Cardiac myofibrillar creatine kinase is not influenced by hypothyroidism.

The cardiac myofibrillar component of the phosphorylcreatine shuttle mechanism enzymatically couples the functionally significant processes of energy utilization (ATPase) with substrate regeneration by creatine kinase (CK). Both components have isoenzyme forms that are transcriptionally regulated. Propylthiouracil-induced (PTU) hypothyroidism reduced rat cardiac contractile protein ATPase activity by shifting isomyosin predominance from the V1 to the V3 form. However, neither CK specific activity or CK isoenzyme composition was altered by PTU treatment. Thus, myofibrillar components of the phosphorylcreatine shuttle, ATPase and CK, are not coordinately regulated under hypothyroid conditions.