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 sarco plasmic reticulum (SR) was investigated using SR-enriched membrane ves icles isolated from the slow-twitch soleus muscle (SM) and the relativ ely 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 a nd 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 analys is of the data revealed age-associated decrease in maximum activity re ached (V-max) and increase in the concentration of Ca2+ giving half of V-max. In contrast, no significant age-related difference was observe d in ATP-supported Ca2+ uptake activity of GM SR. The Ca2+ stimulated adenosinetriphosphatase (ATPase) activities and the amount of Ca2+-ATP ase 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 pro tein, calsequestrin in SM and GM. SR. In isometrically contracting SM, the time to peak force, half-relaxation time, and contraction duratio n were significantly prolonged in the aged compared with adult, wherea s 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 skel etal muscle contractile properties and SR function are muscle specific . Furthermore, the data strongly suggest that impairment in SR Ca2+ pu mp function, apparently due to uncoupling of ATP hydrolysis from Ca2transport, contributes to the age-associated slowing of relaxation in the soleus muscle.