Stimulatory effect of endogenous orexin A on gastric emptying and acid secretion independent of gastrin

Orexin A (OXA) increases food intake and inhibits fasting small bowel motility in rats. The aim of this study was to examine the effect of exogenous OXA and endogenous OXA on gastric emptying, acid secretion, glucose metabolism and distribution of orexin immunoreactivity in the stomach. Rats equipped with a gastric fistula were subjected to intravenous (IV) infusion of OXA or the selective orexin-1 receptor (OX1R) antagonist SB-334867-A during saline or pentagastrin infusion. Gastric emptying was studied with a liquid non-nutrient or nutrient, using 51Cr as radioactive marker. Gastric retention was measured after a 20-min infusion of OXA or SB-334867-A. Plasma concentrations of OXA, insulin, glucagon, glucose and gastrin were studied. Immunohistochemistry against OXA, OX1R and gastrin in gastric tissue was performed. OXA alone had no effect on either acid secretion or gastric emptying. SB-334867-A inhibited both basal and pentagastrin-induced gastric acid secretion and increased gastric retention of the liquid nutrient, but not PEG 4000. Plasma gastrin levels were unchanged by IV OXA or SB-334867-A. Plasma OXA levels decreased after intake of the nutrient meal and infusion of the OX1R antagonist. Only weak effects were seen on plasma glucose and insulin by OXA. Immunoreactivity to OXA and OX1R were found in the mucosa, myenteric cells bodies and varicose nerve fibers in ganglia and circular muscle of the stomach. In conclusion, endogenous OXA influences gastric emptying of a nutrient liquid and gastric acid secretion independent of gastrin. This indicates a role for endogenous OXA, not only in metabolic homeostasis, but also in the pre-absorptive processing of nutrients in the gut.     

Effect of peripherally administered ghrelin on gastric emptying and acid secretion in the rat

Ghrelin is a gut peptide that is secreted from the stomach and stimulates food intake. There are ghrelin receptors throughout the gut and intracerebroventricular ghrelin has been shown to increase gastric acid secretion. The aim of the present study was to examine the effects of peripherally administered ghrelin on gastric emptying of a non-nutrient and nutrient liquid, as well as, basal and pentagastrin-stimulated gastric acid secretion in awake rats. In addition, gastric contractility was studied in vitro. Rats equipped with a gastric fistula were subjected to an intravenous infusion of ghrelin (10-500 pmol kg(-1) min(-1)) during saline or pentagastrin (90 pmol kg(-1) min(-1)) infusion. After administration of polyethylene glycol (PEG) 4000 with 51Cr as radioactive marker, or a liquid nutrient with (51)Cr, gastric retention was measured after a 20-min infusion of ghrelin (500 pmol kg(-1) min(-1)). In vitro isometric contractions of segments of rat gastric fundus were studied (10(-9) to 10(-6) M). Ghrelin had no effect on basal acid secretion, but at 500 pmol kg(-1) min(-1) ghrelin significantly decreased pentagastrin-stimulated acid secretion. Ghrelin had no effect on gastric emptying of the nutrient liquid, but significantly increased gastric emptying of the non-nutrient liquid. Ghrelin contracted fundus muscle strips dose-dependently (pD2 of 6.93+/-0.7). Ghrelin IV decreased plasma orexin A concentrations and increased plasma somatostatin concentrations. Plasma gastrin concentrations were unchanged during ghrelin infusion. Thus, ghrelin seems to not only effect food intake but also gastric motor and secretory function indicating a multifunctional role for ghrelin in energy homeostasis.     

Multiple subcutaneous injections of somatostatin induce tachyphylaxis of the suppression of plasma insulin, but not glucagon, in the rat

The time course of pancreatic effects of somatostatin was studied over a period of 2 h in unanesthetized unrestrained rats after administration of the peptide by intravenous infusion and by single and multiple subcutaneous injections. During infusion of 10 and 30 micrograms/kg per min, somatostatin continuously suppressed plasma insulin and plasma glucagon. Plasma glucose was significantly increased at the lower dose, but not affected at the higher dose. Single subcutaneous injections of 0.3 and 3 mg/kg decreased plasma insulin and glucagon dose-dependently for 20-60 min without affecting plasma glucose. Multiple subcutaneous injections of somatostatin (one to four doses of 3 mg/kg, administered at intervals of 30 min) caused an initial decrease of plasma insulin (at 30 min), a rebound-increase at 60 and 90 min, and a final return to control values by 120 min. Plasma glucagon remained continuously suppressed. Plasma glucose increased significantly at 60 and 90 min and tended to return towards control values thereafter. In conclusion, pancreatic B cells - but not A cells - of the rat develop tachyphylaxis to somatostatin within 2 h after multiple subcutaneous injections of the peptide. By this mode of administration, 'selective' suppression of plasma glucagon can be achieved with somatostatin in the rat.     

