Oxidative myocytes of heart and skeletal muscle express abundant sarcomeric mitochondrial creatine kinase

Sarcomeric mitochondrial creatine kinase catalyzes the reversible transfer of a high energy phosphate between ATP and creatine. To study cellular dist ribution of the kinase, we performed immunocytochemical studies using a pep tide antiserum specific for the kinase protein. Our results demonstrated th at the sarcomeric mitochondrial creatine kinase gene is abundantly expresse d in heart and skeletal muscle, with no protein detected in other tissues e xamined, including brain, lung, liver, spleen, kidney, bladder, testis, sto mach, intestine, and colon. RNA blot study showed that there is no detectab le expression of the kinase mRNA in the thymus gland. In heart and skeletal muscle, the kinase protein is expressed in atrial and ventricular cardiomy ocytes and a subpopulation of skeletal myofibres. In skeletal muscle, fast myosin heavy chain co-localization studies demonstrated that the sarcomeric mitochondrial creatine kinase is highly expressed in type 1, slow-oxidativ e and type 2A, fast-oxidative-glycolytic myofibres. We conclude that the ki nase gene is abundantly expressed in oxidative myocytes of heart and skelet al muscle and may contribute to oxidative capacity of these cells.