Chronic exercise prevents characteristic age-related changes in rat cardiac contraction

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Abstract
Prolonged contraction duration (CD) and enhanced dynamic stiffness coefficient measured during twitch (.alpha.) have consistently been observed in isometric cardiac muscle from senescent rat compared with that from the adult rat. To determine whether chronic exercise would alter CD or .alpha., male rats at 3-4 and 18-19 mo. were run in motorized wheels (0.7 km/h) for 0.5-h periods 5 days/wk for 5 mo. and, with age-matched controls, were killed at 8-9 and 23-24 mo. of age. Trabeculae carneae removed from the left ventricle were stimulated to contract isometrically at 24 min-1 at 29.degree. C over a range of muscle lengths (0.9-1.0 Lmax) and perfusate extracellular Ca concentrations ([Ca2+]e, 0.25-2.5 mM). In control muscles developed force and maximum rate of development were not age related; CD was prolonged 23% (P < 0.01), and .alpha. was 35% greater (P < 0.01) in muscles from senescent vs. younger adult rats. The exercise protocol did not modify any twitch parameter in the young group. In senescent rats, exercise reduced CD and .alpha. to levels present in the younger rats: at Lmax ([Ca2+]e = 2.5 mM) CD is 199 .+-. 5 ms in senescent vs. 193 .+-. 4 in adult; .alpha. is 1.5 .+-. 0.13 is senescent vs. 1.24 .+-. 0.11 in adult. The effect of exercise in modifying CD and .alpha. was observed at all muscle lengths and perfusate [Ca2+]. Apparently, the effectiveness of exercise in modifying the cardiac contractions is highly dependent on the age of the animal and the alterations in stiffness and CD that are characteristic of senescent myocardium are not fixed but can be modified by a mild chronic exercise protocol.