Localization and effects of orexin on fasting motility in the rat duodenum

The orexins [orexin A (OXA) and orexin B (OXB)] are novel neuropeptides that increase food intake in rodents. The aim of this study was to determine the distribution of orexin and orexin receptors (OX1R and OX2R) in the rat duodenum and examine the effects of intravenous orexin on fasting gut motility. OXA-like immunoreactivity was found in varicose nerve fibers in myenteric and submucosal ganglia, the circular muscle, the mucosa, submucosal and myenteric neurons, and numerous endocrine cells of the mucosa. OXA neurons displayed choline acetyltransferase immunoreactivity, and a subset contained vasoactive intestinal peptide. OXA-containing endocrine cells were identified as enterochromaffin (EC) cells based on the presence of 5-hydroxytryptamine immunoreactivity. OX1R was expressed by neural elements of the gut, and EC cells expressed OX2R. OXA at 100 and 500 pmol x kg(-1) x min(-1) significantly increased the myoelectric motor complex (MMC) cycle length compared with saline. Similarly, OXB increased the MMC cycle length at 100 pmol x kg(-1) x min(-1), but there was no further effect at 500 pmol x kg(-1) x min(-1). We postulate that orexins may affect the MMC through actions on enteric neurotransmission after being released from EC cells and/or enteric neurons.     

Indomethacin does not affect endogenous glucose production in type 2 diabetes mellitus.

In healthy subjects, basal endogenous glucose production is partly regulated by paracrine intrahepatic factors. It is currently unknown whether paracrine intrahepatic factors also influence the increased basal endogenous glucose production in patients with type 2 diabetes mellitus. Administration of indomethacin to patients with type 2 diabetes mellitus stimulates endogenous glucose production and inhibits insulin secretion. Our aim was to evaluate whether this stimulatory effect on glucose production is solely attributable to inhibition of insulin secretion. In order to do this, we administered indomethacin to 5 patients with type 2 diabetes during continuous infusion of somatostatin to block endogenous insulin and glucagon secretion and infusion of basal concentrations of insulin and glucagon in a placebo-controlled study. Endogenous glucose production was measured 3 hours after the start of the somatostatin, insulin and glucagon infusion, for 4 hours after administration of placebo/indomethacin, by primed, continuous infusion of [6,6-(2)H(2)] glucose. At the time of administration of placebo or indomethacin, there were no significant differences in plasma glucose concentrations and endogenous glucose production rates between the two experiments (16.4 +/- 2.09 mmol/l vs. 16.6 +/- 1.34 mmol/l and 17.7 +/- 1.05 micromol/kg/min and 17.0 +/- 1.06 micromol/kg/min), control vs. indomethacin). Plasma glucose concentration did not change significantly in the four hours after indomethacin or placebo administration. Endogenous glucose production in both experiments was similar after both placebo and indomethacin. Mean plasma C-peptide concentrations were all below the detection limit of the assay, reflecting adequate suppression of endogenous insulin secretion by somatostatin. There were no differences in plasma concentrations of insulin (76 +/- 5 vs. 74 +/- 4 pmol/l) and glucagon (69 +/- 8 vs. 71 +/- 6 ng/l) between the studies with levels remaining unchanged in both experiments. Plasma concentrations of cortisol, epinephrine, and norepinephrine were similar in the two studies and did not change significantly. We conclude that indomethacin stimulates endogenous glucose production in patients with type 2 diabetes mellitus by inhibition of insulin secretion.     

Interactions between oxytocin, glucagon and glucose in normal and streptozotocin-induced diabetic rats

