Changes In Gluconeogenesis Research Articles

Gluconeogenesis in Isolated Hepatic Parenchymal Cells

Abstract A pure preparation of rat liver parenchymal cells was prepared by collagenase digestion of livers from fasted (24 hours) Sprague-Dawley rats, and the general features of gluconeogenesis in these cells have been elucidated. Rates of gluconeogenesis by a 5% suspension of cells were studied over a range of concentrations (0 to 11.3 mm) of different substrates. Rates were most rapid with concentrations of d-glyceraldehyde and dihydroxyacetone up to 1.5 mm and d-fructose up to 3.7 mm. d-Glyceraldehyde inhibited gluconeogenesis at 1.5 to 2.0 mm. At greater than 1.5 mm, dihydroxyacetone continued to be converted but at a slower rate. At 3.7 to 7.6 mm, fructose was converted at a constant maximal rate; slight inhibition occurred at 11.3 mm. The maximal rate of gluconeogenesis from glycerol was observed at 0.75 mm and was half the maximal rate observed for fructose. Alanine and serine were converted to glucose very slowly; pyruvate and lactate were converted at rates between those from glycerol and from the amino acids. When lactate or pyruvate was incubated with the liver cells at 2 to 3 mm or greater, the rate of gluconeogenesis became approximately constant. When lactate and pyruvate were incubated together in ratios varying from 2 to 6, gluconeogenesis occurred rapidly and in proportion to the substrate concentration. In contrast to perfused liver, isolated cells subjected to the extreme oxidized or reduced state caused by incubation with pyruvate alone or lactate alone, respectively, were incapable of rapid carbohydrate synthesis. The liver cells were responsive to glucagon stimulation of gluconeogenesis from d-glyceraldehyde and dihydroxyacetone: with d-glyceraldehyde as substrate, glucagon caused a 50 % increase, but the effect was not reproducible. The effects of monobutyryl adenosine 3':5'-monophosphate (cyclic AMP) on metabolism of dihydroxyacetone and a mixture of lactate and pyruvate (3:1) also were studied. The nucleotide caused a 30% increase in gluconeogenesis from dihydroxyacetone but much less of a change in gluconeogenesis from lactate-pyruvate. Thus, the properties of our cell system permitted a discrimination between a readily demonstrable cyclic AMP effect on dihydroxyacetone metabolism and an effect (reported by others) on pyruvate metabolism which was not as demonstrable. These results are consistent with the view that the actions of glucagon on the gluconeogenic pathway are not unifocal.

Gluconeogenesis

Gluconeogenesis is reviewed from the standpoints of its functions and relationships to general metabolism, the biochemical mechanisms involved in its control, and its alterations in various physiologic and pathologic conditions. Its functions are discussed in relationship to cerebral metabolism, the Cori cycle, the alanine cycle, and ammonia production by the kidney. The pathways of glucose synthesis from major physiologic substrates are outlined. Control mechanisms for gluconeogenesis, which appear to operate under physiologic conditions, are discussed. Particular attention is paid to regulation of the following processes or enzymes: supply of substrate from peripheral tissues, substrate uptake by the liver, transport of metabolites across the mitochondrial membranes, pyruvate carboxylase, pyruvate dehydrogenase, the Krebs cycle, P-enolpyruvate carboxykinase, pyruvate kinase, fructose-1, 6-diphosphatase, P-fructokinase, glucose 6-phosphatase, glucokinase, phosphorylase, and glycogen synthetase. In addition, the effects of changes in ATP, ADP, AMP, cyclic AMP, the [free NADH] / [free NAD +] ratio, Ca ++, and H + are considered. Changes in gluconeogenesis in various situations are described with particular reference to studies in man. An attempt is made to identify hormonal and other factors responsible for alterations in metabolism. The situations considered include the following: brief and prolonged starvation, diabetes and insulin treatment, fetal and neonatal development, pregnancy and lactation, exercise, obesity, glucocorticoid deficiency and excess, alcohol hypoglycemia, hypoxia, and fatty acid infusion. The effects of certain hypoglycemic agents on gluconeogenesis are also described. Some general conclusions are drawn regarding the possible roles and mechanisms of action of glucagon, insulin, adrenal steroids, the sympathetic nervous system, and fatty acids in the control of gluconeogenesis in vivo. Areas needing further research are outlined.

