Effects of oral vs. i.v. glucose administration on splanchnic and extrasplanchnic O2 uptake and blood flow

Oxygen and Glucose as Stimulation Agents for BOLD Functional MR Imaging of Rabbit Liver: A Feasibility Study

PURPOSE

To assess the feasibility of using oxygen and glucose as stimulating agents in blood-oxygen-level-dependent (BOLD) Functional Magnetic Resonance Imaging (fMRI) of rabbit liver and analyze the impacts by blood flow.

METHODS

Pure oxygen inhalation, intravenous injection and oral administration of glucose were given to 11 New Zealand white rabbits to compare the differences of liver T₂*, aortic flow (AF), portal vein flow (PVF), aortic area (AA) and portal vein area (PVA) before and at 5 min, 10 min, 20 min, 30 min after administrations. AF and PVF were acquired by two dimensional (2D) Phase Contrast MR (2D-PCMR). The impacts of AF and PVF upon BOLD fMRI were analyzed.

RESULTS

AF and PVF declined at 5 min after oxygen inhalation and were significantly different from baseline, then reverted to baseline. No significant difference was observed in liver T₂*, AA and PVA before and after oxygen inhalation. AF, PVF, AA and PVA showed no significant difference before and after glucose intravenous injection, while liver T₂* increased gradually with significant difference. AF and liver T₂* were significantly different before and after glucose oral administration and increased gradually, AA was significantly different before and after glucose administration at 10 min and 20 min. PVF and PVA started to be different from baseline at 10 min. Greatest variation of T₂* (19.6%) was induced by glucose oral administration after 30 min.

CONCLUSION

Rabbit liver T₂* increasing by glucose intravenous injection is possibly associated with glycogen synthesis, provides the possibility to evaluate liver function. Glucose oral administration demonstrated an optimal comparative effect of raising T₂*, however, resulted from the superposition of increased glycogen synthesis and blood flow. Inhalation of pure oxygen didn't alter the rabbit liver T₂*, which may possibly result from an offset between the increased concentration of oxyhemoglobin and decreased blood flow.

Fructose ingestion acutely elevates blood pressure in healthy young humans

Overconsumption of fructose, particularly in the form of soft drinks, is increasingly recognized as a public health concern. The acute cardiovascular responses to ingesting fructose have not, however, been well-studied in humans. In this randomized crossover study, we compared cardiovascular autonomic regulation after ingesting water and drinks containing either glucose or fructose in 15 healthy volunteers (aged 21–33 yr). The total volume of each drink was 500 ml, and the sugar content 60 g. For 30 min before and 2 h after each drink, we recorded beat-to-beat heart rate, arterial blood pressure, and cardiac output. Energy expenditure was determined on a minute-by-minute basis. Ingesting the fructose drink significantly increased blood pressure, heart rate, and cardiac output but not total peripheral resistance. Glucose ingestion resulted in a significantly greater increase in cardiac output than fructose but no change in blood pressure and a concomitant decrease in total peripheral resistance. Ingesting glucose and fructose, but not water, significantly increased blood pressure variability and decreased cardiovagal baroreflex sensitivity. Energy expenditure increased by a similar amount after glucose and fructose ingestion, but fructose elicited a significantly greater increase in respiratory quotient. These results show that ingestion of glucose and fructose drinks is characterized by specific hemodynamic responses. In particular, fructose ingestion elicits an increase in blood pressure that is probably mediated by an increase in cardiac output without compensatory peripheral vasodilatation.

The contribution of visceral adipose tissue to splanchnic cortisol production in healthy humans

Cortisol is regenerated from cortisone by 11beta-hydroxysteroid dehydrogenase type 1 (11HSD1), amplifying glucocorticoid action in adipose tissue and liver. 11HSD1 inhibitors are being developed for type 2 diabetes and may be most effective in obesity, where adipose 11HSD1 is increased. However, the magnitude of regeneration of cortisol in different tissues in humans is unknown, hindering understanding of the pathophysiological and therapeutic importance of 11HSD1. In eight healthy men, we infused 9,11,12,12-(2)H4-cortisol and measured tracer enrichment in the hepatic vein as an indicator of total splanchnic cortisol generation. Oral cortisone (25 mg) was then given to measure first-pass hepatic cortisol generation. In steady state, splanchnic cortisol production was 45 +/- 11 nmol/min when arterialized plasma cortisone concentration was 92 +/- 7 nmol/l. Extrapolation from hepatic cortisol generation after oral cortisone suggested that, at steady state, the liver contributes 15.2 nmol/min and extrahepatic splanchnic tissue contributes 29.8 nmol/min to the total splanchnic cortisol production. We conclude that tissues draining into the portal vein, including visceral adipose tissue, contribute substantially to the regeneration of cortisol. Thus, in addition to free fatty acids and adipokines, the portal vein delivers cortisol to the liver, and inhibition of 11HSD1 in visceral adipose tissue may indeed be valuable in ameliorating insulin resistance in obesity.

Effects of insulin on adipose tissue blood flow in man

Adipose tissue blood flow (ATBF) rises after nutrient ingestion. It is not clear whether this is due to insulin. The aim of this study was to investigate the role of insulin in the regulation of subcutaneous ATBF. We have investigated the role of insulin in the regulation of ATBF in normal, healthy subjects in a three-step procedure to determine the functional level at which insulin may potentially exert its effect. Fifteen subjects were studied on two occasions. On the first visit, 75 g oral glucose was given. In the second, similar plasma concentrations of insulin and glucose were achieved by dynamic intravenous infusions of insulin and glucose. The increase in ATBF after oral glucose (4.2 +/- 1.4 ml min(-1) (100 g tissue)(-1), P = 0.01) was significantly greater (P < 0.05) than that after intravenous infusions (1.5 +/- 0.6 ml min(-1) (100 g tissue)(-1) P < 0.05). For the local delivery of potentially vasoactive substances and simultaneous measurement of ATBF, we describe a novel combination of methods, which we have called 'microinfusion'. We have used this technique to show that locally infused insulin, even at pharmacological concentrations, had no demonstrable effect on ATBF in nine subjects. We conclude that whilst insulin does not have a direct effect on ATBF, it is likely to be an important mediator, possibly acting via sympathetic activation. In the postprandial state, other candidate peptides and hormones are also likely to play important roles.