Oxytocin has been suggested to have glucoregulatory functions in rats, man and other mammals. The hyperglycemic actions of oxytocin are believed to be mediated indirectly through changes in pancreatic function. The present study examined the interaction between glucose and oxytocin in normal and streptozotocin (STZ)-induced diabetic rats, under basal conditions and after injections of oxytocin. Plasma glucose and endogenous oxytocin levels were significantly correlated in cannulated lactating rats (r = 0.44, P less than 0.01). To test the hypothesis that oxytocin was acting to elevate plasma glucose, adult male rats were injected with 10 micrograms/kg oxytocin and killed 60 min later. Oxytocin increased plasma glucose from 6.1 +/- 0.1 to 6.8 +/- 0.2 mM (P less than 0.05), and glucagon from 179 +/- 12 to 259 +/- 32 pg/ml (P less than 0.01, n = 18). There was no significant effect of oxytocin on plasma insulin, although the levels were increased by 30%. A lower dose (1 microgram/kg) of oxytocin had no significant effect on plasma glucose or glucagon. To eliminate putative local inhibitory effects of insulin on glucagon secretion, male rats were made diabetic by i.p. injection of 100 mg/kg STZ, which increased glucose to greater than 18 mM and glucagon to 249 +/- 25 pg/ml (P less than 0.05). In these rats, 10 micrograms/kg oxytocin failed to further increase plasma glucose, but caused a much greater increase in glucagon (to 828 +/- 248 pg/ml) and also increased plasma ACTH. A specific oxytocin analog, Thr4,Gly7-oxytocin, mimicked the effect of oxytocin on glucagon secretion in diabetic rats. The lower dose of oxytocin also increased glucagon levels (to 1300 +/- 250 pg/ml), but the effect was not significant. A 3 h i.v. infusion of 1 nmol/kg per h oxytocin in conscious male rats significantly increased glucagon levels by 30 min in normal and STZ-rats; levels returned to baseline by 30 min after stopping the infusion. Plasma glucose increased in the normal, but not STZ-rats. The relative magnitude of the increase in glucagon was identical for normal and diabetic rats, but the absolute levels of glucagon during the infusion were twice as high in the diabetics. To test whether hypoglycemia could elevate plasma levels of oxytocin, male rats were injected i.p. with insulin and killed from 15-180 min later. Plasma glucose levels dropped to less than 2.5 mM by 15 min. Oxytocin levels increased by 150-200% at 30 min; however, the effect was not statistically significant.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of exogenous insulin on plasma free carnitine levels during exercise in normal man

Preliminary data from our laboratory have shown that the decrease in plasma free carnitine levels normally found during prolonged exercise is blunted in type 1 diabetic man. This study was designed to test the hypothesis that this might be due to the sustained peripheral hyperinsulinemia seen during exercise in diabetics treated by subcutaneous insulin. Ten male subjects underwent 90 min of cycle ergometry at 60% of their maximal oxygen uptake capacity on two occasions, one with and the other without a constant 0.13 mU.kg-1.min-1 i.v. insulin infusion. Blood samples were taken at rest, during exercise, and after exercise for measurement of plasma glucose, insulin, C-peptide, free fatty acids, and carnitine. Plasma glucose dropped significantly (p less than 0.01) from basal during both infusions, but values at 30, 45, and 60 min of exercise were lower (p less than 0.05) during insulin infusion compared with the saline infusion. Exercise produced a significant (p less than 0.01) fall in plasma insulin in both infusions. However, from 30 to 90 min of exercise, the plateau insulin level was higher during the insulin infusion compared with the saline infusion (91.4 +/- 3.0 vs. 32.9 +/- 3.0 pmol/L; p less than 0.001). Plasma C-peptide decreased significantly (p less than 0.01) during exercise and recovery in both infusions, but values between infusions were not significantly different. Plasma free fatty acids increased significantly (p less than 0.01) at 90 min of exercise during the saline infusion, while during the insulin infusion this was noted during recovery only.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gastric inhibitory polypeptide does not inhibit gastric emptying in humans

