Long-term treatment with the dipeptidyl peptidase IV inhibitor P32/98 causes sustained improvements in glucose tolerance, insulin sensitivity, hyperinsulinemia, and beta-cell glucose responsiveness in VDF (fa/fa) Zucker rats

The incretins, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are responsible for >50% of nutrient-stimulated insulin secretion. After being released into the circulation, GIP and GLP-1 are rapidly inactivated by the circulating enzyme dipeptidyl peptidase IV (DP IV). The use of DP IV inhibitors to enhance these insulinotropic hormonal axes has proven effective on an acute scale in both animals and humans; however, the long-term effects of these compounds have yet to be determined. Therefore, we carried out the following study: two groups of fa/fa Zucker rats (n = 6 each) were treated twice daily for 3 months with the DP IV inhibitor P32/98 (20 mg.kg(-1).day(-1), p.o.). Monthly oral glucose tolerance tests (OGTTs), performed after drug washout, revealed a progressive and sustained improvement in glucose tolerance in the treated animals. After 12 weeks of treatment, peak OGTT blood glucose values in the treated animals averaged 8.5 mmol/l less than in the controls (12.0 +/- 0.7 vs. 20.5 +/- 1.3 mmol/l, respectively). Concomitant insulin determinations showed an increased early-phase insulin response in the treated group (43% increase). Furthermore, in response to an 8.8 mmol/l glucose perfusion, pancreata from controls showed no increase in insulin secretion, whereas pancreata from treated animals exhibited a 3.2-fold rise in insulin secretion, indicating enhanced beta-cell glucose responsiveness. Also, both basal and insulin-stimulated glucose uptake were increased in soleus muscle strips from the treated group (by 20 and 50%, respectively), providing direct evidence for an improvement in peripheral insulin sensitivity. In summary, long-term DP IV inhibitor treatment was shown to cause sustained improvements in glucose tolerance, insulinemia, beta-cell glucose responsiveness, and peripheral insulin sensitivity, novel effects that provide further support for the use of DP IV inhibitors in the treatment of diabetes.

Effect of glucagon-like peptide 1 on non-insulin-mediated glucose uptake in the elderly patient with diabetes

An important cause of elevated glucose levels in elderly patients with diabetes is an alteration in non-insulin-mediated glucose uptake (NIMGU). Glucagon-like peptide 1 (GLP-1) is an intestinal insulinotropic hormone. It has been proposed that this hormone also lowers glucose levels by enhancing NIMGU. This study was conducted to determine whether GLP-1 augments NIMGU in elderly patients with diabetes, a group in which NIMGU is known to be impaired. Studies were conducted on 10 elderly patients with type 2 diabetes (aged 75 +/- 2 years, BMI 27 +/- 1 kg/m(2)) who underwent paired 240-min glucose clamp studies. In each study, octreotide was infused to suppress endogenous insulin release, and tritiated glucose methodology was used to measure glucose production and disposal rates. For the first 180 min, no glucose was infused. From 180 to 240 min, glucose was increased to 11 mmol/l using the glucose clamp protocol. In the GLP-1 study, GLP-1 was infused from 30 to 240 min. In a subsequent control study, insulin was infused using the glucose clamp protocol from 30 to 240 min to match the insulin levels that occurred during the GLP-1 infusion study. During hyperglycemia, GLP-1 enhanced glucose disposal (control study: 2.52 +/- 0.19 mg x kg(-1) x min(-1); GLP-1 study: 2.90 +/- 0.17 mg x kg(-1) x min(-1); P < 0.0001). Hepatic glucose output was not different between studies. We conclude that GLP-1 may partially reverse the defect in NIMGU that occurs in elderly patients with diabetes.

Glucagon-like peptide-1 (7-37) augments insulin-mediated glucose uptake in elderly patients with diabetes

BACKGROUND

Glucagon-like peptide-1 (GLP-1) is an intestinal insulinotropic hormone that augments glucose-induced insulin secretion in patients with type 2 diabetes. It has also been proposed that a substantial component of the glucose-lowering effects of GLP-1 occurs because this hormone enhances insulin-mediated glucose disposal. However, interpretations of the studies have been controversial. This study determines the effect of GLP-1 on insulin-mediated glucose disposal in elderly patients with type 2 diabetes.

