Changes with age of myocardial creatine phosphokinase in the male Fischer rat

Myocardial creatine phosphokinase (CPK) was extracted from crushed tissue in 0.01 M Tris buffer at pH 9.0 and purified by successive ammonium chloride and ethanol precipitations. The CPK activity was estimated by measuring the rate of phosphocreatine formation by the transphosphorylation of creatine by ATP. There was a slight rise in CPK activity from 1 to 2 months of age. Thereafter, there was a progressive fall in the CPK activity of 14% by 7-9 months of age, 20% by about one year of age, and 25% by 16-24 months of age. These data indicate that, with advancing age, there is an inability to rapidly restore adequate cellular levels of the high-energy substrate ATP during muscular contraction. Since the loss with age of myocardial CPK parallels a strong biochemical correlate for known declining cardiac function in aging mammals, including man.