The insulinotropic gut hormone gastric inhibitory polypeptide (GIP) has been demonstrated to inhibit gastric acid secretion and was proposed to possess "enterogastrone" activity. GIP effects on gastric emptying have not yet been studied. Fifteen healthy male volunteers (23.9 +/- 3.3 yr, body mass index 23.7 +/- 2.3 kg/m(2)) were studied with the intravenous infusion of GIP (2 pmol.kg(-1).min(-1)) or placebo, each administered to the volunteers on separate occasions from -30 to 360 min in the fasting state. At 0 min, a solid test meal (250 kcal containing [(13)C]sodium octanoate) was served. Gastric emptying was calculated from the (13)CO(2) exhalation rates in breath samples collected over 360 min. Venous blood was drawn in 30-min intervals for the determination of glucose, insulin, C-peptide, and GIP (total and intact). Statistical calculations were made by use of repeated-measures ANOVA and one-way ANOVA. During the infusion, GIP rose to steady-state concentrations of 159 +/- 15 pmol/l for total and 34 +/- 4 pmol/l for intact GIP (P < 0.0001). Meal ingestion further increased GIP concentrations in both groups, reaching peak levels of 265 +/- 20 and 82 +/- 9 pmol/l for total and 67 +/- 7 and 31 +/- 9 pmol/l for intact GIP during the administration of GIP and placebo, respectively (P < 0.0001). There were no differences in glucose, insulin, and C-peptide between the experiments with the infusion of GIP or placebo. Gastric half-emptying times were 120 +/- 9 and 120 +/- 18 min (P = 1.0, with GIP and placebo, respectively). The time pattern of gastric emptying was similar in the two groups (P = 0.98). Endogenous GIP secretion, as derived from the incremental area under the curve of plasma GIP concentrations in the placebo experiments, did not correlate to gastric half-emptying times (r(2) = 0.15, P = 0.15 for intact GIP; r(2) = 0.21, P = 0.086 for total GIP). We conclude that gastric emptying does not appear to be influenced by GIP. The secretion of GIP after meal ingestion is not suppressed by its exogenous administration. The lack of effect of GIP on gastric emptying underlines the differences between GIP and the second incretin glucagon-like peptide 1.     

The glucagon-like peptide-1 metabolite GLP-1-(9-36) amide reduces postprandial glycemia independently of gastric emptying and insulin secretion in humans

Glucagon-like peptide 1 (GLP-1) lowers glycemia by modulating gastric emptying and endocrine pancreatic secretion. Rapidly after its secretion, GLP-1-(7-36) amide is degraded to the metabolite GLP-1-(9-36) amide. The effects of GLP-1-(9-36) amide in humans are less well characterized. Fourteen healthy volunteers were studied with intravenous infusion of GLP-1-(7-36) amide, GLP-1-(9-36) amide, or placebo over 390 min. After 30 min, a solid test meal was served, and gastric emptying was assessed. Blood was drawn for GLP-1 (total and intact), glucose, insulin, C-peptide, and glucagon measurements. Administration of GLP-1-(7-36) amide and GLP-1-(9-36) amide significantly raised total GLP-1 plasma levels. Plasma concentrations of intact GLP-1 increased to 21 +/- 5 pmol/l during the infusion of GLP-1-(7-36) amide but remained unchanged during GLP-1-(9-36) amide infusion [5 +/- 3 pmol/l; P < 0.001 vs. GLP-1-(7-36) amide administration]. GLP-1-(7-36) amide reduced fasting and postprandial glucose concentrations (P < 0.001) and delayed gastric emptying (P < 0.001). The GLP-1 metabolite had no influence on insulin or C-peptide concentrations. Glucagon levels were lowered by GLP-1-(7-36) amide but not by GLP-1-(9-36) amide. However, the postprandial rise in glycemia was reduced significantly (by approximately 6 mg/dl) by GLP-1-(9-36) amide (P < 0.05). In contrast, gastric emptying was completely unaffected by the GLP-1 metabolite. The GLP-1 metabolite lowers postprandial glycemia independently of changes in insulin and glucagon secretion or in the rate of gastric emptying. Most likely, this is because of direct effects on glucose disposal. However, the glucose-lowering potential of GLP-1-(9-36) amide appears to be small compared with that of intact GLP-1-(7-36) amide.     

Both GLP-1 and GIP are insulinotropic at basal and postprandial glucose levels and contribute nearly equally to the incretin effect of a meal in healthy subjects

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are both incretin hormones regulating postprandial insulin secretion. Their relative importance in this respect under normal physiological conditions is unclear, however, and the aim of the present investigation was to evaluate this. Eight healthy male volunteers (mean age: 23 (range 20-25) years; mean body mass index: 22.2 (range 19.3-25.4) kg/m2) participated in studies involving stepwise glucose clamping at fasting plasma glucose levels and at 6 and 7 mmol/l. Physiological amounts of either GIP (1.5 pmol/kg/min), GLP-1(7-36)amide (0.33 pmol/kg/min) or saline were infused for three periods of 30 min at each glucose level, with 1 h "washout" between the infusions. On a separate day, a standard meal test (566 kcal) was performed. During the meal test, peak insulin concentrations were observed after 30 min and amounted to 223+/-27 pmol/l. Glucose+saline infusions induced only minor increases in insulin concentrations. GLP-1 and GIP infusions induced significant and similar increases at fasting glucose levels and at 6 mmol/l. At 7 mmol/l, further increases were seen, with GLP-1 effects exceeding those of GIP. Insulin concentrations at the end of the three infusion periods (60, 150 and 240 min) during the GIP clamp amounted to 53+/-5, 79+/-8 and 113+/-15 pmol/l, respectively. Corresponding results were 47+/-7, 95+/-10 and 171+/-21 pmol/l, respectively, during the GLP-1 clamp. C-peptide responses were similar. Total and intact incretin hormone concentrations during the clamp studies were higher compared to the meal test, but within physiological limits. Glucose infusion alone significantly inhibited glucagon secretion, which was further inhibited by GLP-1 but not by GIP infusion. We conclude that during normal physiological plasma glucose levels, glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide contribute nearly equally to the incretin effect in humans, because their differences in concentration and potency outweigh each other.     

