Gluconeogenesis from acetone in diabetic rats

Previous investigations have demonstrated that acetone is a true, if minor precursor of glucose in vivo. In diabetic rats 1.30% of the carbon atoms of circulating glucose arises from acetone, whereas 0.67% does in normal 3-day fasted animals. Calculated from these fractions and the turnover rate of glucose, 48 micrograms/kg. min acetone-carbon is converted to glucose-carbon in diabetic and 16 micrograms/kg. min in normal rats. In both groups of rats the labelling of plasma lactate was stronger than that of glucose. In view of these results we conclude that: the transfer of C-atoms from acetone to glucose increases in diabetes; acetone remains a minor source of glucose even in ketonemic diabetic rats.     

Kinases and phosphatases of hepatic glycogen metabolism during fasted to refed transition in normal and streptozotocin-induced diabetic rats.

Normal and streptozotocin-induced diabetic rats were fasted for 24 hours and refed for 4 hours. Changes in the activities of glycogen metabolizing enzymes in liver were followed during this period. In normal rats, hepatic glycogen content increased gradually after the onset of food intake. The percent of active glycogen synthase increased to a peak value at 1h which coincided with a significant (P less than 0.02) increase in synthase phosphatase activity. Phosphorylase alpha and the percent of alpha increased significantly (P less than 0.01) after the meal which correlated with similar increases in cAMP-dependent protein kinase and phosphorylase kinase activities. Activation of enzymes involved in both synthesis and degradation of glycogen during fasted to refed transition indicate a probable substrate cycling. In diabetic livers, there was marked decrease in the activities of glycogen metabolizing enzymes and their levels did not alter significantly in response to the meal indicating a poor turnover of glycogen.     

Suppression of methionine-induced hyperhomocysteinemia by glycine and serine in rats

The hyperhomocysteinemia induced by a dietary addition of 1% methionine was significantly suppressed by the concurrent addition of 1% glycine or 1.4% serine to the same degree. The methionine-induced increase in the hepatic concentration of methionine metabolites was significantly suppressed by glycine and serine, but the hepatic cystathionine beta-synthase activity was not enhanced by these amino acids. When the methionine-supplemented diet was changed to the methionine plus glycine or serine diet, the plasma homocysteine concentration rapidly decreased during and after the first day. The hyperhomocysteinemia induced by an intraperitoneal injection with methionine was also suppressed by concurrent injection with glycine or serine, although the effect of serine was significantly greater than that of glycine. These results indicate that glycine and serine were effective for suppressing methionine-induced hyperhomocysteinemia: serine and its precursor glycine are considered to have elicited their effects mainly by stimulating cystathionine synthesis by supplying serine, another substrate for cystathionine synthesis.     

Oxidative stress in normal and diabetic rats.

Parameters related to oxidative stress were studied in a group of 10 Wistar diabetic rats and 10 control rats. The levels of total erythrocyte catalase activity in the diabetic animals were significantly (p<0.001) greater than the control levels. The diabetic animals presented an amount of vitamin E far greater (p<0.0001) than the controls, as was also the case for the vitaminE/polyunsaturated fatty acid (PUFA) and vitaminE/linoleic acid (C18:2) ratios. Greater vitaminE/triglyceride (TG) ratio, however, appeared in the control group. The corresponding vitamin A ratios (vitaminA/TG, vitaminA/PUFA, vitaminA/C 18:2) were higher in the control group. Our work corroborates the findings that fatty acid metabolism presents alterations in the diabetes syndrome and that the antioxidant status is affected.     

Reduced glucose transport and increased binding of insulin in adipocytes from diabetic and fasted rats

1. Animals made diabetic by injection of streptozotocin or animals after 3 days of fasting show decreased insulin levels and a decrease in mean cell diameter of adipocytes from epidydymal fat pads in comparison with cells from normal animals. 2. 14CO2 production from D-[U-14C]glucose is impaired in diabetic and fasted animals both in presence or in absence of a concentration of insulin stimulating 14CO2 production maximally. 3. Insulin binding is increased in adipocytes from diabetic and fasted animals due to changes in affinity. 4. Transport studies show that basal and insulin stimulated 2-deoxy[1-14C]-glucose transport is decreased in absolute terms due to a decrease in V and an increase in Km. 5. The relative stimulatory effect of insulin is impaired in adipocytes of diabetic and fasted animals. 6. A shift of the maximal effect of insulin to lower insulin levels is seen in these cells.     

