Rapid Conversion by Insulin of Hepatic Intermediary Metabolism from Glucose Production to Glucose Utilization in the Liver of Alloxan-diabetic Rats

Abstract

The concentration of free fatty adds in the blood, the tissue concentrations of hepatic cyclic AMP, long-chain acyl-CoA (LCA), adenine nucleo tides, inorganic phosphate, the intermediates of the Embden-Meyerhof pathway and the citric acid cycle (including acetyl-CoA and free CoA), and the cytoplasmic and mitochondrial redox couples were determined in alloxan-diabetic rats 15, 30, 60, and 90 minutes after insulin injection. From the meared values, the energy state of the adenine nucleotide system in the liver, the redox state of the NAD system, and the concentrations of malate, oxaloacetate, citrate, and α-ketoglutarate were estimated for the cytoplasmic and mitochondrial compartment by the use of established calculation methods. Insulin injection into alloxan-diabetic rats caused a rapid decline in the concentration of serum free fatty acids and a coordinate decline in hepatic LCA concentration. The latter alteration was accompanied by an increase in the cytoplasmic and a synchronous decrease in the mitochondrial ATP/ADP×P ratio. Simultaneously, the redox state of the cytoplasmic NAD system was shifted towards the oxidized state. When the appropriate data were plotted against each other, a highly significant correlation between the concentration of free fatty acids in the blood and that of LCA in the liver, between the hepatic LCA concentration and the cytoplasmic energy state, and between the cytoplasmic energy state and the cytoplasmic redox state was obtained. These findings are interpreted to support the hypothesis derived from in-vitro experiments that insulin, because of the concentration of hepatic LCA, might affect the translocation of adenine nucleotides between the cytoplasmic and the mitochondrial compartment, thereby regulating the cytoplasmic energy state and, consequently, the redox state of the cytoplasmic NAD system. Insulin injection into alloxan-diabetic rats provoked rapid, coordinate changes in the concentration of the intermediates of both the citric acid cycle and the Embden-Meyerhof chain, indicating the altered substrate flow through these pathways. Thosemetabolit s known to modulate the activity of key regulatory enzymes in vitro were analyzed with respect to their proposed regulatory function. As to the insulin-induced shift from pyruvate carboxylatlon to pyruvate decarboxylatIon, the data reveal that the decrease in pyruvate carboxylase activity can be attributed to the decrease in the intramitochondrial ATP/ADP ratio and the simultaneous fall in acetyl-CoA concentration, while the coordinate increase in pyruvate dehydrogenase activity can be ascribed to the decline in the concentration of LCA and, consequently, in the ratios of ATP/ADP, NADH/NAD, and acetyl-CoA/CoA within the mitochondria.