CAATPASE CONTENT IS LOWER IN CARDIAC SARCOPLASMIC-RETICULUM ISOLATED FROM OLD RATS

The rate of oxalate-facilitated ATP-dependent calcium uptake by the ca lcium pump, calcium adenosinetriphosphatase (CaATPase), is 30-40% slow er in the sarcoplasmic reticulum (SR) isolated from the hearts of sene scent Fischer 344 male rats. To determine the underlying mechanism, ca rdiac SR was isolated from 11- to 12-mo-old (adult) and 22- to 24-mo-o ld (senescent) male Fischer 344 rats. The yield of SR and contaminatio n by other membrane organelles were similar between the groups. The ra te of calcium uptake by the homogenate and isolated SR was 28-44% slow er (P < 0.05) in the senescent group. In the isolated SR the calculate d maximal velocity (V(max)) of CaATPase activity as a function of vary ing concentrations of ATP or calcium was 20-30% lower (P < 0.05) in th e senescent group; however, the affinities for both calcium and ATP of CaATPase activity were unaltered. The lower V(max) was matched by a d ecreased (P < 0.05) content of calcium-dependent phosphoenzyme (EP) in the SR isolated from the senescent rats. Thus the ratio of enzyme act ivity to phosphoenzyme content (V(max)/EP) was similar between the gro ups. The immunoreactive CaATPase protein was 22 +/- 2% lower in the SR from the senescent rats. Taken together the data indicate that the ma jor mechanism underlying the slower calcium transport by cardiac SR is olated from old rats is a lower content of the CaATPase protein.