Oral glycine administration attenuates diabetic complications in streptozotocin-induced diabetic rats

Diabetes mellitus is a disease characterized by impaired glucose metabolism that leads to retinopathy, brain micro-infarcts and other complications. We have previously shown that oral glycine administration to diabetic rats inhibits non-enzymatic glycation of hemoglobin and diminishes renal damage. In this work, we evaluated the capacity of the amino acid glycine (1% w/v, 130 mM) to attenuate diabetic complications in streptozotocin (STZ)-induced diabetic Wistar rats and compared them with non-treated or taurine-treated (0.5% w/v, 40 mM) diabetic rats. Glycine-treated diabetic rats showed an important diminution in the percentage of animals with opacity in lens and microaneurysms in the eyes. Interestingly, there was a diminished expression of O-acetyl sialic acid in brain vessels compared with untreated diabetic rats (P<0.05). Additionally, peripheral blood mononuclear cells isolated from glycine-treated diabetic rats showed a better proliferative response to PHA or ConA than those obtained from non-treated diabetic rats (P<0.05). Glycine-treated rats had a less intense corporal weight loss in comparison with non-treated animals. Our results suggest that administration of glycine attenuates the diabetic complications in the STZ-induced diabetic rat model, probably due to inhibition of the non-enzymatic glycation process.     

Gluconeogenesis and P-enolpyruvate carboxykinase in liver and kidney of long-term fasted quails

The activity of cytoplasmic and mitochondrial phosphoenolpyruvate carboxykinase (PEPCK) in kidney and liver, and in vivo gluconeogenic activity, were determined during different phases of prolonged fasting in quails. The fasting-induced changes in the activity of kidney cytoplasmic PEPCK were positively correlated with the changes in gluconeogenesis. Both activities increased at the initial phase (I) of fasting to levels 65% to 100% higher than fed values, and decreased during the protein-sparing period (phase II), although remaining higher than in fed birds. At the catabolic final phase (III) both kidney cytoplasmic PEPCK activity and gluconeogenesis increased markedly, attaining levels 115% to 150% higher than fed values. The activity of liver cytoplasmic PEPCK, present in appreciable amounts in quails, did not change during phases I and II of fasting, but increased to levels 60% higher than fed values at the final phase (III). Plasma glucose levels at phase III did not differ significantly from those at phases I and II. In both kidney and liver the activity of the mitochondrial PEPCK was not significantly affected by fasting. The data suggest that the kidney cytoplasmic PEPCK is the main enzyme responsible for gluconeogenesis adjustments during food deprivation in quails, and that this function is complemented at the final phase by enzyme present in liver cytosol. Accepted: 14 April 2000     

Fine structure of hepatocytes from fasted and fed rats.

The fine structure of hepatocytes from rats maintained on a controlled feeding schedule are described. Liver samples were processed for electron microscopy, histochemistry and chemical determinations of glycogen at precise time-intervals following a 30-hour fast and a 2-hour meal. Hepatocytes from 30-hour-fasted rats with extremely low hepatic glycogen levels were devoid of glycogen particles. Centrilobular cells showed areas of the cytoplasm rich in vesicles of smooth endoplasmic reticulum (SER) while periportal hepatocytes contained less extensive regions of SER. Soon after feeding the fasted rats, glycogen particles appeared in regions of the cell rich in SER. Centrilobular hepatocytes contained numerous glycogen areas which were infiltrated with tubules of SER, while periportal cells showed dense glycogen deposits with SER restricted to the periphery of the masses of glycogen. Throughout glycogen deposition each glycogen particle was closely associated with membranes of SER until maximum glycogen deposition was achieved 12 hours after initiation of feeding. At this point SER was reduced to the lowest amounts of the time-periods studied. During stages of glycogen depletion SER proliferated and reached the highest concentration measured in this study. Tubules of SER were present throughout the glycogen masses of centrilobular hepatocytes, whereas in periportal cells the organelle was restricted to the periphery of the glycogen masses. It is concluded that SER is associated with glycogen particles in rat hepatocytes during both deposition and depletion of glycogen.     