METHODS

Studies were conducted on 8 elderly patients with type 2 diabetes (age range, 76 +/- 1 years; body mass index, 28 +/- 1 kg/m(2)). Each subject underwent two 180-minute euglycemic (insulin infusion rate, 40 mU/m(2)/min) insulin clamps in random order. Glucose production (Ra) and disposal (Rd) rates were measured using tritiated glucose methodology. In one study, glucose and insulin alone were infused. In the other study, a primed-continuous infusion of GLP-1 was administered at a final rate of 1.5 pmol x kg(-1) x min(-1) from 30 to 180 minutes.

RESULTS

Glucose values were similar between the control and GLP-1 infusion studies. 120- to 180-minute insulin values appeared to be higher during the GLP-1 infusion study (control, 795 +/- 63 pmol/l; GLP-1, 1140 +/- 275 pmol/l; p = not significant [NS]). The higher insulin values were largely due to 2 subjects who had substantial insulin responses to GLP-1 despite euglycemia and hyperinsulinemia. The 120- to 180-minute insulin values were similar in the other 6 subjects (control, 746 +/- 35 pmol/l; GLP-1, 781 +/- 41 pmol/l; p = NS). Basal (control, 2.08 +/- 0.05 mg/kg/min; GLP-1, 2.13 +/- 0.04 mg/kg/min; p = NS) and 120- to 180-minute (control, 0.50 +/- 0.18 mg/kg/min; GLP-1, 0.45 +/- 0.14 mg/kg/min; p = NS) Ra was similar between studies. The 120- to 180-minute Rd values were higher during the GLP-1 infusion studies (control, 4.73 +/- 0.39 mg/kg/min; GLP-1, 5.52 +/- 0.43 mg/kg/min; p <.01). When the 2 subjects who had significant insulin responses to GLP-1 during the euglycemic clamp were excluded, the 120- to 180-minute Rd values were still higher in the GLP-1 infusion study (control, 5.22 +/- 0.32 mg/kg/min; GLP-1, 6.05 +/- 0.37 mg/kg/min; p <.05).

CONCLUSIONS

We conclude that GLP-1 may enhance insulin sensitivity in elderly patients with diabetes.

Effect of aging and diabetes on the enteroinsular axis

BACKGROUND

The current studies were designed to examine the effect of aging and diabetes on the enteroinsular axis.

METHODS

Healthy young control subjects (n = 10 young; age 23 +/- 1 years; body mass index [BMI] 24 +/- 1 kg/m(2)), healthy elderly subjects (n = 10; age 80 +/- 2 years; BMI 26 +/- 1 kg/m(2)), and elderly patients with type 2 diabetes (n = 10; age 76 +/- 2 years; BMI 26 +/- 2 kg/m(2)) underwent a 3-hour oral glucose tolerance test (glucose dose 40 gm/m(2)).

RESULTS

Insulin responses were not different between young controls and elderly patients with diabetes but were significantly lower in elderly patients with diabetes and young controls than in elderly controls (young control: 178 +/- 27 pM; elderly control: 355 +/- 57 pM; elderly diabetes: 177 +/- 30 pM; p <.05 elderly control vs young control and elderly diabetes). Total glucagon-like peptide 1 (GLP-1) responses were not significantly different between young and elderly controls and patients with diabetes (young control: 15 +/- 2 pM; old control: 8 +/- 2 pM; elderly diabetes: 12 +/- 3 pM; p = ns). Active GLP-1 responses were also not different between young and elderly controls and patients with diabetes (young control: 5 +/- 1 pM; old control: 6 +/- 1 pM; elderly diabetes: 7 +/- 1 pM; p = ns). However, the difference between total and active GLP levels was significantly greater in the young controls (young control: 10 +/- 2 pM; old control: 2 +/- 2 pM; elderly diabetes: 4 +/- 2 pM; p <.05, young vs elderly). Glucose-dependent insulinotropic polypeptide responses were not different between young and elderly controls and between elderly controls and patients with diabetes but were significantly higher in elderly patients with diabetes than in young controls (young control: 97 +/- 12 pM; elderly control: 121 +/- 16 pM; elderly diabetes: 173 +/- 27 pM; p <.05, young vs elderly diabetes). Glucagon responses were reduced in elderly controls but were similar in young controls and elderly patients with diabetes (young control: 15 +/- 1 pM; elderly control: 9 +/- 1 pM; elderly diabetes: 16 +/- 1 pM; p <.01 elderly control vs young control and elderly diabetes). Dipeptidyl peptidase IV levels were lower in both elderly controls and patients with diabetes when compared with young controls (young control: 0.17 +/- 0.01; elderly control: 0.15 +/- 0.01; elderly diabetes: 0.15 +/- 0.01 DeltaOD/20 minutes; p <.05, elderly vs young).

