Age-associated slowing of cardiac relaxation related to the decline in the Ca2+ 
pump function of cardiac sarcoplasmic reticulum (SR) has been previously 
described. It is unclear if the decreased Ca2+ pump function results from a 
lower amount of Ca2(+)-ATPase protein or a decreased pumping activity of the 
enzyme. To determine if these alterations could be mediated by changes in the 
amount of the protein itself, the level of the messenger RNA (mRNA) coding for 
the Ca2(+)-ATPase of the SR of Fischer rat hearts (4- and 30-month-old rats) 
were quantitated with a Northern blotting technique. We observed that the levels 
of SR Ca2(+)-ATPase mRNA were 60% lower in old rats as compared with young rats, 
suggesting that a quantitative reduction in the levels of the corresponding 
protein could occur during aging to explain the delayed diastolic relaxation 
documented in old animals as opposed to a change in the specific activity of 
this enzyme. The thyroid hormone responsiveness of SR Ca2(+)-ATPase mRNA has 
been previously established. We have found in this study that the thyroxine 
levels were consistently lower in old rats; however, this difference was 
relatively small (4.3 +/- 0.7 and 3.1 +/- 0.8 micrograms/dl [mean +/- SD), 
respectively, in young and old rats). In addition, no age-induced decrease in 
3,5,3'-triiodothyronine levels was observed, suggesting that the aging process 
itself may be responsible for the changes in SR Ca2(+)-ATPase mRNA levels.