The adenosine triphosphatase and calcium ion-transporting activities of the sarcoplasmic reticulum of developing musce.

1. The ATPase (adenosine triphosphatase) specific activity and the total nitrogen content of the myofibrillar fraction per g. wet weight of rabbit longissimus dorsi muscle increased steadily during the late foetal stages and the first few weeks after birth. 2. The ATPase specific activity of the sarcoplasmic-reticular fraction isolated by a sucrose-density-gradient procedure rose to a sharp peak 8-10 days after birth and then declined to the adult value, which was about 25% of the maximum. 3. The peak in ATPase activity was a feature of the sarcoplasmic reticulum isolated from muscle, and the time at which it occurred in relation to birth was related to the degree of development and the activity pattern of the muscle. 4. The peak in ATPase activity of the sarcoplasmic reticulum occurred at an earlier age if newborn animals were made to exercise earlier than was normal. 5. The ;extra' ATPase associated with the sarcoplasmic reticulum and the ability to concentrate Ca(2+) increased in a similar manner over the period of development studied. 6. It is postulated that the Ca(2+)-transport system of the sarcoplasmic reticulum consists of two components, namely the ATPase and the system coupling this enzyme to Ca(2+) transport. During development the ATPase develops first and has almost reached maximum activity in the longissimus dorsi muscle of the rabbit after 8-10 days. Subsequently the activity of the coupling system rises rapidly, leading to an increase in the capacity and efficiency of Ca(2+) transport.