In vitro study on the contribution of the rat intestine-pancreas to glucose homeostasis

Hepatic glucose uptake, gluconeogenesis and the regulation of glycogen synthesis

Hepatic glycogen is replenished during the absorptive period postprandially. This repletion is prompted partly by an increased hepatic uptake of glucose by the liver, partly by metabolite and hormonal signals in the portal vein, and partly by an increased gluconeogenic flux to glycogen (glyconeogenesis). There is some evidence that the direct formation of glycogen from glucose and that formed by gluconeogenic pathways is linked. This includes: (i) the inhibition of all glycogen synthesis, in vivo, when gluconeogenic flux is blocked by inhibitors; (ii) a dual relationship between glucose concentrations, lactate uptake by the liver and glycogen synthesis (by both pathways) which indicates that glucose sets the maximal rates of glycogen synthesis while lactate uptake determines the actual flux rate to glycogen; (iii) the decrease of both gluconeogenesis and glycogen synthesis by the biguanide, metformin; and (iv) correlations between increased gluconeogenesis and liver glycogen in obese patients and animal models. The degree to which the liver extracts portal glucose is not entirely agreed upon although a preponderance of evidence points to about a 5% extraction rate, following meals, which is dependent on a stimulation of glucokinase. This enzyme may be linked to the expression of other enzymes in the gluconeogenic pathway. Perivenous cells in the liver may induce additional gluconeogenesis in the periportal cells by increasing glycolytically produced lactate. A number of potential mechanisms therefore exist which could link glycogen synthesis from glucose and gluconeogenic substrate.

A new role for enteric glucagon-37: acute stimulation of glucose absorption in rat small intestine

Glucagon-37 is secreted by intestinal L-cells following carbohydrate uptake. It is known to inhibit gastric acid secretion (hence also named oxyntomodulin) and appears to increase intracellular cyclic AMP concentrations. Since cyclic AMP could enhance intestinal glucose absorption, a possible stimulatory effect of glucagon-37 on glucose transport was examined. Glucagon-37 acutely increased glucose absorption in the isolated, vascularly perfused small intestine and in isolated enterocytes of the rat. In these cells the stimulation by glucagon-37 could be completely blocked by the cAMP antagonist Rp-cAMPS and was therefore mediated by cAMP. The stimulation of intestinal glucose absorption by glucagon-37 appears to be a major new physiological function.

Experimental streptozotocin-reduced diabetes and intestinal glucose metabolism in the rat, in vivo and in vitro

The intestine has a high glycolytic activity, but its metabolic role could be altered in diabetes mellitus. The aim of the present work was to investigate in vivo the glucose retained and the lactate produced by the intestine of normal and diabetic rats and in vitro the effect of different arterial glucose concentrations on glucose utilization and lactate, alanine, and pyruvate production in normal and diabetic rats when the glucose is supplied to the intestine exclusively via the vascular route. In vivo, the normal and diabetic rats retained similar percentages of the arterially supplied glucose (14.7 +/- 2.4, respectively). In vitro, when the preparations were perfused under hyperglycemic conditions, the glucose consumed, as a fraction of the quantity infused, was significantly lower (P < 0.05) in the diabetic (247.0 +/- 22.8 mumol/mmol infused glucose) than in normal (315 +/- 16.3 mumol/mmol infused glucose rats) rats. The lactate produced was significantly higher in diabetic than in normal rats whether the preparations were perfused under isoglycemic (P < 0.01; 1916.4 +/- 124.0 vs vs 1284 +/- 67.7 mumol/mmol consumed glucose) or hyperglycemic (P < 0.05; 1356.4 +/- 199.7 vs 898.0 +/- 87.3 mumol/mmol consumed glucose) conditions. There was significantly (P < 0.05) greater alanine release from the diabetic (123.7 +/- 21.8 mumol/mmol consumed glucose) than from the normal (40.7 +/- 10.3 mumol/mmol consumed glucose) rat preparations perfused under isoglycemic conditions.

