ALTERED TURNOVER OF CALCIUM REGULATORY PROTEINS OF THE SARCOPLASMIC-RETICULUM IN AGED SKELETAL-MUSCLE

We have measured the in vivo protein turnover for the major calcium re gulatory proteins of the sarcoplasmic reticulum from the skeletal musc le of young adult (7 months) and aged (28 months) Fischer 344 rats. Fr om the time course of the incorporation and decay of protein-associate d radioactivity after a pulse injection of [C-14]leucine and correctin g for leucine reutilization, in young rats, the apparent half-lives fo r calsequestrin, the 53-kDa glycoprotein, and ryanodine receptor are 5 .4 +/- 0.4, 6.3 +/- 1.3, and 8.3 +/- 1.3 days, respectively. A half-li fe of 14.5 +/- 2.5 days was estimated for the Ca-ATPase isolated from young muscle. Differences in protein turnover associated with aging we re determined using sequential injection of two different isotopic lab els ([C-14]leucine and [H-3]leucine) to provide an estimate of protein synthesis and degradation within the same animal. The Ca-ATPase and r yanodine receptor isolated from aged muscle exhibits 27 +/- 5% and 25 +/- 3% slower protein turnover, respectively, relative to that from yo ung muscle. In contrast, the 53-kDa glycoprotein exhibits a 25 +/- 5% more rapid turnover in aged SR, while calsequestrin exhibits no age-de pendent alteration in turnover. Statistical analysis comparing the sen sitivity of various methods for discriminating different rates of prot ein turnover validates the approach used in this study and demonstrate s that the use of two isotopic labels provides at least a 6-fold more sensitive means to detect age-related differences in protein turnover relative to other methods.