Effects of aging on the work output and efficiency of rat papillary muscle

Objectives: This study aimed to investigate the effect of aging on the work output and efficiency of rat papillary muscle. Methods: The mechanical and energetic properties of left ventricular papillary muscle preparations isolated from 6-, 15-, and 27- to 32-month-old Sprague–Dawley rats were measured in myothermic experiments at 27°C at a stimulus frequency of 0.167 Hz. Results: We found that the basal metabolism measured in quiescent papillary muscles was significantly reduced in the 27- to 32-month-old group (4.9 mWg−1 compared to 7.7 and 7.0 mWg−1 in the 6- and 15-month groups). In isotonic experiments, the work output (at a range of afterloads) was significantly depressed for the 27- to 32-month group being only 52% of the work output of the 6-month group. This outcome was due to a decrease in both the extent of muscle shortening only, 66% of 6- and 15-month data, and in the maximum force developed. The reduced work was accompanied by a parallel decrease in energy consumption (enthalpy) and hence, the net mechanical efficiency (work/active enthalpy×100%) was not altered. A force–length– area (FLA) analysis was applied to the isotonic data and an energy: FLA regression line was obtained for each preparation. We found that there were no significant differences in either the intercept or slope of the energy: FLA relation with age. Contractile efficiency (39±3%) in the 27- to 32-month group was not significantly different to that found in the 6-month (43±4%) or 15-month (40±3% group). Conclusion: There are no changes in the mechanical performance or efficiency of cardiac muscle from young (6-month-old) or adult (15-month-old) rats but in the aged and senescent rats (27–32-month-old) there is a pronounced decline in stress development and shortening ability leading to a fall in work output. Mechanical and contractile efficiency however remain unchanged in old age and the data resembles that obtained in pressure overload hypertrophy.