Effect of insulin and gliclazide on glucose utilization by a perfused intestine-pancreas preparation isolated from diabetic and non-diabetic rats

The effect of insulin (40 microU/ml) and gliclazide (200 micrograms/ml) on intestinal glucose metabolism was investigated by using an in vitro perfused intestine-pancreas preparation isolated from normal or streptozotocin-diabetic rats. Glucose, lactate and alanine were measured enzymatically in the portal effluent. The glucose retained by the perfused preparation was reduced (P < 0.05) in diabetic versus control rats. The portal lactate levels were not modified, but alanine portal levels were increased (P < 0.05) in diabetic versus control rats. In the diabetic rats, the level of glucose retained by the preparations was increased (P < 0.05) by the presence of insulin, and insulin plus gliclazide in the perfusate. In the presence of insulin and/or gliclazide, the portal lactate levels were not modified, but the alanine levels were reduced (P < 0.05) to normal values. In preparations from non-diabetic rats, the level of glucose retained was increased (P < 0.05) by gliclazide and insulin plus gliclazide, without modification of the portal lactate and alanine levels. In conclusion, the results show that both insulin and gliclazide increased glucose utilization by perfused intestine-pancreas preparations isolated from diabetic rats. The effect was enhanced when both substances were present simultaneously in the perfusion medium.

Substrates and the regulation of hepatic glycogen metabolism

Glycogen metabolism is a complex process which depends on the metabolic circumstances and the hormonal milieu. In this overview an intriguing new possibility has been emphasized--the possible central role of lactate in coordinating, with glucose, the net synthesis of glycogen. Since lactate changes acutely under many physiological circumstances, it would be a logical candidate for a signal which communicates to the liver the metabolic states of the periphery. It would then acutely determine the synthetic rate of glycogen synthesis within the range determined by the glucose concentrations which in turn could be said to reflect the nutritional state of the system. Interestingly, after oral glucose loading, portal glucose levels would be about 25% higher (Radziuk et al., 1978) relative to arterial. As seen from Figs 8 and 9 however the glycogen synthetic rate appears very sensitive to glucose (at a given lactate uptake). Everything else being assumed equal therefore, more glycogen would be synthesized than during intravenous loading with an equivalent peripheral concentration. This is indeed the case (Shulman and Rossetti, 1989). On the other hand, during equivalent loads, peripheral glucose levels are higher and the same quantity of glycogen is synthesized (Radziuk, 1989a, 1989b). If lactate is typical of other glucogenic substrates, then it is also logical that mixed meals with higher levels of portal substrate would maximize glycogen synthetic rates. Similarly, in diabetes where hyperglycemia and hyperlactatemia prevail, gluconeogenesis plays a predominant role in glycogen synthesis (Giaccari and Rossetti, 1992).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of acarbose on the intestinal metabolism of glucose in vitro

The effect of acarbose on the intestinal metabolism of glucose was investigated using an in vitro perfused preparation of the isolated rat small intestine-pancreas. In preparations perfused without intraluminal sucrose administration, the total glucose recovered in the portal effluent and the portal values of lactate, pyruvate and alanine did not depend on whether or not acarbose [1.5 mg/kg body weight (b.w.)] was present in the intestinal lumen. The intestinal glucose and lactate contents were very low at the end of the experiment, and identical with or without acarbose. Insulin and glucagon concentrations remained constant during the whole perfusion period. After intraluminal administration of sucrose a clear increase in portal glucose concentration was observed, which was severely reduced by acarbose administration no changes in portal levels of lactate, pyruvate, alanine, insulin and glucagon were observed. The intestinal content of sucrose at the end of the study was significantly higher in the presence of acarbose (1.5 mg/kg b.w.), while the glucose concentration was low both with and without acarbose (0.20 +/- 0.08 vs 0.29 +/- 0.09 mmol/l respectively). These results suggest that acarbose does not influence the metabolic utilization of the glucose being translocated from the lumen.