Effects of aging on sarcoplasmic reticulum function and contraction duration in skeletal muscles of the rat

The impact of aging on the Ca2+ pump function of skeletal muscle sarcoplasmic reticulum (SR) was investigated using SR-enriched membrane vesicles isolated from the slow-twitch soleus muscle (SM) and the relatively fast-twitch gastrocnemius muscle (GM) isolated from adult (6-8 mo old) and aged (26-28 mo old) Fischer 344 rats. In addition, isometric twitch characteristics of SM and GM were determined in situ in adult and aged rats under anesthesia. The rates of ATP-supported Ca2+ uptake by SM SR was markedly lower (similar to 50%) in the aged compared with adult at varying Ca2+ (0.11-8.24 mu M) concentrations. Kinetic analysis of the data revealed age-associated decrease in maximum activity reached (V-max) and increase in the concentration of Ca2+ giving half of V-max. In contrast, no significant age-related difference was observed in ATP-supported Ca2+ uptake activity of GM SR. The Ca2+ stimulated adenosinetriphosphatase (ATPase) activities and the amount of Ca2+-ATPase protein did not vary significantly with aging in SM or GM SR. Also, no significant age-related difference was observed in the content of the ryanodine receptor (Ca2+-release channel) or the Ca2+ binding protein, calsequestrin in SM and GM. SR. In isometrically contracting SM, the time to peak force, half-relaxation time, and contraction duration were significantly prolonged in the aged compared with adult, whereas there was no age-related difference in maximum developed force. None of these isometric twitch parameters differed significantly with age in the GM. These results demonstrate that the effects of aging on skeletal muscle contractile properties and SR function are muscle specific. Furthermore, the data strongly suggest that impairment in SR Ca2+ pump function, apparently due to uncoupling of ATP hydrolysis from Ca2+ transport, contributes to the age-associated slowing of relaxation in the soleus muscle.