Blood platelets as a possible source of creatine kinase in rat plasma and serum.

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.