Effects of glucagon and dibutyryl cyclic AMP on the phosphorylase activity and gluconeogenesis in rat liver slices

Regulatory mechanisms of hepatic phosphorylase in fetal and neonatal livers of rats

Active phosphorylase was determined in rat livers during the end of the fetal period and the first days of life. This enzyme increases between the 16th day of gestation and birth. After birth, another increase in observed that takes place with 6 h. In the neonatal liver, the rapid increase in active phosphorylase, is inhibited by high levels of blood glucose, but is unaffected by actinomycin D. In fetal liver glucagon administration is followed after 5 h by an increase in total phosphorylase with only a small increase in active phosphorylase; this effect is blocked by actinomycin D. The fetal changes are interpreted as de novo synthesis, whereas the neonatal increase is due to an activation of inactive phosphorylase. Both enzymatic changes appear to be regulated by glycogen. The role of phosphorylase in the regulation of glycogen metabolism in neonatal liver is discussed.

Metabolic responses of rats to chronic theophylline ingestion

Studies of the metabolic responses of rats to 0.2% dietary theophylline were conducted. Young male rats were fed theophylline-containing diets or control diets for 8 wk and killed after 22 hr without food. Levels of serum lipids, glucose, and ketones were determined as were hepatic glycogen, lipid, calcium, magnesium, and zinc levels. Epididymal fat pad lipid content, as well as kidney cation levels and hepatic mitochondrial respiration, was also determined. Chronic theophylline feeding lowered blood glucose levels, serum triglyceride levels, hepatic lipid and calcium levels, and the state III respiration with succinate as the substrate. Kidney calcium and magnesium levels were elevated in theophylline-fed rats as were the serum cholesterol levels. The relationship of these metabolic responses to theophylline feeding is discussed as a function of the theophylline effect on tissue cation levels.

Gluconeogenesis in human liver tissue. An in vitro method for evaluation of glyconeogenesis in man

Human liver tissue was obtained as surgical biopsies in 29 subjects operated on for uncomplicated gallstone disease. Liver slices were incubated in Krebs-Ringer bicarbonate buffer solution, pH 7.4, with 17 l-amino acids, lactate, glycerol, and glucose at various concentrations. The incorporation rate of alanine, lactate, and glycerol into glucose, glycogen, and CO2 was determined by use of 14C-labeled precursors. The gluconeogenetic rate of all substrates was increased 10-35 times by increasing precursor concentration in the medium. Insulin at a physiological concentration (300 mU/l) and dexamethasone (0.001 mmol/l) had slight but significant effects on the incorporation rate of alanine into glucose and glycogen, respectively. Glucagon had no effect. Glucose in the incubation medium did not influence the incorporation rate of precursors into glucose, glycogen, or CO2, suggesting that glucose was not of importance for the regulation of the gluconeogenesis. The gluconeogenetic rate of a precursor was not dependent on or influenced by the presence of other precursors. The gluconeogenesis in human liver slices at physiological concentrations of precursors was 5-20% of the maximal rate reported for the rat liver. When the precursor concentration in the medium was increased, the gluconeogenetic rate increased to values close to those reported for the rat liver in vitro and for man in vivo. This in vitro preparation of human liver seems to be valid for evaluation of gluconeogenesis in man.