Short-term modulation of glycogen metabolism, glycolysis and gluconeogenesis by physiological oxygen concentrations in hepatocyte cultures.

Abstract

The influence of different oxygen concentrations (0% to 20%, v/v) on the main pathways of carbohydrate metabolism was studied in rat hepatocyte cultures. Cells resembling the periportal or the perivenous cell type were obtained after 48 h culture under different hormonal conditions; they are referred to as 'periportal' or 'perivenous', respectively. Using radiochemical techniques the metabolic rates of the two cell types were measured between 48 h and 50 h under different oxygen tensions. Standard physiological substrates of 5 mM glucose, 2 mM lactate and endogenous glycogen were used. In cells incubated under 4% O2, mimicking hepatovenous oxygen levels, compared to cells assayed under 13% O2, mimicking arterial levels, glycogen degradation to free glucose and to lactate was faster while glycogen synthesis was slower; glycolysis, from glucose to lactate, was faster and gluconeogenesis, from lactate to glucose, was only slightly slower. Under anoxic conditions glycogen breakdown was maximal and glycogen synthesis minimal; gluconeogenesis was also minimal, but glycolysis was not maximal, it reached its peak rate at 4% O2. CO2-formation increased up to 6% and then stayed essentially constant at higher O2 tensions. Net glycogen metabolism: in 'perivenous' cells net glycogen synthesis was observed above, net glycogen degradation below 4% O2. In 'periportal' cells, which had a very low glycogen content, net glycogen metabolism was very small. Net glucose metabolism: in 'perivenous' cells net glucose formation was only seen under anoxic conditions. Net glucose utilization was observed at about the same rate under all physiological O2 tensions. In 'periportal' cells net glucose formation increased clearly up to 6% O2 and then remained almost constant. Net lactate metabolism: in 'perivenous' cells net lactate formation occurred below, and net lactate utilization above, 6% O2. In 'periportal' cells a pronounced net lactate utilization was observed under all physiological O2 tensions. Net flow between glucose-6-phosphate and pyruvate was observed in the glycolytic direction in 'perivenous' and in the gluconeogenic direction in 'periportal' cells except under anoxic conditions. When O2 tensions were lowered, the percentage of 'futile cycling' was decreased in the 'perivenous', glycolytic hepatocytes; conversely, it was increased in the 'periportal', gluconeogenic hepatocytes. It is concluded that physiological oxygen concentrations modulate hepatic carbohydrate metabolism and that they contribute further to the proposed metabolic differences between periportal and perivenous cells in vivo.