CONCLUSIONS

We conclude that normal aging and diabetes are associated with multiple changes in the enteroinsular axis.

A new pathway for glucose-dependent insulinotropic polypeptide (GIP) receptor signaling: evidence for the involvement of phospholipase A2 in GIP-stimulated insulin secretion

The hormone glucose-dependent insulinotropic polypeptide (GIP) is an important regulator of insulin secretion. GIP has been shown to increase adenylyl cyclase activity, elevate intracellular Ca(2+) levels, and stimulate a mitogen-activated protein kinase pathway in the pancreatic beta-cell. In the current study we demonstrate a role for arachidonic acid in GIP-mediated signal transduction. Static incubations revealed that both GIP (100 nm) and ATP (5 microm) significantly increased [(3)H]arachidonic acid ([(3)H]AA) efflux from transfected Chinese hamster ovary K1 cells expressing the GIP receptor (basal, 128 +/- 11 cpm/well; GIP, 212 +/- 32 cpm/well; ATP, 263 +/- 35 cpm/well; n = 4; p < 0.05). In addition, GIP receptors were shown for the first time to be capable of functionally coupling to AA production through Gbetagamma dimers in Chinese hamster ovary K1 cells. In a beta-cell model (betaTC-3), GIP was found to elicit [(3)H]AA release, independent of glucose, in a concentration-dependent manner (EC(50) value of 1.4 +/- 0.62 nm; n = 3). Although GIP did not potentiate insulin release under extracellular Ca(2+)-free conditions, it was still capable of elevating intracellular cAMP and stimulating [(3)H]AA release. Our data suggest that cAMP is the proximal signaling intermediate responsible for GIP-stimulated AA release. Finally, stimulation of GIP-mediated AA production was shown to be mediated via a Ca(2+)-independent phospholipase A(2). Arachidonic acid is therefore a new component of GIP-mediated signal transduction in the beta-cell.

Identification of a bioactive domain in the amino-terminus of glucose-dependent insulinotropic polypeptide (GIP)

The incretins are a class of hormones released from the small bowel that act on the endocrine pancreas to potentiate insulin secretion in a glucose-dependent manner. Due to the requirement for an elevated glucose concentration for activity, the incretins, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1, have potential in the treatment of non-insulin-dependent diabetes mellitus. A series of synthetic peptide GIP fragments was generated for the purpose of elucidating the bioactive domain of the molecule. Peptides were screened for stimulation of cyclic AMP (cAMP) accumulation in Chinese hamster ovary cells transfected with the rat islet GIP receptor. Of the GIP fragments tested, GIP(1-14) and GIP(19-30) demonstrated the greatest cAMP-stimulating ability over the range of concentrations tested (up to 20 microM). In contrast, GIP fragments corresponding to amino acids 15-42, 15-30, 16-30 and 17-30 all demonstrated weak antagonism of GIP(1-42) activity. Competitive-binding displacement studies indicated that these peptides were low-affinity ligands for the GIP receptor. To examine biological activity in vivo, a bioassay was developed in the anesthetized rat. Intravenous infusion of GIP(1-42) (1 pmol/min/100 g) with a concurrent intraperitoneal glucose load (1 g/kg) significantly reduced circulating blood glucose excursions through stimulation of insulin release. Higher doses of GIP(1-14) and GIP(19-30) (100 pmol/min/100 g) also reduced blood glucose excursions.