Glutamine and glucose metabolism in thymocytes from normal and spontaneously diabetic BB rats.

Metabolism of glutamine and glucose was studied in thymocytes from normal rats and BB rats with the spontaneous autoimmune diabetic syndrome to assess their potential roles as fuels. The major measured products from glucose were lactate and, to a lesser extent, CO2, and pyruvate. Glutamine had no effect on the rates of their production from glucose. Glutamine was metabolized to ammonia, aspartate, glutamate, and CO2, with aspartate being the major product of carbons from glutamine in the absence of glucose. Glucose markedly decreased the formation of ammonia, aspartate, and CO2 from glutamine, but increased that of glutamate, with an overall decrease in glutamine utilization by 55%. More glutamate than aspartate was produced from glutamine in the presence of glucose. The potential production of ATP from glucose was similar to that when glutamine was present alone. However, glucose markedly decreased production of ATP from glutamine, but not vice versa. This resulted in ATP production from glucose being 2.5 times that from glutamine when both substrates were present. The oxidation of glucose to CO2 via the Krebs cycle accounts for 75-80% of glucose-derived ATP production. Cellular ATP levels markedly decreased in the absence of exogenous substrates, but were constant throughout a 2-h incubation in the presence of glutamine, glucose, or both. There were no differences in thymocyte glucose or glutamine metabolism between normal and diabetic BB rats, in contrast to previous findings in peripheral lymphoid organs. Our results suggest that glucose is a more important fuel than glutamine for "resting" thymocytes, again in contrast to the cells of peripheral lymphoid organs in which glutamine is as important as glucose as a fuel.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dichloroacetate-induced changes in liver of normal and diabetic rats.

Dichloroacetate (DCA) was administered orally to normal (nondiabetic) and streptozotocin-diabetic rats in a dose of 1000 mg/day/kg rat wt. One group of diabetic animals received DCA both orally and intraperitoneally. DCA therapy lowered the blood glucose values of diabetic animals but did not alter values in nondiabetic rats. The hepatic activity of glucokinase and pyruvate kinase were significantly lower in both DCA-treated nondiabetic and DCA-treated diabetic animals than values observed for untreated animals. However, DCA therapy was accompanied by remarkable increases in the activities of glucose-6-phosphate dehydrogenase and malic enzyme in both nondiabetic and diabetic animals. Glucose-6-phosphate dehydrogenase was 3-fold higher in DCA-treated nondiabetic animals whereas malic enzyme activity was 10-fold higher in the treated animals than observed in the untreated animals. Similar changes, although smaller in magnitude, were observed for these enzymes in the DCA-treated diabetic animals. Although DCA therapy was accompanied by a significant increase in the wet weights of the liver for both nondiabetic and diabetic animals, no morphological changes were seen by light or electron microscopy. Our observations coupled with those of previous investigators suggest that DCA therapy may have an important role in pyruvate metabolism and may lower the blood glucose concentration by inhibiting hepatic gluconeogenesis.     

Phosphoinositide metabolism in adipocytes from fed and fasted rats

Phosphoinositide turnover was investigated in adipocytes from fed and 48 hour fasted rats. Insulin stimulated phosphoinositide synthesis both in adipocytes from fed and fasted rats. Fasting enhanced this effect of insulin 2-fold. Hydrolysis of phosphoinositides to inositol phosphates was not activated by insulin, neither transient after 2 minutes nor after 60 minutes stimulation. Under similar conditions, alpha 1-adrenergic receptor stimulation induced a pronounced inositol phosphate production. Thus, it is suggested that phosphoinositide hydrolysis is not involved in insulin action. The alpha 1-adrenergic effect was similar in adipocytes from fed and fasting rats.