A myothermic technique has been used to investigate the mechanics and energetics of left ventricular papillary muscles from 6-, 15-, 22- and 27-32-month-old Sprague-Dawley rats. 2. There was a significant increase in the left ventricular mass to body mass (LVM:BM) ratio in the senescent. 27-32-month-old group of animals compared with the younger animals (P<0.05). 3. The maximum stress developed in the senescent groups was reduced by almost 40% in comparison with the stress developed by the 6-, 15- and 22-month-old groups (P<0.001). The mean rise time, half relaxation time and half width were increased significantly (P<0.05) in the 22-month-old group but, unexpectedly, this effect was not seen in the senescent group. 4. Heat production per beat versus total stress relationships were obtained in two different ways to determine the magnitude of the activation heat and the isometric economy (given by the slope of the relationship). The activation heat was not significantly different between groups with either method, but there was a significant increase (P<0.001) in the economy with which stress was developed in the senescent group in comparison with the 6- and 15-month-old groups. 5. A combination of forskolin (2.5-6.5 micromol/L) and high Ca2+ (7.5 mmol/L) was used to increase the energy usage per beat. In the 6- and 15-month-old groups, these agents caused a four-fold increase in the activation heat magnitude compared with a less than two-fold increase in the 22-month-old and senescent groups (P<0.001). There was no effect of forskolin/high calcium on the slope of the heat:total stress relationship. 6. The data suggest that, under conditions known to increase cardiac contractility, there is a reduced ability to cycle calcium in the 22-month-old and senescent groups relative to the young adult 6-month-old and adult 15-month-old groups.