Defective glucose-dependent insulinotropic polypeptide receptor expression in diabetic fatty Zucker rats

Glucose-dependent insulinotropic polypeptide (GIP) is a peptide hormone that is released postprandially from the small intestine and acts in concert with glucagon-like peptide (GLP)-1 to potentiate glucose-induced insulin secretion from the pancreatic beta-cell. In type 2 diabetes, there is a decreased responsiveness of the pancreas to GIP; however, the insulin response to GLP-1 remains intact. The literature suggests that the ineffectiveness of GIP in type 2 diabetes may be a result of chronic homologous desensitization of the GIP receptor. Yet, there has been no conclusive evidence suggesting that GIP levels are elevated in diabetes. The hypothesis of the present study is that one cause of decreased responsiveness to GIP in type 2 diabetes is an inappropriate expression of the GIP receptor in the pancreatic islet. This hypothesis was tested using a strain of diabetic fatty Zucker rats. The obese rats displayed basal GIP levels similar to the control animals; however, they were unresponsive to a GIP infusion (4 pmol.min(-1). kg(-1)), whereas the lean animals displayed a significant reduction in blood glucose (GIP levels, 50% control after 60 min, P < 0.05) as well as a significant increase in circulating insulin. GIP also potently stimulated first-phase insulin secretion from isolated perifused islets (10.3 +/- 3.0 x basal), and GIP and GLP-1 potentiated insulin secretion from the perfused pancreas (6 x control area under the curve [AUC]) from lean animals. GIP yielded no significant effect in the Vancouver diabetic fatty Zucker (VDF) rat pancreases, whereas GLP-1 elicited an eightfold increase of insulin secretion from the perfused VDF pancreas. Islets from lean animals subjected to static incubations with GIP showed a 2.2-fold increase in cAMP, whereas GIP failed to increase islet cAMP in the VDF islets. Finally, the expression of both GIP receptor mRNA and protein was decreased in islets from VDF rats. These data suggest that the decreased effectiveness of GIP in the VDF rat and in type 2 diabetes may be a result of a decreased receptor expression in the islet.

Metabolism of glucagon by dipeptidyl peptidase IV (CD26)

Glucagon is a 29-amino acid polypeptide released from pancreatic islet alpha-cells that acts to maintain euglycemia by stimulating hepatic glycogenolysis and gluconeogenesis. Despite its importance, there remains controversy about the mechanisms responsible for glucagon clearance in the body. In the current study, enzymatic metabolism of glucagon was assessed using sensitive mass spectrometric techniques to identify the molecular products. Incubation of glucagon with purified porcine dipeptidyl peptidase IV (DP IV) yielded sequential production of glucagon(3-29) and glucagon(5-29). In human serum, degradation to glucagon(3-29) was rapidly followed by N-terminal cyclization of glucagon, preventing further DP IV-mediated hydrolysis. Bioassay of glucagon, following incubation with purified DP IV or normal rat serum demonstrated a significant loss of hyperglycemic activity, while a similar incubation in DP IV-deficient rat serum did not show any loss of glucagon bioactivity. Degradation, monitored by mass spectrometry and bioassay, was blocked by the specific DP IV inhibitor, isoleucyl thiazolidine. These results identify DP IV as a primary enzyme involved in the degradation and inactivation of glucagon. These findings have important implications for the determination of glucagon levels in human plasma.