Thermogenesis induced by intravenous infusion of hypertonic solutions in the rat

1. Intravenous administration of 20-60 % glucose, 3.2-9.7 % NaCl or 20 % mannitol solutions (1.66 ml kg(-1)) for 5 min increased oxygen consumption in urethane-anaesthetized rats, whereas administration of physiological saline had no effect. Administration of 7.7-18.3 % urea slightly increased the oxygen consumption, but the increase was significantly smaller than that measured after the administration of other hypertonic solutions. The magnitude of the thermogenic effect correlated with the osmolality of the applied solutions. These results suggest that the thermogenesis was caused mainly by changes in osmolality rather than by a specific action of the different solute molecules. 2. Neither pretreatment with the ganglion blocker hexamethonium (20 mg kg(-1), I.P.) or the beta-adrenergic antagonist propranolol (10 mg kg(-1), I.P.), nor bilateral cervical vagotomy or bilateral adrenalectomy had any effect on the osmotically induced thermogenesis. Therefore, the autonomic nervous system and the adrenal gland were not involved in this metabolic response. 3. In response to osmotic stimulation, the temperature of the skeletal muscle increased significantly, whereas that of brown adipose tissue did not change and that of the colon and liver decreased. Accordingly, the site of osmotic thermogenesis is probably in the skeletal muscle, although osmotic stimulation was not accompanied by electromyographic activity and was not blocked by pretreatment with muscle relaxants such as dantrolene sodium or pancuronium bromide, or with the Na(+)-Cl(-) co-transport inhibitor bumetanide. 4. The increases in plasma osmolality observed after the administration of 20 % (1.3 osmol kg(-1)) glucose and 4.1 % (1.3 osmol kg(-1)) NaCl were 4.50 +/- 0.88 and 5.57 +/- 0.71 mosmol kg(-1), respectively. Since the slight increase in osmolality is well within the physiological range of changes that occur after food ingestion, diet-induced thermogenesis may have a component that is mediated by an increase in plasma osmolality, which results from the prandial increase in circulating nutrients.

Metabolic handling of intraduodenal vs. intravenous glucose in humans

To determine whether the route of glucose administration affects whole body glucose metabolism, 14 healthy volunteers were randomly infused with intraduodenal (id) or intravenous (iv) glucose at 6 mg x kg(-1) x min(-1) for 180 min. Infused glucose was labeled with [2-(3)H]glucose in a first series of paired experiments designed to characterize kinetic parameters to be used in a second series of experiments in which [3-(3)H]- and [U-(14)C]glucose labeling was used to characterize the metabolic fate of infused glucose. Experiments with [2-(3)H]glucose showed that, after a lag period of only 20 min, id absorption averaged 105 +/- 3% of infusion. During the final hour of id and iv infusion of [3-(3)H]glucose, tissue uptake averaged 98 +/- 3 and 107 +/- 4% of infusion, respectively, and was equally divided between glycolysis ((3)H(2)O production) and storage (uptake-glycolysis). Glucose oxidation ((14)CO(2)), total carbohydrate oxidation (indirect calorimetry), and net carbohydrate balance were also similar, but the thermic effect of glucose was significantly greater after id infusion. Because insulin and estimated portal vein glucose levels were similar during the final 80 min of both infusions, our results suggest that hepatic glucose storage (and therefore muscle storage estimated as whole body minus liver storage) is not affected by the route of glucose administration.

Thermogenesis induced by osmotic stimulation of the intestines in the rat

Infusion of 5-20% glucose, 1.8-3.6% NaCl, 20% methylglucose, 20% fructose, or 5-10% solutions of various amino acids (10 ml x kg(-1)) into the duodenum induced dose-dependent thermogenesis in urethane-anaesthetized rats. In contrast, infusion of 0.9% NaCl, distilled water, or safflower oil had no effect on the metabolic rate. Infusion of 7.2% urea induced a small and transient increase in the metabolic rate. These results suggested that the thermogenesis was caused mainly by changes in osmolality rather than by a specific action of the different solute molecules. The respiratory exchange ratio increased after the infusion of glucose, fructose, glycine, or serine, did not change after the infusion of NaCl, methylglucose, safflower oil, or distilled water, and decreased after infusion of arginine. Therefore, there was no relationship between substrate utilization and the occurrence of thermogenesis. Intestinal infusion of 3.6% NaCl elevated the plasma osmolality, with a plateau increase of approximately 20 mosmol x kg(-1). However, intravenous infusion of the same amount of NaCl induced a significantly smaller thermogenic response, although it elevated the plasma osmolality with a time course and magnitude similar to those obtained after the intestinal infusion. Infusion of NaCl into the hepatic portal vein or the peritoneal cavity also produced a significantly small thermogenic response. These results suggested an intestinal or mesenteric location for osmoreceptors. To test for possible stimulation of intestinal osmoreceptors after intake of a normal meal, we measured the osmolality of the intestinal contents. The osmolality of the duodeno-jejunal contents was 600-800 mosmol kg-1, whereas the plasma osmolality was 306 +/- 1 mosmol x kg(-1), which suggests that the intestinal osmoreceptors are stimulated after meals and are involved in diet-induced thermogenesis.