Absence of a memory effect for the insulinotropic action of glucagon-like peptide 1 (GLP-1) in healthy volunteers.

BACKGROUND/AIMS: The term memory effect refers to the phenomenon that B cell stimuli retain some of their insulinotropic effects after they have been removed. Memory effects exist for glucose and sulfonylureas. It is not known whether there is a B-cell memory for incretin hormones such as GLP-1. SUBJECTS/METHODS: Eight healthy young volunteers were studied on four occasions in the fasting state. In one experiment, placebo was administered (a). in three more experiments (random order), synthetic GLP-1 (7 - 36 amide) at 1.2 pmol/kg/min was administered over a period of three hours. At 0 min, a bolus of glucose was injected intravenously (0.33 g/kg body weight). GLP-1 was infused from (b). - 60 to 120 min, (c). - 210 to - 30 min, or (d). - 300 to - 120 min. Glucose (glucose oxidase), insulin, C-peptide, GLP-1, and glucagon (immunoassays) were determined. Statistical analysis was carried out by ANOVA and appropriate post hoc tests. RESULTS: GLP-1 plasma levels during the infusion periods were elevated to 89 +/- 9, 85 +/- 13, and 89 +/- 6 pmol/l (p < 0.0001 vs. placebo, 10 +/- 1 pmol/l). Glucose was eliminated faster (p < 0.0001), with an enhanced negative rebound (p = 0.014), and insulin and C-peptide increments were greater after intravenous glucose administration (p < 0.0001) if GLP-1 was administered during the injection of the glucose bolus, but not if GLP-1 had been administered until 120 or 30 min before the glucose load. There was a trend towards higher insulin concentrations (p = 0.056) five minutes after glucose with GLP-1 administered until - 30 min before the glucose load. Glucagon was suppressed by exogenous glucose, but increased significantly (p = 0.013) during the induction of reactive hypoglycemia after glucose injection during GLP-1 administration. CONCLUSION: 1). No memory effect appears to exist for insulinotropic actions of GLP-1, in line with clinical data. 2). Reactive hypoglycemia causes a prompt rise in glucagon despite pharmacological circulating concentrations of GLP-1. 3). Similar studies should be performed in Type 2-diabetic patients, because exposure to GLP-1 might recruit dormant pancreatic B cells to become glucose-competent, and this might contribute to the overall antidiabetogenic effect of GLP-1 in such patients.     

Effects of ghrelin on insulin and glucagon secretion: a study of isolated pancreatic islets and intact mice

We combined in vitro and in vivo methods to investigate the effects of ghrelin, a novel gastric hormone, on insulin and glucagon release. Studies of isolated mouse islets showed that ghrelin concentrations in the physiological range (0.5-3 nmol l(-1)) had no effect on glucose-stimulated insulin release, while low ghrelin concentrations (1-100 pmol l(-1)) inhibited and high (0.1 and 1 micromol l(-1)) stimulated. The insulin response to glucose was enhanced in the presence of a high ghrelin concentration (100 nmol l(-1)). Glucagon release was stimulated by ghrelin (0.1 pmol l(-1) to 1 micromol l(-1)); this effect was maintained in the presence of glucose (0-20 mmol l(-1)). In intact mice, basal plasma insulin was suppressed by 1 and 10 nmol kg(-1) of ghrelin, 2 and 6 min after i.v. injection. Ghrelin (0.2-10 nmol kg(-1) i.v.) suppressed also the glucose-stimulated insulin response and impaired the glucose tolerance (at a ghrelin dose of 3.3 nmol kg(-1)). Ghrelin (1 or 10 nmol kg(-1) i.v.) inhibited the insulin response to the phospholipase C stimulating agent carbachol and enhanced the insulin response to the phosphodiesterase inhibitor isobutyl-methylxanthine (IBMX) but did not affect the response to the membrane-depolarizing amino acid l-arginine. These observations suggest that the inhibitory effect of ghrelin on glucose-induced insulin release is in part exerted on phospholipase C pathways (and not on Ca(2+)entry), while the stimulatory effect of high doses of ghrelin depends on cyclic AMP. In contrast to the spectacular glucagon-releasing effect of ghrelin in vitro, ghrelin did not raise plasma glucagon. Carbachol, IBMX and l-arginine stimulated glucagon release. These responses were impaired by ghrelin, suggesting that it suppresses the various intracellular pathways (phospholipase C, cyclic AMP and Ca(2+)), that are activated by the glucagon secretagogues. Together these observations highlight (but do not explain) the different effects of ghrelin on glucagon release in vitro and in vivo. The results show that ghrelin has powerful effects on islet cells, suggesting that endogenous ghrelin may contribute to the physiological control of insulin and glucagon release. However, the narrow "window" of circulating ghrelin concentrations makes this doubtful.     