Role of glucose in chronic desensitization of isolated rat islets and mouse insulinoma (betaTC-3) cells to glucose-dependent insulinotropic polypeptide

It is well documented that the release of insulin from isolated perifused islets attenuates over time, despite a continued glucose stimulation. In the current study we have shown that potentiation of insulin release by the intestinal hormone glucose-dependent insulinotropic polypeptide (GIP) is also attenuated after its continuous application. In less than 20 h of maintained stimulus with either hyperglycaemia (11.0 mM glucose) or GIP (10 nM) under hyperglycaemic conditions, insulin release returned to basal values. This was not due to loss of islet viability or reduction in the releasable pool of insulin granules, as 1 mM isobutylmethylxanthine was able to stimulate equivalent insulin release under both conditions. Further examination of chronic GIP desensitization was examined in cultured mouse insulinoma (betaTC-3) cells. GIP-stimulated cAMP production was not greatly affected by the prevailing glucose conditions, suggesting that the glucose dependence of GIP-stimulated insulin release occurs distally to the increase in intracellular cAMP in betaTC-3 cells. The GIP-stimulated cAMP response curve after desensitization was of similar magnitude at all glucose concentrations, but GIP pretreatment did not affect forskolin-stimulated cAMP production. Desensitization of the cAMP response in betaTC-3 cells was shown not to involve induction of dipeptidyl peptidase IV or pertussis toxin-sensitive G-proteins, activation of protein kinase C or protein kinase A, or modulation of phosphodiesterase activity. Homologous desensitization of the insulin-potentiating activity of GIP was found to affect both GIP-stimulated and forskolin-stimulated insulin release, indicating desensitization of distal steps in the stimulus-exocytosis cascade.

Effect of ageing and diabetes on glucose-dependent insulinotropic polypeptide and dipeptidyl peptidase IV responses to oral glucose

AIMS

Glucose-dependent insulinotropic polypeptide (GIP) acts on the pancreas to potentiate glucose-induced insulin secretion (enteroinsular axis). GIP is rapidly inactivated in vivo by the enzyme dipeptidyl dipeptidase IV (DPP-IV). The current studies were designed to examine the effect of ageing, obesity and diabetes on GIP and DPP-IV responses to oral glucose.

METHODS

Healthy controls (nine middle-aged, age 42 +/- 2 years, body mass index (BMI) 33 +/- 1 kg/m2; nine elderly, age 71 +/- 1 years, BMI 30 +/- 1 kg/m2) and patients with Type 2 diabetes (12 middle-aged, age 44 +/- 2 years, BMI 34 +/- 2 kg/m2; 19 elderly, age 74 +/- 1 years, BMI 31 +/- 1 kg/m2) underwent a 3-h oral glucose tolerance test (OGTT) (glucose dose 40 g/m2).

RESULTS

Insulin responses were similar in elderly controls and patients with diabetes, but were lower in middle-aged patients with diabetes than in controls (308 +/- 65 vs. 640 +/- 109 pM, P < 0.05). GIP responses were similar in controls and patients with diabetes in each age group, but were higher in elderly controls (middle-aged 45 +/- 13; elderly 112 +/- 13 pM, P < 0.01) and patients with diabetes (middle-aged 55 +/- 10; elderly 99 +/- 10 pM, P < 0.01). DPP-IV levels were lower in patients with diabetes in both middle-aged (control 0.241 +/- 0.015; diabetes 0.179 +/- 0.017 delta OD/20 min, P < 0.05) and elderly groups (control 0.223 +/- 0.019; diabetes 0.173 +/- 0.010 delta OD/20 min, P < 0.05).

CONCLUSIONS

It was concluded that ageing in obese subjects is associated with enhanced GIP responses to oral glucose. In addition, DPP-IV activity is reduced in middle-aged and elderly obese patients with diabetes.

Dipeptidyl peptidase IV (DPIV/CD26) degradation of glucagon. Characterization of glucagon degradation products and DPIV-resistant analogs

