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J Biol Chem. 1988 Feb 15;263(5):2285-91.

NMR measurements of in vivo myocardial glycogen metabolism.

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Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06510.


Using 13C and 1H NMR we measured the rate of glycogen synthesis (0.23 +/- 0.10 mumol/min gram wet weight tissue (gww) in rat heart in vivo during an intravenous infusion of D-[1-13C]glucose and insulin. Glycogen was observed within 10 min of starting and increased linearly throughout a 50-min infusion. This compared closely with the average activity of glycogen synthase I (0.22 +/- 0.03 mumol/min gww) measured at physiologic concentrations of UDP-glucose (92 microM) and glucose-6-phosphate (110 microM). When unlabeled glycogen replaced D-[1-13C]glucose in the infusate after 50 min the D-[1-13C]glycogen signal remained stable for another 60 min, indicating that no turnover of the newly synthesized glycogen had occurred. Despite this phosphorylase a activity in heart extracts from rats given a 1 h glucose and insulin infusion (3.8 +/- 2.4 mumol/min gww) greatly exceeded the total synthase activity and if active in vivo should promote glycogenolysis. We conclude that during glucose and insulin infusion in the rat: (a) the absolute rate of myocardial glycogen synthesis can be measured in vivo by NMR; (b) glycogen synthase I can account for the observed rates of heart glycogen synthesis; (c) there is no futile cycling of glucose in and out of heart glycogen; and (d) the activity of phosphorylase a measured in tissue extracts is not reflected in vivo. These studies raise the question whether significant regulation of phosphorylase a activity in vivo is mediated by factors in addition to its phosphorylation state.

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