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KMID : 0378019600030030039
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New Medical Journal
1960 Volume.3 No. 3 p.39 ~ p.44
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Metabolism of C^(14)-labeled Carbohydrate in the Dog
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Abstract
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1. With the aid of carbon 14-labeled glucose, the metabolism of glucose by the isolated beating dog heart has been investigated in a series of five experiments. In four of these, radioactive glucose was infused at a constant rate in order to maintain a constant specific activity of plasma glucose and a constant plasma glucose level. In the remaining experiment, radioactive glucose was administered in a single initial dose, the blood glucose level being allowed to decline thereafter.
Two control experiments were performed with blood only in the same perfusion system and under the same experimental conditions as those employed in the perfused heart studies. These controls were necessary to assess contribution of blood glucose metabolism in the heart perfusion experiments.
2. In the control -experiments CO2 production by blood decreased rapidly with time from 0.4 millimoles per 100 ml. blood per hour initially to 0.04 mM per 100 ml. blood per hour after four hours. However, percentage of CO2 derived from glucose increased in a linear fashion from 6 to 74% during the four-hour experimental period. On the basis of these results, all data on respiratory CO2 from isolated heart perfusion experiments have been corrected for blood metabolism of glucose.
3. In the isolated heart experiments, the percentage of respiratory COs which was derived from plasma glucose ranged from 18 to 43% with a mean of, 33%. From these data it seems reasonable to infer that the isolated beating heart, perfused under normoglycemic conditions, readily derives a very_ substantial portion or its oxidative energy from blood glucose.
4. The mean value for the percentage of cardiac glycogen derived from plasma glucose was 18.9% for four experiments where glucose was adiministered by a constant infusion method. A much lower value, 2.4 %, was for glycogen isolated from a heart receiving only a single initial injection of labeled glucose.
Calculations made for the turnover rate of cardiac glycogen yielded values ranging from 4.5 to 7.7% of glycogen replaced per hour by labeled glucose. An average of 27.6 mg of glycogen per 100 grams of wet heart tissue was found to turn over per hour. Calculations of the half time for glycogen turnover (t1/2) gave values ranging from 9.2 to 15.5 hours, with a mean of 11.3 hours.
These data on rates of cardac glycogen replacement by labeled plasma glucose provide evidence for the view that heart glycogen may be much less metabolically inert than has been previously supposed by many authors. The appreciable labeling of cardiac glycogen by glucose also establishes this substrate as a major blood precursor, probably the principal one, for heart glycogen.
It has been pointed out that calculations of turnover are based on the assumption of a well-mixed, homogeneous reservior of glycogen and that this assumption may not be completely valid for the turnover of cardiac glycogen.
5. In the four constant infusion experiments, average values of 18% of the measured total glucose consumption by the isolated heart were found to be accounted for by conversion to CO2 and to lactate, respectively. In no case did the sum of glucose conversion to CO2 and to lactate exceed 50% of the glucose disappearing from the perfusing blood. Thus, since blood pyruvate and cardiac glycogen remained in essentially steady states, at least one half of the disappearing glucose is converted to metabolites and products other than CQ2, -lactate, pyruvate and glycogen.
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