Creatine kinase and mitochondrial respiration in hearts of trout, cod and freshwater turtle

The importance of the creatine kinase system in the cardiac muscle of ectothermic vertebrates is unclear. Mammalian cardiac muscle seems to be structurally organized in a manner that compartmentalizes the intracellular environment as evidenced by the substantially higher mitochondrial apparent Km for ADP in skinned fibres compared to isolated mitochondria. A mitochondrial fraction of creatine kinase is functionally coupled to the mitochondrial respiration, and the transport of phosphocreatine and creatine as energy equivalents of ATP and ADP, respectively, increases the mitochondrial apparent ADP affinity, i.e. lowers the Km. This function of creatine kinase seems to be absent in hearts of frog species. To find out whether this applies to hearts of ectothermic vertebrate species in general, we investigated the effect of creatine on the mitochondrial respiration of saponin-skinned fibres from the ventricle of rainbow trout, Atlantic cod and freshwater turtle. For all three species, the apparent Km for ADP appeared to be substantially higher than for isolated mitochondria. Creatine lowered this Km in trout and turtle, thus indicating a functional coupling between mitochondrial creatine kinase and respiration. However, creatine had no effect on Km in cod ventricle. In conclusion, the creatine kinase-system in trout and turtle hearts seems to fulfil the same functions as in the mammalian heart, i.e. facilitating energy transport and communication between cellular compartments. In cod heart, however, this does not seem to be the case.