Short-term regulation of glycolysis by vasoactive intestinal peptide in epithelial cells isolated from rat small intestine

Suprachiasmatic Nucleus Neuropeptides and Their Control of Endogenous Glucose Production

Defective control of endogenous glucose production is an important factor responsible for hyperglycaemia in the diabetic individual. During the past decade, progressively more evidence has appeared indicating a strong and potentially causal relationship between disturbances of the circadian system and defects of metabolic regulation, including glucose metabolism. The detrimental effects of disturbed circadian rhythms may have their origin in disturbances of the molecular clock mechanisms in peripheral organs, such as the pancreas and liver, or in the central brain clock in the hypothalamic suprachiasmatic nuclei (SCN). To assess the role of SCN output per se on glucose metabolism, we investigated (i) the effect of several SCN neurotransmitters on endogenous glucose production and (ii) the effect of SCN neuronal activity on hepatic and systemic insulin sensitivity. We show that silencing of SCN neuronal activity results in decreased hepatic insulin sensitivity and increased peripheral insulin sensitivity. Furthermore, both oxytocin neurones in the paraventricular nucleus of the hypothalamus (PVN) and orexin neurones in the lateral hypothalamus may be important targets for the SCN control of glucose metabolism. These data further highlight the role of the central clock in the pathophysiology of insulin resistance.

Glucose and glutamine provide similar proportions of energy to mucosal cells of rat small intestine

The objectives of this study were to establish a reliable method for quantifying glycolytic flux in intestinal epithelial cells, to determine the proportion of energy provided to small intestine epithelial cells by glucose vs. glutamine, and to determine whether there was an energetic advantage to having both substrates present simultaneously. There was substantial retention of 3H in alanine and lactate when [2-(3)H]glucose was used as tracer for quantifying glycolysis, and the magnitude of the 3H retention was influenced by the presence of other substrates and metabolites. Detritiation was at least 99% complete, however, when [3-(3)H]glucose was used as tracer in this system and the tritium was recovered as 3H2O. Glycolytic flux was six- to sevenfold higher in cells of the proximal than distal small intestine but was not significantly different for young adult (4 mo) vs. aged adult (24 mo) rats. Net ATP production from exogenous substrates was higher when both glucose and glutamine were present simultaneously than when either substrate was present alone, and glucose was calculated to provide 50-60% of the net ATP produced from these two substrates. Most of the energy produced from glucose was produced via the anaerobic metabolic pathways (78% for glucose alone, 95% with glucose and glutamine). Net energy production was calculated to be 10% lower in cells from aged animals than in those from young animals, since CO2 production from these major substrates was lower in cells from aged animals.

Sepsis increases production of total secreted proteins, vasoactive intestinal peptide, and peptide YY in isolated rat enterocytes

The effect of sepsis on the synthesis of endogenous and secretory proteins, including vasoactive intestinal peptide (VIP) and peptide YY (PYY), was determined in enterocytes from jejunum of rats. Sepsis was induced by cecal ligation and puncture (CLP). Control rats were sham-operated. Total endogenous and secreted protein synthesis was assessed in incubated jejunal enterocytes by measuring incorporation of 3H-phenylalanine into protein. Release of VIP and PYY into the medium of incubated enterocytes and cellular levels of the gut peptides were measured by radioimmunoassay. Sixteen hours after CLP, synthesis rates of both endogenous and secreted proteins were increased, and this effect of sepsis was most pronounced in cells from the lower parts of the villi and crypts. Enterocytes from septic rats released more VIP and PYY into the incubation medium, and approximately half of the peptides they released were newly synthesized VIP and PYY. Intracellular levels of VIP and PYY were increased as early as 4 hours after induction of sepsis. Our results suggest that sepsis stimulates the synthesis of endogenous and secretory proteins, including certain gut peptides, in small intestine mucosa. This is consistent with previous observations of increased circulating levels of VIP, PYY and other gastrointestinal hormones during sepsis. The biological significance of increased synthesis of gut peptides and other intestinal proteins during sepsis remains to be determined.

Fructose 2,6-bisphosphate levels and modulation of glycolysis by histamine, cholecystokinin, and forskolin in isolated rabbit gastric glands

In isolated rabbit gastric glands incubated in the presence of 1 mmol/L glucose, the content of fructose 2,6-bisphosphate (F-2,6-P2) was 5.7 +/- 0.5 pmol/mg dry weight. This value was progressively incremented by increasing glucose concentration in the incubation medium, and was almost doubled at 10 mmol/L glucose. Under these conditions, a close correlation could be established between the levels of F-2,6-P2 and the rate of L-lactate formation (r = .98; P less than .05). Both histamine (0.1 mmol/L) and cholecystokinin octapeptide (CCK-OCT; 0.1 mumol/L) increased L-lactate production, without significant changes in either F-2,6-P2 concentration or the amount of 6-phosphofructo-2-kinase in active form. In contrast, forskolin, which markedly increased the glandular content of cyclic adenosine monophosphate (cAMP), partially blocked glucose consumption and caused a significant reduction in both F-2,6-P2 levels and the proportion of 6-phosphofructo-2-kinase in active form. Furthermore, forskolin partially blocked the rate of glucose uptake by isolated gastric glands. Our results suggest a regulatory role of F-2,6-P2 in the control of the glycolytic flux in response to glucose, but not in its response to histamine or CCK-OCT.

Effect of streptozotocin diabetes on the glycolytic flux and on fructose 2,6-bisphosphate levels in isolated rat enterocytes

In epithelial cells isolated from rat small intestine and incubated in the presence of 1 mM glucose, streptozotocin-induced diabetes reduced, by 46 and 29%, respectively, the rates of both glucose utilization and L-lactate formation. These effects were accompanied by a significant decrease of enterocyte fructose 2,6-bisphosphate concentration (about 50%) and of the glycolytic flux through the reaction catalyzed by 6-phosphofructo 1-kinase. The diminution of enterocyte fructose 2,6-bisphosphate levels caused by diabetes occurred in spite of an increase of hexose 6-phosphate concentration, and was associated with a reduction in the amount of active form of 6-phosphofructo 2-kinase; total activity of this enzyme was not significantly modified. Diabetes also caused an acceleration in the rate of 3-O-methyl-D-(14C) glucose uptake and increased hexokinase activity in enterocytes. Lactate dehydrogenase, pyruvate kinase and 6-phosphofructo 1-kinase activities were not found to be significantly different in epithelial cells isolated from control or diabetic animals. Our results indicate that a reduction of the glycolytic flux in enterocytes could collaborate to increase intestinal glucose absorption in the diabetic state.