Control of energy transport in cardiac muscle
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Summary1. Creatine-2-14C was injected into rats at different times after x- or sham-irradiation.2. Muscle creatine and phosphocreatine, as well as creatine from blood, kidney and urine, were isolated and their respective specific activities determined. Urinary creatinine was also isolated and its 14C-activity determined.3. Coincident with the maximal excretion of creatine, the specific activities of muscle creatine and phosphocreatine decreased after x-irradiation.4. Immediately after injecting the x-irradiated rats with creatine-2-14C, the specific activity of urinary creatine increased, whereas that of creatinine decreased, but at a somewhat later time.5. It is postulated that defective utilization of creatine by muscle constitutes the cause of radiation-induced creatinuria.
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Sufficient ATP concentrations maintain physiological processes and protect tissue from hypoxic damage. With decreasing oxygen concentration, ATP synthesis relies increasingly on the presence of phosphocreatine.The effect of exogenously applied creatine on phosphocreatine and ATP concentrations was studied under control and anoxic conditions.Pregnant mice were fed orally with creatine monohydrate (2 g/kg body weight/day). Brainstem slices from these mice pups were compared with those from pups of non-creatine supplemented pregnant mice. Measurements were performed under normoxic and anoxic conditions. In addition, brainstem slices from non-creatine treated mice pups were incubated for 3 hours in control artificial cerebrospinal fluid (CSF) (n = 10) or in artificial CSF containing 200 microM creatine (n = 10). ATP and phosphocreatine contents were determined enzymatically in single brainstem slices.ATP concentrations were in the same range in all preparations. However, there was a significant increase of phosphocreatine in the brainstems from pups of creatine fed mice when compared with the brainstems of pups from non-creatine treated mice or in non-incubated brainstems of control animals. After 30 minutes anoxia, ATP as well as phosphocreatine concentrations remained significantly higher in creatine pretreated slices compared with controls.The data indicate that exogenous application of creatine is effective in neuroprotection.
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To measure the actual percentage of intracellular free creatine participating in the process of energy transport, the incorporation of [1-14C]creatine into the "free" creatine and phosphocreatine (PCr) pools in spontaneously beating isolated rat atria, under various conditions, was examined. The atria were subjected to three consecutive periods, control, anoxia, and postanoxic recovery, in medium containing tracers of [1-14C]creatine. The tissue content and specific activity of creatine and PCr were determined at the end of each period. The higher specific activity found for tissue PCr (1.87 times) than creatine, independent of the percentage of total intracellular creatine that was present as free creatine, provides evidence for the existence of two separate pools of free creatine. Analysis of the data shows that in the normal oxygenated state approximately equal to 9% of the total intracellular creatine is actually free to participate in the process of energy transport (shuttle pool). About 36% of the total creatine is bound to unknown intracellular components and the rest exists as PCr. The creatine that was taken up and the creatine that was released from the breakdown of PCr have much greater access to the site of phosphorylation than the rest of the intracellular creatine. A sharp increase in the specific activity of residual PCr on prolongation of anoxic time was also observed. This provides evidence for a nonhomogeneous pool of PCr, for the most recently formed (radioactive) PCr appeared to be hydrolyzed last.
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