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 iso lated from 6-, 15-, and 27- to 32-month-old Sprague-Dawley rats were measur ed in myothermic experiments at 27 degrees C at a stimulus frequency of 0.1 67 Hz. Results: We found that the basal metabolism measured in quiescent pa pillary muscles was significantly reduced in the 27- to 32-month-old group (4.9 mW g(-1) compared to 7.7 and 7.0 mW g(-1) in the 6- and 15-month group s). In isotonic experiments, the work output (at a range of afterloads) was significantly depressed for the 27- to 32-month group bring only 52% of th e work output of the 6-month group. This outcome was due to a decrease in b oth 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 paral lel decrease in energy consumption (enthalpy) and hence, the net mechanical efficiency (work/active enthalpy X 100%) was not altered. A force-length- area (FLA) analysis was applied to the isotonic data and an energy: FLA reg ression line was obtained for each preparation. We found that there were no significant differences in either the intercept or slope of the energy: FL A relation with age. Contractile efficiency (39+/-3%) in the 27- to 32-mont h 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 me chanical performance or efficiency of cardiac muscle from young (6-month-ol d) or adult (15-month-old) rats but in the aged and senescent rats (27-32-m onth-old) there is a pronounced decline in stress development and shortenin g ability leading to a fall in work output. Mechanical and contractile effi ciency however remain unchanged in old age and the data resembles that obta ined in pressure overload hypertrophy. (C) 2000 Elsevier Science B.V. All r ights reserved.