Hepatic gluconeogenesis and glycolysis in chickens fed "carbohydrate-free" diets

Dietary fat mediates hyperglycemia and the glucogenic response to increased protein consumption in an insect, Manduca sexta L

Many insects display non-homeostatic regulation over blood sugar level. The concentration of trehalose varies dramatically depending on physiological and nutritional state. In the absence of dietary carbohydrate, blood trehalose in larvae of the lepidopteran insect Manduca sexta is maintained by gluconeogenesis and is dependent on dietary protein consumption. In the present study, the effect of dietary fat on the glucogenic response of insects to increased dietary protein was examined by NMR analysis of (2-13C)pyruvate metabolism. Last instar larvae were maintained on a carbohydrate-free chemically defined artificial diet having variable levels of casein with and without corn oil. Gluconeogenic flux, the ratio of the rate of gluconeogenesis to the rate of glycolysis, was estimated from the 13C distribution in trehalose arising by gluconeogenesis and the 13C enrichment of alanine due to pyruvate cycling. Insects grew well on carbohydrate-free diets and growth increased with increasing dietary protein level. At all dietary protein levels, larvae grew better on diets with fat. Without dietary fat, larvae were glucogenic but displayed low blood trehalose concentrations, <30 mM, regardless of protein consumption. When fat was included in the diet, however, gluconeogenic flux and blood trehalose level increased sharply in response to increased dietary protein level, with trehalose concentrations >50 mM at higher levels of protein consumption. When offered a choice of a high carbohydrate and a high protein diet, larvae maintained on diets with fat displayed a food preference related to blood sugar level. Those with low blood sugar fed on carbohydrate, while those with high blood sugar preferred protein. Trehalose synthesized from (2-13C)pyruvate exhibited asymmetry in the 13C distribution in individual glucose molecules, indicating a disequilibrium at the triose phosphate isomerase-catalyzed step of the gluconeogenic pathway. In trehalose from larvae on diets with fat, the asymmetric 13C distribution was higher than in trehalose from insects on diets lacking fat. This may partially result from isotopic disequilibrium when unenriched glycerol is metabolized to dihydroxyacetone phosphate following fat hydrolysis. The asymmetry in 13C distribution, however, also occurred in insects on diets without fat and decreased with increased gluconeogenic flux suggesting that true disequilibrium between the triose phosphates is the principal reason for the asymmetry.

Rates of entry of alanine and glycerol and their contribution to glucose synthesis in fasted chickens

Rates of entry of alanine and glycerol and their contribution to glucose synthesis were studied in 48-h fasted White Leghorn cockerels using primed constant intravenous infusions of L-[U-14C]alanine and [U-14C]glycerol. Entry rates of alanine and glycerol were 112 and 401 mumol/h per kg body weight, respectively. Of the total glucose irreversible loss (entry rate), 1.5 and 6.6% were derived from alanine and glycerol, respectively. The possibilities of the operation of a glyoxylate cycle and omega-oxidation of fatty acids in the fasted chicken are discussed.

A technique for the isolation of chicken hepatocytes and their use in a study of gluconeogenesis

A new technique was developed for the isolation of chicken liver parenchymal cells. Glucose produced from 10 mM lactate was proportional to the amount of cells present. In the time-course study, gluconeogenesis from lactate and fructose was linear up to 60 min. Fructose proved to be the best substrate. Fructose was converted to glucose at the highest rate; this was followed by lactate, pyruvate, and xylitol. Alanine, glycerol, propionate, alpha-ketoglutarate, and succinate proved to be poor substrates. There was no statistical difference between the results obtained with hepatocytes obtained from fed or fasted chickens. The isolated hepatocytes responded to glucagon, dibutyryl-cAMP, and epinephrine. The dose-response for glucagon was a sigmoid-curve and the half-maximum stimulation was given by approximately 1 x 10(-2) micrometers hormone. The same type of curve was obtained with dibutyryl-cAMP, but the half-maximum stimulation was achieved at around 1.0 micrometer. The response to epinephrine was marginal. In the time-course experiment, prior to glucagon stimulation, glucose accumulated at a linear rate (slope = .2484). After the addition of the hormone, the level of cAMP increased by about 30% in the first minute and reached a peak (100%) in about 2 min; thereafter, it decreased to the level prior to the stimulation by the hormone. Two minutes after the addition of glucagon there was a significant increase in the rate of gluconeogenesis; this continued for another 3 min and then at a slower pace (slope = .2566).

Intracellular distribution of hepatic and renal gluconeogenic enzymes in embryonic and growing chickens

The intracellular distribution of hepatic and renal gluconeogenic enzymes in 20-day-old chicken embryos and 4-week-old chickens (Gallus domesticus: New Hampshire male X Columbian female) has been studied. Pyruvate carboxylase, fructose-1,6-diphosphatase, and glucose-6-phosphatase were found primarily in the mitochondrial, cytosolic, and microsomal fractions, respectively. Phosphenolpyruvate carboxykinase was present not only in the mitochondria but also in the cytosol of the chicken liver and the kidney. The intracellular distribution of the liver enzyme differed from that of the kidney enzyme in chicken embryos as well as in growing chickens.

