Variations of muscle mitochondrial creatine kinase activity in mitochondrial diseases

Intracellular diffusion restrictions in isolated cardiomyocytes from rainbow trout

BACKGROUND

Restriction of intracellular diffusion of adenine nucleotides has been studied intensively on adult rat cardiomyocytes. However, their cause and role in vivo is still uncertain. Intracellular membrane structures have been suggested to play a role. We therefore chose to study cardiomyocytes from rainbow trout (Oncorhynchus mykiss), which are thinner and have fewer intracellular membrane structures than adult rat cardiomyocytes. Previous studies suggest that trout permeabilized cardiac fibers also have diffusion restrictions. However, results from fibers may be affected by incomplete separation of the cells. This is avoided when studying permeabilized, isolated cardiomyocytes. The aim of this study was to verify the existence of diffusion restrictions in trout cardiomyocytes by comparing ADP-kinetics of mitochondrial respiration in permeabilized fibers, permeabilized cardiomyocytes and isolated mitochondria from rainbow trout heart. Experiments were performed at 10, 15 and 20 degrees C in the absence and presence of creatine.

RESULTS

Trout cardiomyocytes hypercontracted in the solutions used for mammalian cardiomyocytes. We developed a new solution in which they retained their shape and showed stable steady state respiration rates throughout an experiment. The apparent ADP-affinity of permeabilized cardiomyocytes was different from that of fibers. It was higher, independent of temperature and not increased by creatine. However, it was still about ten times lower than in isolated mitochondria.

CONCLUSIONS

The differences between fibers and cardiomyocytes suggest that results from trout heart fibers were affected by incomplete separation of the cells. However, the lower ADP-affinity of cardiomyocytes compared to isolated mitochondria indicate that intracellular diffusion restrictions are still present in trout cardiomyocytes despite their lower density of intracellular membrane structures. The lack of a creatine effect indicates that trout heart lacks mitochondrial creatine kinase tightly coupled to respiration. This argues against diffusion restriction by the outer mitochondrial membrane. These results from rainbow trout cardiomyocytes resemble those from other low-performance hearts such as neonatal rat and rabbit hearts. Thus, it seems that metabolic regulation is related to cardiac performance, and it is likely that rainbow trout can be used as a model animal for further studies of the localization and role of diffusion restrictions in low-performance hearts.

Creatine transporter and mitochondrial creatine kinase protein content in myopathies

Total creatine or phosphocreatine, or both, are reduced in the skeletal muscle of patients with inflammatory myopathy, mitochondrial myopathy, and muscular dystrophy/congenital myopathy. We used Western blotting techniques to measure skeletal muscle creatine transporter protein and sarcomeric mitochondrial creatine kinase (mtCK) protein content in patients with inflammatory myopathy (N = 8), mitochondrial myopathy (N = 5), muscular dystrophy (N = 7), and congenital myopathy (N = 3), as compared to a control group without a neuromuscular diagnosis (N = 8). Creatine transporter protein content was lower for all groups compared to control subjects (P < 0.05; P < 0.01 for congenital myopathy). Mitochondrial CK (mtCK) was lower for inflammatory myopathy (P < 0.05), higher for mitochondrial myopathy (P < 0.05), not different for muscular dystrophy, and markedly lower for the congenital myopathy group (P < 0.01), compared to control subjects. Together, these data suggest that the reduction in total creatine or phosphocreatine in patients with certain myopathies is correlated with creatine transporter and not mtCK protein content. This further supports the belief that creatine monohydrate supplementation may benefit patients with low muscle creatine stores, although the reduction in creatine transporter protein may have implications for dosing.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.