Over the past decade, numerous studies have been targeted at defining structure-activity relationships of glucagon. Recently, we have found that glucagon(1-29) is hydrolyzed by dipeptidyl peptidase IV (DPIV) to produce glucagon(3-29) and glucagon(5-29); in human serum, [pyroglutamyl (pGlu)(3)]glucagon(3-29) is formed from glucagon(3-29), and this prevents further hydrolysis of glucagon by DPIV (H.-U. Demuth, K. Glund, U. Heiser, J. Pospisilik, S. Hinke, T. Hoffmann, F. Rosche, D. Schlenzig, M. Wermann, C. McIntosh, and R. Pederson, manuscript in preparation). In the current study, the biological activity of these peptides was examined in vitro. The amino-terminally truncated peptides all behaved as partial agonists in cyclic AMP stimulation assays, with Chinese hamster ovary K1 cells overexpressing the human glucagon receptor (potency: glucagon(1-29) > [pGlu(3)]glu- cagon(3-29) > glucagon(3-29) > glucagon(5-29) > [Glu(9)]glu- cagon(2-29)). In competition binding experiments, [pGlu(3)]glucagon(3-29) and glucagon(5-29) both demonstrated 5-fold lower affinity for the receptor than glucagon(1-29), whereas glucagon(3-29) exhibited 18-fold lower affinity. Of the peptides tested, only glucagon(5-29) showed antagonist activity, and this was weak compared with the classical glucagon antagonist, [Glu(9)]glucagon(2-29). Hence, DPIV hydrolysis of glucagon yields low affinity agonists of the glucagon receptor. As a corollary to evidence indicating that DPIV degrades glucagon (Demuth, et al., manuscript in preparation), DPIV-resistant analogs were synthesized. Matrix-assisted laser desorption/ionization-time of flight mass spectrometry was used to assess DPIV resistance, and it allowed kinetic analysis of degradation. Of several analogs generated, only [D-Ser(2)] and [Gly(2)]glucagon retained high affinity binding and biological potency, similar to native glucagon in vitro. [D-Ser(2)]Glucagon exhibited enhanced hyperglycemic activity in a bioassay, whereas [Gly(2)]glucagon was not completely resistant to DPIV degradation.

Characterization of the carboxyl-terminal domain of the rat glucose-dependent insulinotropic polypeptide (GIP) receptor. A role for serines 426 and 427 in regulating the rate of internalization

Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal hormone involved in the regulation of insulin secretion. In non-insulin-dependent diabetes mellitus insulin responses to GIP are blunted, possibly due to altered signal transduction or reduced receptor number. Site-directed mutagenesis was used to construct truncated GIP receptors to study the importance of the carboxyl-terminal tail (CT) in binding, signaling, and receptor internalization. Receptors truncated at amino acids 425, 418, and 405, expressed in COS-7 or CHO-K1 cells, exhibited similar binding to wild type receptors. GIP-dependent cAMP production with the 405 mutant was decreased in COS-7 cells. Maximal cAMP production in CHO-K1 cells was reduced with all truncated forms. Binding was undetectable with a receptor truncated at amino acid 400; increasing tail length by adding 5 alanines restored binding and signaling. Mutants produced by alanine scanning of residues 394-401, adjacent to transmembrane domain 7, were all functional. CT truncation by 30 or more amino acids, mutation of serines 426/427, singly or combined, or complete CT serine knockout all reduced receptor internalization rate. The majority of the GIP receptor CT is therefore not required for signaling, a minimum chain length of approximately 405 amino acids is needed for receptor expression, and serines 426 and 427 are important for regulating rate of receptor internalization.

Improved glucose tolerance in rats treated with the dipeptidyl peptidase IV (CD26) inhibitor Ile-thiazolidide

The incretins glucose-dependent insulinotropic polypeptide (GIP1-42) and truncated forms of glucagon-like peptide-1 (GLP-1) are hormones released from the gut in response to ingested nutrients, which act on the pancreas to potentiate glucose-induced insulin secretion. These hormones are rapidly inactivated by the circulating enzyme dipeptidyl peptidase IV ([DPIV] CD26). This study describes the effect on glucose tolerance and insulin secretion of inhibiting endogenous DPIV in the rat using Ile-thiazolidide, a specific DPIV inhibitor. High-performance liquid chromatography (HPLC) analysis of plasma following in vivo administration of 125I-labeled peptides showed that inhibition of DPIV by about 70% prevented the degradation of 90.0% of injected 125I-GLP-17-36 after 5 minutes, while only 13.4% remained unhydrolyzed in rats not treated with the DPIV-inhibiting agent after only 2 minutes. Ile-thiazolidide treatment also increased the circulating half-life of intact GLP-17-36 released in response to intraduodenal (ID) glucose (as measured by N-terminal specific radioimmunoassay [RIA]). In addition, inhibition of DPIV in vivo resulted in an earlier increase and peak of plasma insulin and a more rapid clearance of blood glucose in response to ID glucose challenge. When considered with the HPLC data, these results suggest that the altered insulin profile is an incretin-mediated response. DPIV inhibition resulting in improved glucose tolerance may have therapeutic potential for the management of type 2 diabetes mellitus.

