The rate of oxalate-facilitated ATP-dependent calcium uptake by the calcium pump, calcium adenosinetriphosphatase (CaATPase), is 30-40% slower in the sarcoplasmic reticulum (SR) isolated from the hearts of senescent Fischer 344 male rats. To determine the underlying mechanism, cardiac SR was isolated from 11- to 12-mo-old (adult) and 22- to 24-mo-old (senescent) male Fischer 344 rats. The yield of SR and contamination by other membrane organelles were similar between the groups. The rate of calcium uptake by the homogenate and isolated SR was 28-44% slower (P < 0.05) in the senescent group. In the isolated SR the calculated maximal velocity (Vmax) of CaATPase activity as a function of varying concentrations of ATP or calcium was 20-30% lower (P < 0.05) in the senescent group; however, the affinities for both calcium and ATP of CaATPase activity were unaltered. The lower Vmax was matched by a decreased (P < 0.05) content of calcium-dependent phosphoenzyme (EP) in the SR isolated from the senescent rats. Thus the ratio of enzyme activity to phosphoenzyme content (Vmax/EP) was similar between the groups. The immunoreactive CaATPase protein was 22 +/- 2% lower in the SR from the senescent rats. Taken together the data indicate that the major mechanism underlying the slower calcium transport by cardiac SR isolated from old rats is a lower content of the CaATPase protein.