Maturation of energy metabolism in the lamb: changes in myosin ATPase and creatine kinase activities.

Studies have been carried out to assess maturation of myofibrillar and mitochondrial proteins in fetal (113 to 140 days gestation), neonatal (30 min to 21 days postpartum), and adult sheep hearts. Ca++-activated myosin ATPase activity was approximately 20% lower in fetal than in adult left ventricular myocardium (1.13 +/- 0.06, n = 12, versus 1.36 +/- 0.07, n = 9, mumoles P1 per g protein per sec; P less than 0.025). In fetal and neonatal hearts (but not in adult hearts), myosin ATPase activity was slightly higher (approximately 14%; P less than 0.001) in right ventricular tissue than in left ventricular tissue. In contrast to these small changes in myosin ATPase activity, large changes indicative of maturation of energy metabolism occurred in the creatine kinase system: between 115 days gestation and 21 days postpartum, total creatine kinase activity increased nearly 8-fold (0.2 to 1.6 IU/mg cardiac mass), the MM-creatine kinase isozyme increased 7-fold (0.2 to 1.5 IU/mg wet weight), and mitochondrial creatine kinase increased more than 25-fold (less than 0.01 to 0.27 IU/mg wet weight). The total creatine pool, but not the ATP pool, increased (from approximately 6 to approximately 15 nmoles/g tissue). Neither the concentration nor isozyme distribution of lactate dehydrogenase, a glycolytic enzyme, changed during this 7-wk period of development.