Intravenous infusion of glucagon-like peptide-1 potently inhibits food intake, sham feeding, and gastric emptying in rats

Glucagon-like peptide-1(7-36)-amide (GLP-1) is postulated to act as a hormonal signal from gut to brain to inhibit food intake and gastric emptying. A mixed-nutrient meal produces a 2 to 3-h increase in plasma GLP-1. We determined the effects of intravenous infusions of GLP-1 on food intake, sham feeding, and gastric emptying in rats to assess whether GLP-1 inhibits food intake, in part, by slowing gastric emptying. A 3-h intravenous infusion of GLP-1 (0.5-170 pmol.kg(-1).min(-1)) at dark onset dose-dependently inhibited food intake in rats that were normally fed with a potency (mean effective dose) and efficacy (maximal % inhibition) of 23 pmol.kg(-1).min(-1) and 82%, respectively. Similar total doses of GLP-1 administered over a 15-min period were less potent and effective. In gastric emptying experiments, GLP-1 (1.7-50 pmol.kg(-1).min(-1)) dose-dependently inhibited gastric emptying of saline and ingested chow with potencies of 18 and 6 pmol.kg(-1).min(-1) and maximal inhibitions of 74 and 83%, respectively. In sham-feeding experiments, GLP-1 (5-50 pmol.kg(-1).min(-1)) dose-dependently reduced 15% aqueous sucrose intake in a similar manner when gastric cannulas were closed (real feeding) and open (sham feeding). These results demonstrate that intravenous infusions of GLP-1 dose-dependently inhibit food intake, sham feeding, and gastric emptying with a similar potency and efficacy. Thus GLP-1 may inhibit food intake in part by reducing gastric emptying, yet can also inhibit food intake independently of its action to reduce gastric emptying. It remains to be determined whether intravenous doses of GLP-1 that reproduce postprandial increases in plasma GLP-1 are sufficient to inhibit food intake and gastric emptying.     

Effect of bile salts on the plasma concentration of immunoreactive vasoactive intestinal polypeptide in man

The effects of bile salts on the release of immunoreactive vasoactive intestinal polypeptide (IR-VIP) were investigated in men using a specific radioimmunoassay. Plasma IR-VIP was determined after extraction by the acid-acetone method (recovery 75 +/- 5%). Oral administration of 400 mg sodium taurocholate caused a rise in plasma IR-VIP from 18.5 +/- 1.3 pmol/l to 31.1 +/- 2.1 pmol/l after 30 min and 39.0 +/- 1.7 pmol/l after 60 min and return to the initial value after 120 min. Oral administration of chenodeoxycholic acid (CDCA) also increased plasma IR-VIP from a basal level of 14.5 +/- 1.5 pmol/l to 36.3 +/- 1.2 pmol/l after 60 min. Oral administration of ursodeoxycholic acid (UDCA) increased plasma IR-VIP from 11.9 +/- 1.1 pmol/l to 25.6 +/- 1.8 pmol/l after 30 min. Perifusion of 1 mM taurocholate stimulated release of IR-VIP from human duodenal mucosa into the perifusate. These results suggest that bile salts may participate, at least in part, in the release of IR-VIP from the gut.     

