Activities of liver pyruvate dehydrogenase complex and 3-hydroxyacyl-CoA dehydrogenase in sand rat (Psammomys obesus)

The effect of insulin on the uptake and metabolic fate of glucose in isolated perfused hearts of dyslipemic rats

Male Wistar rats chronically (15 weeks) fed a sucrose-rich diet (SRD; 63% w/w) developed hypertriglyceridemia and impaired glucose homeostasis. Hearts from these animals were isolated and perfused using the Langendorff recirculating method. Glucose at levels similar to those found in the animal in vivo was used as the only exogenous substrate. The hearts were perfused for 30 minutes in the presence or absence of insulin (30 mU/mL) in the perfusion medium. In the absence of the hormone, glucose uptake was impaired and the glucose utilization was reduced, with a significant increase of lactate release. Glucose oxidation, which was estimated from the activation state of the enzyme pyruvate dehydrogenase complex (PDHc), was depressed mainly due to both an increase of PDH kinase and a decrease of PDHa (active form of PDHc) activities. Although the addition of insulin in the perfusion medium improved the above parameters, it was unable to normalize them. The present results suggest that at least two different mechanisms might contribute to insulin resistance and to the impaired glucose metabolism in the perfused hearts of the dyslipemic SRD-fed animals: (1) reduced basal and insulin-stimulated glucose uptake and its utilization or (2) increased availability and oxidation of lipids (low PDHa and high PDH kinase activities), which in turn decrease glucose uptake and utilization. Thus, this nutritional experimental model may be useful to study how impaired glucose homeostasis, increases plasma free fatty acid levels and hypertriglyceridemia could contribute to heart tissue malfunction.

Low rate of glucose 6-phosphate hydrolysis in liver cells is a physiological feature of non-diabetic wild sand rats (Psammomys obesus)

OBJECTIVE

In this study we have compared glucose metabolism and liver gluconeogenesis in wild adult desert gerbil Psammomys obesus fed with their natural halophilic plants and Wistar rats fed on a laboratory chow. Psammomys obesus is a natural model of insulin resistance when fed a rodent laboratory chow.

METHODS

Basal glucose and insulin were determined in plasma of fasting animals. Hepatocyte gluconeogenesis from lactate-plus-pyruvate was investigated in perifused hepatocytes by assessing simultaneously glucose synthesis rate and intracellular oxaloacetate, phosphoenolpyruvate, 3-phosphoglycerate, fructose 6-phosphate and glucose 6-phosphate (G6P) under true steady state conditions.

RESULTS

Fasting blood glucose (2.8 +/- 0.1 vs 4.8 +/- 0.4 mmol.L(- 1)) and plasma insulin concentration (129 +/- 14 vs 150 +/- 21 pmol.L(-1)) were significantly lower in Psammomys as compared to albino rats. Maximal gluconeogenic rate was also lower in Psammomys (2.3 +/- 0.3 vs 5.1 +/- 0.3 micromol x min(-1) x g dry cells(-1)). This effect was related to a slower hydrolysis of G6P.

CONCLUSION

A lower G6P hydrolysis in Psammomys as compared to wistar was the main difference between the two groups of liver cells. Such feature may represent the major metabolic adaptation permitting Psammomys to survive despite its severe restrictive natural conditions. Indeed, a low G6P hydrolysis allows an insulin resistance state, with a high lipogenic activity, but associated with low blood glucose. The rise in blood glucose occurring when Psammomys are fed with exogenous carbohydrates perturbs such delicate metabolic equilibrium, resulting thus in a diabetic state because of the deleterious effect of hyperglycemia.

Role of skeletal muscle on impaired insulin sensitivity in rats fed a sucrose-rich diet: effect of moderate levels of dietary fish oil

In the present study we investigated: (1) the contribution of the skeletal muscle to the mechanisms underlying the impaired glucose homeostasis and insulin sensitivity present in dyslipemic rats fed a sucrose-rich diet (SRD) over a long period of time and (2) the effect of fish oil on these parameters when there was a stable hypertriglyceridemia before the source of fat (corn oil) in the diet was replaced by isocaloric amounts of cod liver oil. Our results show an increased triglyceride content in the gastrocnemius muscle with an impaired capacity for glucose oxidation in the basal state and during euglycemic clamp. This was mainly due to a decrease of the active form of pyruvate dehydrogenase complex (PDHa) and an increase of PDH kinase activities. Hyperglycemia, normoinsulinemia, and diminished peripheral insulin sensitivity also were found. Even though there were no changes in the insulin levels, the former metabolic abnormalities were completely reversed when the source of fat was changed from corn oil to cod liver oil. The data also suggest that in the gastrocnemius muscle of rats fed a SRD over an extended period, an increased availability and oxidation of the lipid fuel, which in turn impairs the glucose oxidation, contributes to the abnormal glucose homeostasis and to the peripheral insulin insensitivity. Moreover, the parallel effect on insulin sensitivity, glucose, and lipid homeostasis attained through the manipulation of dietary fat (n-3) in the SRD suggests a role of n-3 fatty acid in the management of dyslipidemia and insulin resistance.

Insulin activation of pyruvate dehydrogenase complex is enhanced by exercise training

We studied the effects of exercise training on the activity of the pyruvate dehydrogenase (PDH) complex in rat gastrocnemius muscle (experiment 1) and the response of the complex to glucose and insulin infusion (euglycemic clamp) in trained and sedentary rats (experiment 2). In experiment 1, half of the rats were randomly allocated as sedentary animals and the other half were trained by voluntary running exercise for 8 weeks. The total activity of the PDH complex was not affected by exercise training, and the activity state (proportion of the active form) of the PDH complex was decreased from 15.0%+/-2.4% to 7.5%+/-1.1% by exercise training. The activity of 3-hydroxyacyl-coenzyme A (CoA) dehydrogenase ([3-HADH] an enzyme in beta-oxidation) was significantly higher in trained versus sedentary rats. In experiment 2, sedentary and trained rats were starved for 24 hours before performing the euglycemic clamp. Glucose and insulin infusion was performed by a euglycemic clamp (insulin infusion rate, 6 mU/kg/min) for 90 minutes. The PDH complex was inactivated to less than 1% in both sedentary and trained rats after 24 hours of starvation. The glucose infusion rate (GIR) during the euglycemic clamp was higher in trained versus sedentary rats. The euglycemic clamp resulted in activation of the PDH complex in both sedentary and trained rats, but the response of the PDH complex to the euglycemic clamp was significantly higher in trained rats (5.8%+/-0.5%) than in sedentary rats (2.9%+/-0.5%). These results suggest that exercise training promotes fatty acid oxidation in association with suppression of glucose oxidation in skeletal muscle under resting conditions, but increases the rate of carbohydrate oxidation when glucose flux into muscle cells is stimulated by insulin.