Regulation of glucose uptake by muscle. 5. Effects of anoxia, insulin, adrenaline and prolonged starving on concentrations of hexose phosphates in isolated rat diaphragm and perfused isolated rat heart

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Abstract
The concentrations of glucose 6-phosphate, fructose 6-phosphate and fructose 1,6-diphosphate have been assayed in extracts of perfused rat heart and isolated rat diaphragm by enzymic methods, and effects of anoxia, insulin and epinephrine on these parameters investigated. In rat hearts perfused aerobically concentrations of glucose 6-phosphate, fructose 6-phosphate and fructose 1,6-diphosphate (in [mu]moles/lOO g) were 23, 4.6 and 2.6. The corresponding concentrations in intact diaphragms were 29, 6.7 and 4.0 and in hemidiaphragms 30, 7.7. and 2.0. In rat heart these concentrations were changed (as per cent of control) as follows anoxia -21, -22 and +63; insulin +9, +17 and +85; anoxia in the presence of insulin -33, -30 and +20. In intact diaphragms these concentrations were changed by anoxia +37, +16 and +33; by insulin +50, +50 and +42; by anoxia in the presence of insulin +50, +86 and +65. In hemidiaphragms these were changed by anoxia +4, +36 and +81; by insulin +26, +22 and +60; by anoxia in the presence of insulin -27, -30 and +33; adrenaline in the presence of insulin +37, +52 and -12. It is concluded that in heart and diaphragm the overall rate of phosphorylation of fructose 6-phosphate to the diphosphate may limit glycolysis under aerobic conditions and that anoxia accelerates glycolysis by increasing the rate of this step in particular. Evidence is presented that phosphorylation of glucose by hexokinase may be regulated in muscle by the intracellular concentration of glucose 6-phosphate (a known inhibitor of muscle hexokinase). On the other hand, monosaccharide transport in muscle does not appear to be regulated by the intracellular concentration of glucose 6-phosphate.