Constant intravenous ghrelin infusion in healthy young men: clinical pharmacokinetics and metabolic effects

Ghrelin levels fluctuate rapidly and dynamically with surges before meal times and postprandial troughs, and ghrelin increases appetite and food intake. Circulating ghrelin correlates negatively with body mass index (BMI), but obese individuals have a reduced postprandial decrease in ghrelin levels. Whether this reflects changes in secretion or clearance of ghrelin is uncertain. We therefore studied the pharmacokinetics of ghrelin in relation to anthropometric and biochemical measures. We also studied the effects of ghrelin on hormones and metabolites. In fasting humans, we used a constant infusion rate of ghrelin lasting 180 min at 5 pmol.kg body wt(-1).min(-1) in a randomized, double-blind, placebo-controlled crossover study. Serum ghrelin (s-ghrelin; total levels) was distributed and eliminated according to a two-compartment model. s-Ghrelin initial half-life was 24 +/- 2 min and terminal half-life 146 +/- 36 min, respectively. Mean residence time (MRT) of ghrelin was 93 +/- 16 min. MRT correlated positively with both BMI (r = 0.51, P < 0.001) and high-density cholesterol (HDL) levels (r = 0.75, P < 0.001). Serum insulin levels remained constant during ghrelin infusion, whereas plasma glucose increased 0.3 +/- 0.1 mmol/l (P < 0.01) and free fatty acid levels more than doubled (to 1.03 +/- 0.08 mmol/l, P < 0.001), translating into a significant reduction of insulin sensitivity (P < 0.001). In conclusion, 1) we describe novel pharmacokinetics of ghrelin that are useful when tailoring ghrelin infusion rates in clinical experiments, 2) BMI and HDL correlate positively with MRT of infused ghrelin, and 3) supraphysiological ghrelin levels impair insulin sensitivity.     

Insulin and glucagon secretion in rats: effects of calcitonin gene-related peptide

Immunoreactive calcitonin gene-related peptide (CGRP) has been shown to occur in intrapancreatic nerves and islet somatostatin cells in the rat. Therefore, we investigated the effects of CGRP on insulin and glucagon secretion in the rat. CGRP was infused i.v. at one of 3 dose levels (4.3, 17 or 68 pmol/min). Infusion of CGRP alone was found to elevate basal plasma levels of both insulin and glucagon. In contrast, CGRP impaired the plasma insulin responses to both glucose (7 mg/min; P less than 0.001) and arginine (8.5 mg/min; P less than 0.001), and inhibited the arginine-induced increase in plasma glucagon concentrations (P less than 0.001). Since CGRP and somatostatin are colocalized within the D-cells, we also infused CGRP and somatostatin together at equimolar dose levels (17 pmol/min), with glucose (7 mg/min). By that, the increase in plasma insulin concentrations decreased more rapidly than during infusion of either peptide alone. Since alpha 2-adrenoceptor activation is known to inhibit glucose-stimulated insulin secretion, we also infused CGRP together with the specific alpha 2-adrenoceptor antagonist yohimbine (37 nmol/min). In that way, the plasma insulin-lowering effect of CGRP was prevented. We have shown in the rat: (1) that CGRP stimulates basal insulin and glucagon secretion; (2) that CGRP inhibits stimulated insulin and glucagon secretion; (3) that CGRP and somatostatin more rapidly induce a potent inhibitory action on glucose-stimulated insulin secretion when given together; and (4) that the alpha 2-adrenoceptor antagonist, yohimbine, counteracts the inhibitory action of CGRP on glucose-stimulated insulin secretion. We suggest that CGRP is of importance for the regulation of insulin and glucagon secretion in the rat. The mechanisms behind the islet effects of CGRP can not be established by the present results, though they apparently require intact alpha 2-adrenoceptors.     

Absorption of intragastrically administered DDAVP in conscious dogs

Plasma concentrations of DDAVP were measured after intragastric administration and intravenous infusion in dogs. Oral ingestion of DDAVP led to a dose dependent increase in peak plasma concentrations as well as area under the curve (AUC). Intravenous infusion of DDAVP (0.13 pmol/l min) resulted in a mean steady-state level of 20.3 pmol/l. Elimination half-lives for oral DDAVP were 77.6 and 76.1 min for low and high doses respectively. T1/2 estimated from the ascending part of the i.v. infusion curve was 50 min. A metabolic clearance rate (MCR) of 3.9 ml/kg . min was assessed from the i.v. steady-state level.     