Effect of fasting and fasting and refeeding on hepatic and renal gluconeogenic enzymes in the chicken

The effect of fasting and fasting and refeeding on hepatic and renal gluconeogenic enzyme activities were studied in six-week-old chickens (Gallus domesticus: New Hampshire male x Columbian female). Hepatic pyruvate carboxylase appeared not to be affected by fasting, but the renal enzyme activity increased in four-day fasted chickens. The hepatic mitochondrial and cytosolic phosphoenolpyruvate carboxykinases were essentially not affected by fasting. The renal mitochondrial phosphoenolpyruvate carboxykinase showed a slight increase in activity only after a four-day fast, but the cytosolic enzyme activity increased markedly already after a two-day fast. Also the activities of the hepatic and renal fructose-1,6-diphosphatase and glucose-6-phosphatase increased markedly on fasting. Refeeding for four days after a four-day fast returned these enzyme activities to near control values.

Effect of dietary energy on hepatic glycogen metabolism in the turkey hen

An experiment was conducted with turkey hens to investigate the effect of substituting 30% of the carbohydrate calories with corn oil, 1,3-butanediol, or glycerol. Birds fed additional corn oil had the lowest liver glycogen concentration. Corn oil increased phosphorylase, a total phosphorylase, and glycogen synthetase I in comparison to the controls. Also, additional corn oil resulted in the highest specific activity of glucose-6-phosphatase. Dietary glycerol caused the highest concentration of liver glycogen. Glycerol increased glycogen synthetase I, but had little effect upon total activity in comparison to butanediol in the diet. Both butanediol and glycerol gave similar phosphorylase a activity, but butanediol increased total activity. The fat-fed and control-fed hens regulated hepatic glycogen concentration through phosphorylase, while glycerol and butanediol-fed hens regulated glycogen through glycogen synthetase. In vitro activation of glycogen synthetase I was deficient in hens fed additional corn oil, indicating a lack of glycogen synthetase phosphatase activity. The order of activation (glycerol greater than butanediol greater than control greater than corn oil) corresponds to the rank of glycogen concentrations

Activity of certain liver enzymes in fast- and slow-growing lines of chickens

The realtionship between growth rate and the metabolic activity of certain liver enzymes was studied using two strains of White Plymouth Rock chickens which had been selected in divergent directions for eight-week body weight. The activities of hexokinase, glucokinase, phosphofructokinase, glucose-6-phosphate dehydrogenase, citrate synthase, glycogen synthetase, glutamate dehydrogenase and aspartate transaminase were measured at 4, 8 and 20 weeks of age. The mean percentage rate of growth of the birds selected for high eight-week body weight exceeded that of the birds selected for low eight-week body weight only during the early growth period. Thereafter, and until sexual maturity, the low-line birds grew at a faster rate, relative to body size. The mature body weight of the high-line birds exceeded that of the low-line birds by a factor of approximately 1.5. A close similarity was noted between the metabolic activity of certain liver enzymes and the growth rate (relative to body size) of the birds studied. At four and eight weeks of age, the faster-growing birds (whether high- or low-line) generally exhibited a greater capacity for glucose phosphorylation and glycolysis, but a poorer capacity for glycogen synthesis, than the slower-growing birds. At twenty weeks, growth rate and metabolic activity were similar in both strains.

Studies on the growth and changes in metabolism of rats fed on carbohydrate-deficient fatty acid-based diets supplemented with graded levels of maize starch

1. Experiments were conducted to determine the effects of the supplementation of carbohydrate-deficient fatty acid-based (FA) diets with 18·7, 37·5, 75, 150 and 300 g maize starch (MS)/kg on the growth and metabolism of the growing rat. Further, the effects of fasting on rats given the FA diet were compared with those given the high-carbohydrate (CHO) diet. Due to the significant decrease in food consumption of rats given the FA diet, the effects of pair-feeding the CHO and the FA diet were also investigated.

2. The isoenergetic replacement of fatty acids with increasing amounts of MS in the FA diet given to rats increased their weight gain and concentration of glucose in their plasma. These increases, however, tended to level off for weight gain and glucose concentration, respectively, at about 18·7 g and 75 g MS/kg diet. The same type of replacement decreased the concentration of ketones in the plasma but the decrease levelled off at the high concentration of 300 g MS/kg diet. Activities of liver glucose-6-phosphatase (EC3.1.3.9) and glucokinase (EC2.7.1.2) decreased and increased respectively with increasing concentration of maize starch in the diet. These changes tended to level off at concentrations of about 75–150 g MS/kg diet.

3. Fasting for 18 h decreased the concentration of glucose in plasma of both the FA- and the CHO-fed rats, while fasting of the CHO-fed rats depressed the concentration of glycogen in the liver but did not influence that of the FA-fed rats. Pair-feeding the CHO diet to the FA diet produced similar weight increases during the 8-week experimental period.

Influence of meal-feeding on some of the effects of dietary carbohydrate deficiency in rats

1. Experiments were conducted to investigate the effects of feeding rats on a diet based on a mixture of maize oil fatty acids as the only source of non-protein energy (fatty acid diet) and the influence on these effects of giving such a diet in a single daily meal lasting 2 h.

2. In comparison with a triglyceride diet in which the non-protein energy was in the form of maize oil, feedingad lib.with the fatty acid diet produced no significant changes in body-weight gain, plasma glucose and plasma ketones concentrations, liver glycogen concentration and protein efficiency ratio.

3. In comparison with the triglyceride diet, meal-feeding with the fatty acid diet produced significantly lower body-weight gain and protein efficiency ratio; moreover, it significantly lowered plasma glucose and liver glycogen concentrations.

4. Rats meal-fed on the fatty acid diet synthesized glucose from protein, as evidenced by the significantly higher liver glycogen concentration detected 6 h after the meal had been eaten, but the increase was significantly lower than in the animals fed on the triglyceride diet. Also, 6 h after the meal had been eaten, the amount of meal remaining in the stomach of rats meal-fed on the fatty acid diet was significantly higher than in those fed on the triglyceride diet.