Recycling of Glucose by Rat Hepatocytes

Abstract

1 The metabolism of glucose labeled uniformly with ¹⁴C, and in positions 2, 3 and 5 with tritium by hepatocytes from fed and fasted rats were studied. Cells were incubated with glucose as sole substrate, or with glucose and a variety of glucose precursors, and uptake or production of glucose, and the utilization of the isotopes was determined. 2 There was no uptake of glucose at concentration of up to 15 mM, and net glucose synthesis in the presence of precursors. ¹⁴C was however recovered in CO₂, lactate and amino acids, and tritium in water. Considerable incorporation into glycogen from ¹⁴C and ³H-labeled glucose occurred at high (above 20 mM) glucose concentrations. 3 The yield in water always exceeded that in ¹⁴C-labeled products. The yield in ³HOH from [2-³H]glucose exceeded that from [5-³H]glucose, and the latter was greater than from [3-³H]glucose. 4 Utilization of labeled glucose does not follow Michaelis-Menten kinetics. The fractional rate of uptake of ¹⁴C and tritium-labeled glucose increases with glucose concentration with a maximum at about 15 mM and then declines. 5 The effect of numerous gluconeogenic substrates on the isotope utilization and the ³H/¹⁴C ratio in glycogen was studied. The uptake of ¹⁴C was always depressed. Addition of lactate and dihydroxyacetone has little effect on the detritiation of [2-³H]glucose, but it is depressed by other substrates. The detritiation of [3-³H]- and [5-³H]glucose is depressed in aluconeogenesis, that from [3-³H]glucose usually more than from [5-³H]glucose. In the presence of lactate detritiation of [3-³H]glucose is about half that from [5-³H]glucose. 6 Equations to calculate the phosphorylation of glucose and fructose 6-phosphate in the presence of futile cycling between glucose and glucose 6-phosphate and fructose 6-phosphate and fructose 1,6-bisphosphate were derived. 7 The estimate of glucose phosphorylation requires determination of the specific activity of glucose 6-phosphate from [2-³H]glucose. It appears that. futile cycling between glucose and glucose 6-phosphate is extensive in cells with a high glycogen content, but is low in cells from starved rats and nearly absent in those from diabetic animals. 8 The estimation of the phosphorylation of fructose 6-phosphate in the presence of cycling requires knowledge of the specific activities of fructose 6-phosphate and fructose 1,6-bisphosphate from [3-³H]glucose. At present there are no adequate data to calculate phosphorylation and recycling of fructose 6-phosphate, but under some conditions the rate may be quite high.