Basal insulin, glucagon, and growth hormone replacement

Conclusions drawn from the pancreatic (or islet) clamp technique (suppression of endogenous insulin, glucagon, and growth hormone secretion with somatostatin and replacement of basal hormone levels by intravenous infusion) are critically dependent on the biological appropriateness of the selected doses of the replaced hormones. To assess the appropriateness of representative doses we infused saline alone, insulin (initially 0.20 mU.kg(-1).min(-1)) alone, glucagon (1.0 ng.kg(-1).min(-1)) alone, and growth hormone (3.0 ng.kg(-1).min(-1)) alone intravenously for 4 h in 13 healthy individuals. That dose of insulin raised plasma insulin concentrations approximately threefold, suppressed glucose production, and drove plasma glucose concentrations down to subphysiological levels (65 +/- 3 mg/dl, P < 0.0001 vs. saline), resulting in nearly complete suppression of insulin secretion (P < 0.0001) and stimulation of glucagon (P = 0.0059) and epinephrine (P = 0.0009) secretion. An insulin dose of 0.15 mU.kg(-1).min(-1) caused similar effects, but a dose of 0.10 mU.kg(-1).min(-1) did not. The glucagon and growth hormone infusions did not alter plasma glucose levels or those of glucoregulatory factors. Thus, insulin "replacement" doses of 0.20 and even 0.15 mU.kg(-1).min(-1) are excessive, and conclusions drawn from the pancreatic clamp technique using such doses may need to be reassessed.     

Effects of muscarinic blockade on the thermic effect of oral or intravenous carbohydrate.

Muscarinic blockade by atropine has been shown to decrease the thermic effect of a mixed meal, but not of intravenous glucose. To further delineate the mechanisms involved in the atropine-induced inhibition of thermogenesis after a meal, plasma substrate and hormone concentrations, energy expenditure (EE) and substrate oxidation rates were measured before and during a continuous glucose infusion (44.4 mumol.kg-1.min-1) with or without atropine. After 2 h of glucose infusion, a 20-g oral fructose load was administered while the glucose infusion was continued. Plasma insulin concentrations attained a plateau at 596 (SEM 100) pmol.l-1 after 120 min of glucose infusion and were not affected by muscarinic blockade; plasma glucose concentrations peaked at 13.3 (SEM 0.5) mmol.l-1 at 90 min and decreased progressively thereafter; no difference was observed with or without atropine. Plasma free fatty acid and glucagon concentrations, with or without atropine, were both decreased to 201 (SEM 18) mumol.l-1 and 74 (SEM 4) ng.l-1, respectively, after 2 h of glucose infusion, and were not further suppressed after oral fructose. Carbohydrate oxidation rates (CHO(ox)) increased to 20.8 (SEM 1.4) mumol.kg-1.min-1 and lipid oxidation rates (Lox) decreased to 1.5 (SEM 0.3) mumol.kg-1.min-1 between 90 and 120 min after the beginning of glucose infusion and were not affected by atropine. Glucose-induced thermogenesis was similar with [6.5% (SEM 1.4%) of basal EE] or without [6.0% (SEM 1.0%), NS) muscarinic blockade during the 30 min preceding fructose ingestion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glucagon-like peptide 1 (GLP-1) suppresses ghrelin levels in humans via increased insulin secretion

INTRODUCTION: Ghrelin is an orexigenic peptide predominantly secreted by the stomach. Ghrelin plasma levels rise before meal ingestion and sharply decline afterwards, but the mechanisms controlling ghrelin secretion are largely unknown. Since meal ingestion also elicits the secretion of the incretin hormone glucagon-like peptide 1 (GLP-1), we examined whether exogenous GLP-1 administration reduces ghrelin secretion in humans. PATIENTS AND METHODS: 14 healthy male volunteers were given intravenous infusions of GLP-1(1.2 pmol x kg(-1) min(-1)) or placebo over 390 min. After 30 min, a solid test meal was served. Venous blood was drawn frequently for the determination of glucose, insulin, C-peptide, GLP-1 and ghrelin. RESULTS: During the infusion of exogenous GLP-1 and placebo, GLP-1 plasma concentrations reached steady-state levels of 139+/-15 pmol/l and 12+/-2 pmol/l, respectively (p<0.0001). During placebo infusion, ghrelin levels were significantly reduced in the immediate postprandial period (p<0.001), and rose again afterwards. GLP-1 administration prevented the initial postprandial decline in ghrelin levels, possibly as a result of delayed gastric emptying, and significantly reduced ghrelin levels 150 and 360 min after meal ingestion (p<0.05). The patterns of ghrelin concentrations in the experiments with GLP-1 and placebo administration were inversely related to the respective plasma levels of insulin and C-peptide. CONCLUSIONS: GLP-1 reduces the rise in ghrelin levels in the late postprandial period at supraphysiological plasma levels. Most likely, these effects are indirectly mediated through its insulinotropic action. The GLP-1-induced suppression of ghrelin secretion might be involved in its anorexic effects.