Glucose-dependent insulinotropic polypeptide stimulation of lipolysis in differentiated 3T3-L1 cells: wortmannin-sensitive inhibition by insulin

GIP is an important insulinotropic hormone (incretin) that has also been implicated in fat metabolism. There is controversy regarding the actions of GIP on adipocytes. In the current study, the existence of GIP receptors and effects of GIP on lipolysis were studied in differentiated 3T3-L1 cells. GIP receptor messenger RNA was detected by RT-PCR and RNase protection assay. Receptors were detected in binding studies (IC50 26.7 +/- 0.7 nM). GIP stimulated glycerol release with an EC50 of 3.28 +/- 0.63 nM. GIP (10(-9)-10(-7) M) +/- IBMX increased cAMP production by 1180-2246%. The adenylyl cyclase inhibitor MDL 12330A (10(-4) M) inhibited GIP-induced glycerol production by >90%, and reduced cAMP responses to basal. Preincubation of 3T3-L1 cells with insulin inhibited glycerol responses to GIP, and the inhibitory effect of insulin was blocked by the phosphatidylinositol 3'-kinase inhibitor, wortmannin. It is concluded that GIP stimulates glycerol release in 3T3-L1 cells primarily via stimulation of cAMP production, and that insulin antagonizes GIP-induced lipolysis in a wortmannin-sensitive fashion. It is suggested that effects of GIP on fat metabolism in vivo may depend upon the circulating insulin level, and that meal-released GIP may elevate circulating fatty acids, thus optimizing pancreatic beta-cell responsiveness to stimulation by glucose and GIP.

Improved glucose tolerance in Zucker fatty rats by oral administration of the dipeptidyl peptidase IV inhibitor isoleucine thiazolidide

The hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide (GLP)-1 act on the pancreas to potentiate glucose-induced insulin secretion (enteroinsular axis). These hormones (incretins) are rapidly hydrolyzed by the circulating enzyme dipeptidyl peptidase IV (DP IV) into biologically inactive NH2-terminally truncated fragments. This study describes the effect of inhibiting endogenous DP IV with a specific DP IV inhibitor, isoleucine thiazolidide (Ile-thiazolidide), on glucose tolerance and insulin secretion in the obese Zucker rat. In initial studies, the specificity of Ile-thiazolidide as an inhibitor of incretin degradation was determined using matrix-assisted laser desorption/ionization-time of flight mass spectrometry. These results showed that inhibiting DP IV activity with Ile-thiazolidide blocked the formation of NH2-terminally truncated GIP and GLP-1. Oral administration of Ile-thiazolidide resulted in rapid inhibition of circulating DP IV levels by 65% in obese and lean Zucker rats. Suppression of DP IV levels enhanced insulin secretion in both phenotypes with the most dramatic effect occurring in obese animals (150% increase in integrated insulin response vs. 27% increase in lean animals). Ile-thiazolidide treatment improved glucose tolerance in both phenotypes and restored glucose tolerance to near-normal levels in obese animals. This was attributed to the glucose-lowering actions of increasing the circulating half-lives of the endogenously released incretins GIP and, particularly, GLP-1. This study suggests that drug manipulation of plasma incretin activity by inhibiting the enzyme DP IV is a valid therapeutic approach for lowering glucose levels in NIDDM and other disorders involving glucose intolerance.

