impair myocardial relaxation. The study found that the parameter B, which measures relaxation time course, did not significantly change with heart rate, suggesting preserved myocardial lusitropy.
Area of Science:
- Cardiology
- Physiology
- Biophysics
Background:
- Heart rate influences calcium handling, affecting myocardial relaxation.
- Understanding the impact of heart rate on relaxation is crucial for cardiac function.
- Previous studies suggested potential impairment of myocardial relaxation at higher heart rates.
Purpose of the Study:
- To investigate the effect of increased heart rate on myocardial relaxation.
- To test the hypothesis that elevated heart rates impair myocardial relaxation.
- To analyze changes in relaxation parameters in response to varying heart rates.
Main Methods:
- Studied blood-perfused canine right ventricular papillary muscles.
- Utilized a servo system for isometric force clamping at a fixed muscle length.
- Fitted force-time data using a mathematical model (Marquardt's algorithm) to analyze parameters A, B, and C.
- Performed linear regression to assess parameter changes with heart rate (100-150 bpm).
Main Results:
- The mathematical model demonstrated a high goodness of fit (mean R²=0.996).
- Parameters A and C, reflecting chronotropic and inotropic states, increased significantly with heart rate in most muscles.
- Parameter B, indicative of relaxation time course, showed no significant change with increased heart rate in most muscles.
Conclusions:
- Increased heart rate does not impair myocardial lusitropy.
- The parameter B effectively measures relaxation independent of total mechanical activity duration.
- Findings suggest preserved myocardial relaxation dynamics even with elevated heart rates.
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(e)1-(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.
Conclusions:
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.
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