Previously, we have shown that potassium and magnesium (K-Mg, 20 mM each) cardioplegia ameliorated cytosolic calcium ([Ca2+]i) accumulation and was associated with enhanced functional recovery after surgically induced global ischemia in the aged heart. K-Mg cardioplegia was also shown to enhance cytosolic cytochrome oxidase I activity and mRNA levels, suggesting that enhanced functional recovery may involve the preservation of high-energy phosphates. To investigate this hypothesis, 31P nuclear magnetic resonance was used to measure serial alterations in phosphocreatine (PCr), inorganic phosphate, nucleoside triphosphate (NTP), intracellular free magnesium (Mgf), and intracellular pH (pHi) in Langendorff-perfused, aged (135 wk) rabbit hearts during preischemia, global ischemia (30 min), and reperfusion (30 min). K-Mg cardioplegia retarded PCr depletion (P < 0.05) and significantly enhanced NTP preservation (P < 0.05) during ischemia and reperfusion. K-Mg cardioplegia also attenuated the increase in Mgf during ischemia (P < 0.05). These results were correlated with amelioration of [Ca2+]i accumulation during ischemia and preservation of left ventricular function after reperfusion and suggest that optimal functional recovery from surgically induced ischemia is provided by K-Mg cardioplegia in the aged myocardium.

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Rates of adenosine triphosphate (ATP) metabolism are higher in cerebral gray matter than in white matter. Like other excitable tissues, brain contains a phosphocreatine (PCr)/creatine kinase (CK)/ATP system including cytosolic (B-CK) and mitochondrial (Mi-CK) isozymes. High B-CK activity is present in white and gray matter while Mi-CK is mostly in gray matter. An in situ localizing ³¹P-NMR technique, one-dimensional chemical shift imaging (1D-CSI), has been used to study the PCr/CK/ATP system in these regions. In the metabolically mature 4-week-old piglet, the PCr/nucleoside triphosphate (NTP) ratio measured by the 1D-CSI technique is at least 50% higher in white than gray matter. Total creatine (Cr), ATP, and total NTP concentrations are the same in rapidly frozen rat white and gray matter, suggesting that the PCr/Cr ratio is much higher in white matter. The PCr increases more in gray than white matter between 4 days and 4 weeks of age in piglet brain. The CK catalyzed reaction rate constant, measured by combining the saturation transfer experiment with the 1D-CSI, is also much higher in white than gray matter at both ages. The postnatal maturational increase in the CK rate constant is greater in gray matter. In summary, these differences in PCr concentration and CK reaction rates and isozymes characterize two physiologically different PCr/CK/ATP systems in gray and white matter.

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