Determination of Glycogen Turnover Rates of the Liver, Cardiac Muscle and Skeletal Muscle of the Dog by" C^14-Glucose Dilution Methnd"

Abstract

Several series of exreriments by Stetten, et aI., 1.2) present current concepts of glycogen turnover in the amimal body. In their earlier study , the turnover rate of glycogen in vivo was estimated from the measurement of stably bound deuterium appearing in glycogen; the DzO concentration in the body water was maintained by a DZO conatant infusion method. This study using the rat showed that deuterium concentration of liver and carcass glycogen increased with time. Assuming the glycogen turnover to be metabolically homogenous, the turnover rates for liver glycogen and carcass glycogen were calculated to be 68% and 1 % per day, respectively. In subsequent experiments employing C14_glucose. however. Stetten and Stetten2• 3) reported that peripheral tiers of glycogen degraded enzymatically show more radioactivity than the -less acce ssible limit dextrin within the core of the glycogen molecules. These results support the heterogeneity of glycogen turnover. The earlier calculation by D떼 based on the assumption of homogeneity must be revised. Lorber. et aI.; ) postulated in their experiment that blood sugar must be the principal precursor for cardiac glycogen and that the incorporation of labeled carbon into cardiac glycogen in the intact amimal must be secondarγ to the formation of blα펴 sugar from administered labeled compounds. On the basis of this assumption , Rhee. et aI5) . calculated the turnover rate of the cardiac glycogen in the isolated dog heart using the CJ4-glucose constant,infusion method. In this experiment, the concentration of CJ4-glucose and blood sugar in the perfusion system was maintained constant by secondary injection of additional labeled glucose. The reported turnover rate was 6.4% per hour for cardiac glycogen which was much higher than earlier reported values measured by D잉 in liver and carcass glycogen in rats. The quantitative measurement of the glycogen turnover rate by tracer techniques has always inco rporated a constant tracer infusion method for assuring the steady incorporation of the tracers from the blood into tissue glycogen This technique requires detrmination of the characteristic disappearance of a single injection of the tracer to calculate the regulation of the infusion so as to maintain a constant concentration of the tracer during the experiment. Even so. it is difficult to maintain a conatant concentration. Data from the single injection method has in the past represented only qualitative aspects of glycogen turnover. This paper presents, however, the quantitative calculation of the turnover rate after a single injection of 04 glucose; Ficks principle as utilized in the indicator dilution method" for determining cardiac output is applied. This method is referred to as the oglucose dilution method".