Effects of fructose concentration on carbohydrate metabolism, heat production and substrate cycling in isolated rat hepatocytes

Abstract

1. Hepatocytes from starved rats were incubated with 5mm-glucose, labelled uniformly with ¹⁴C and specifically with ³H at positions 1, 2, 3 or 6, and with fructose at concentrations of 2.5, 7.5 or 25mm. 2. In the absence of other substrates only 1% of the radioactivity initially present in [U-¹⁴C]glucose appeared in the metabolic products, CO₂, lactate, pyruvate, amino acids and glycogen. 3. Fructose at 2.5mm caused a 30% increase in the glucose concentration and a 4-fold increase in the apparent oxidation of [U-¹⁴C]-glucose. 4. The formation of ³H₂O from [1-³H]-, [2-³H]-, [3-³H]- or [6-³H]-glucose was 2.4, 4.3, 2.15 or 1.6% respectively in the control incubations and 4.1, 10.4, 7.7 or 5.1% with 2.5mm-fructose. 5. Fructose at 7.5 and 25mm decreased the ³H₂O yields to less than the control values, but had no apparent effect on the amount of [U-¹⁴C]glucose metabolized. 6. In the incubations with 5mm-glucose and 25mm-fructose there were significant decreases in heat production, O₂ consumption and in the ratio of O₂ uptake to heat output. 7. Fructose at 2.5mm caused a 64% increase in heat output, but only a 43% increase in O₂ uptake. 8. The radioisotopic and calorimetric data demonstrate that physiological concentrations of fructose greatly increase metabolism in hepatocytes from starved rats. These data also indicate increased cycling at glucose/glucose 6-phosphate and at fructose 6-phosphate/fructose 1,6-bisphosphate in the presence of 2.5mm-fructose, although the rates of cycling were actually decreased relative to the amount of glucose catabolized. 9. At concentrations of 2.5, 7.5 and 25mm, fructose depressed hepatocyte ATP concentrations by 20, 65 and 80% respectively. Although fructose at 7.5 and 25mm increased glucose and lactate release, O₂ consumption, production of heat and formation of³H₂O from [1-³H]-, [2-³H]-, [3-³H]- or [6-³H]-glucose were lowered to values equal to, or less than, controls. These effects probably reflect a severe derangement of hepatic metabolism due to excess phosphorylation of fructose when present at high concentrations.