Summary
The creatine kinase (CK) activity in rat plasma was found to be correlated with that
in serum or platelets. Rat serum contained higher activity not only of CK but also
of lactate dehydrogenase than the corresponding plasma. The plasma CK activity decreased
with increasing age of rats. Only the brain-type isoenzyme of CK was found in plasma,
serum and platelets. Contamination of brain-type with muscle-type isoenzyme was detected
in rather exceptional cases in which the activity was high among old rats. These results
suggest the possible contribution of blood platelets to the CK activity of circulating
plasma as well as to the activity in serum in rats.