Metabolism of [2-14C] pyruvate in normal, acromegalic and hgh-treated human subjects.

ABSTRACT In 4 studies on 3 acromegalic patients, who had normal iv glucose tolerance and high insulin response to infused glucose (Al), the oxidation to 14CO2 of [2-14C] pyruvate (injected intravenously in trace amount after overnight fast) was not different from that in 9 studies of 9 nonacromegalic »high insulin responders« (Ni). In 4 studies on 3 other acromegalic patients, who had low glucose tolerance and less insulin response to glucose (A2), the formation of 14CO2 was reduced to ½–⅔ that of Al or N1 and was about proportionate to the reduction in glucose tolerance. In A2 the 14CO2 formation was slightly lower than the mean for 10 studies with 7 non-acromegalic subjects, who were »low insulin responders« with normal or low glucose tolerance (N2). Among non-acromegalics expiration of 14CO2 was significantly lower in N2 than in N1. Among 4 non-acromegalic subjects treated with human growth hormone for 3–4 days one had a marked reduction in pyruvate oxidation, while all had a decrease in glucose tolerance. Analysis of 14C in blood glucose at 60 minutes after injection of [2-14C]pyruvate suggested that slightly more total 14C-glucose was present in A2 than N1 without any differences between A2 and N2 or N1 and N2. Two out of 4 studies in A1 showed lower than normal amounts of 14Cglucose. No change in 14C-glucose occurred after administration of HGH. The findings suggest that impairment of pyruvate oxidation accompanies a lowered glucose tolerance in acromegalics with a diabetic tendency. Changes in gluconeogenesis from pyruvate appear to be minimal.

Acute effects of hydrocortisone on gluconeogenesis from 14C- and 3H-labeled substrates in intact rats.

At short time intervals (2 and 4 hr) after a single intraperitoneal injection of hydrocortisone (20 mg/rat), gluconeogenesis was studied by intraperitoneal injection in trace amount of one of the following pairs of compounds: DL-lactate-2-14C, DL-lactate-2-3H; DL-malate- 3-14C, DL-malate-2-3H; succinate-2,2′-14C, succinate-2,2′-3H; or glycerol-l,3-14C, glycerol- 2-3H. Conversion of radioactivity to CO2, blood glucose and liver glycogen was measured. With all 14C-labeled substrates there was no marked change of 14CO2 after hydrocortisone. Incorporation of both 14C and 3H from lactate and malate into glucose and glycogen was increased slightly at 2 hr and increased significantly at 4 hr after hydrocortisone. There was no change in incorporation of 14C or 3H from succinate or glycerol. Transfer of 3H from lactate and malate to glucose was somewhat more stimulated than was that of 14C. The observed changes in gluconeogenesis from certain of the labeled substrates are qualitatively similar, but less extensive,...

THE INFLUENCE OF CORTISOL ON FOOD INTAKE AND GLUCOSE METABOLISM IN SHEEP.

SUMMARY Some metabolic effects of cortisol were investigated in non-pregnant ewes fed either ad lib. or on a restricted ration. Cortisol (25 mg./day) administered for a period of 21 days stimulated the voluntary food intake of fat ewes offered food ad lib. Higher levels of cortisol (50 and 75 mg./day) administered for similar periods subsequently had little additional effect on voluntary food intake, but an even greater cortisol dose (150 mg./day) resulted in a marked decline in voluntary food intake. Ewes fed a constant restricted ration did not refuse food at any time during cortisol administration. Urinary nitrogen excretion was increased during cortisol administration but sequential increases in the cortisol dose failed to increase urinary nitrogen excretion proportionately above the level attained during administration of 25 mg. cortisol/day. The blood glucose level was elevated progressively as the level of cortisol administered was increased. Similarly, tolerance for a glucose load was impaired progressively. Only very small changes in blood ketones and plasma free fatty acid levels were observed during cortisol administration. The changes in blood glucose during cortisol administration cannot be accounted for by changes in gluconeogenesis, and it is suggested that they reflect a progressive impairment of glucose utilization relative to the blood glucose level, though not a reduction in total glucose utilization.