ATPase activity and force production in myofibrils and twitch characteristics in intact muscle from neonatal, adult, and senescent rat myocardium

To determine whether age-associated alterations in the cardiac muscle twitch could be related to altered myofibrillar ATPase activity or to an altered force-pCa relationship, these variables were measured in rat cardiac preparations across a broad age range. Between 2 and 6 months, maximum ATPase activity in detergent treated myofibrils decreased approximately two fold (0.166±0.01 v. 0.078±0.02 μm Pi/min·mg protein, P<0.001), but did not change with further aging (12 or 24 months). The Ca²⁺-dependent force in thin 'Triton skinned' papillary muscles was not age-related. ATPase activity and force exhibited identical Ca²⁺ sensitivity from the submicromolar to micromolar range: for ATPase activity pCa for 50% activity averaged 6.1 and Hill coefficients averaged 4.5; pCa for 50% force development was 6.1 and Hill coefficients of the force-pCa relation averaged 4.5; no age differences in these parameters were observed. In the intact muscles prior to skinning, neither twitch force nor the maximum rate of force production were age-related; however, indices of the time course of contraction, time to peak force, half relaxation time, and their sum, increased progressively, changing by approximately 30% from 2 to 24 months (P<0.001). since the decline in ATPase activity occurred over the maturational period only, and did not change with further ageing, while the twitch duration changed progressively with age, it is concluded that the twitch prolongation of the senescent myocardium cannot be directly related to the age-related decline in myofibrillar ATPase activity.