Enhanced glucose-dependent insulinotropic polypeptide secretion and insulinotropic action in glucagon-like peptide 1 receptor -/- mice

Incretins are gastrointestinal hormones that act on the pancreas to potentiate glucose-stimulated insulin secretion. Despite the physiological importance of the enteroinsular axis, disruption of glucagon-like peptide (GLP)-1 action is associated with only modest glucose intolerance in GLP-1 receptor -/- (GLP-1R -/-) mice. We show here that GLP-1R -/- mice exhibit compensatory changes in the enteroinsular axis via increased glucose-dependent insulinotropic polypeptide (GIP) secretion and enhanced GIP action. Serum GIP levels in GLP-1R -/- mice were significantly elevated versus those in +/+ control mice after an oral glucose tolerance test (369 +/- 40 vs. 236 +/- 28 pmol/l; P < or = 0.02). Furthermore, GIP perfusion of mice pancreas and isolated islets in the presence of elevated glucose concentrations elicited a significantly greater insulin response in GLP-1R -/- than in +/+ mice (P < or = 0.02-0.05). In contrast, no significant perturbation in the insulin response to perfused glucagon was detected under conditions of low (4.4 mmol/l) or high (16.6 mmol/l) glucose in GLP-1R -/- mice. Total pancreatic insulin but not glucagon content was significantly reduced in GLP-1R -/- compared with in +/+ mice (77 +/- 9 vs. 121 +/- 10 pmol/mg protein; P < or = 0.005). These observations suggest that upregulation of the GIP component of the enteroinsular axis, at the levels of GIP secretion and action, modifies the phenotype resulting from interruption of the insulinotropic activity of GLP-1 in vivo.

The Xenopus proglucagon gene encodes novel GLP-1-like peptides with insulinotropic properties

The proglucagon gene encodes several hormones that have key roles in the regulation of metabolism. In particular, glucagon-like peptide (GLP-1), a potent stimulus of insulin secretion, is being developed as a therapy for the treatment of non-insulin-dependent diabetes mellitus. To define structural moieties of the molecule that convey its insulinotropic activity, we have cloned and characterized the proglucagon gene from the amphibian, Xenopus laevis. Unexpectedly, these cDNAs were found to encode three unique glucagon-like-1 peptides, termed xenGLP-1A, xenGLP-1B, and xenGLP-1C in addition to the typical proglucagon-derived hormones glucagon and GLP-2. xenGLP-1A, -1B, and -1C were synthesized and tested for their ability to bind and activate the human GLP-1 receptor (hGLP-1R), and to stimulate insulin release from rat pancreas. All three Xenopus GLP-1-like peptides bind effectively to the hGLP-1R and stimulate cAMP production. Surprisingly, xenGLP-1B(1-30) demonstrated higher affinity for the hGLP-1R than hGLP-1 (IC50 of 1.1 +/- 0.4 nM vs. 4.4 +/- 1.0 nM, respectively, P < 0.02) and was equipotent to hGLP-1 in stimulating cAMP production (EC50 of 0.17 +/- 0.02 nM vs. 0.6 +/- 0. 2 nM, respectively, P > 0.05). Further studies demonstrated that hGLP-1, xenGLP-1A, -1B, and -1C stimulate comparable insulin release from the pancreas. These results demonstrate that despite an average of nine amino acid differences between the predicted Xenopus GLPs and hGLP-1, all act as hGLP-1R agonists.

Localization of the domains involved in ligand binding and activation of the glucose-dependent insulinotropic polypeptide receptor

The receptors for the two structurally related insulinotropic hormones Glucose-dependent Insulinotropic Polypeptide (GIP) and Glucagon-Like Peptide-1 (GLP-1) share approximately 40% sequence identity and demonstrate complete specificity for their endogenous ligands, while utilizing similar second messenger pathways. In the current study chimeric GIP-GLP-1 receptors were prepared, and the effect of domain-exchange on ligand binding and adenylyl cyclase activation examined. A chimera (CH-2) consisting of the first 132 amino acids of the external N-terminal (NT) domain bound 125I-GIP with high affinity (27.77 +/- 11.85 nM). However, for receptor coupling to cAMP production it was necessary to extend the NT into the first transmembrane (TM-1) region (CH-3: IC50 = 9.04 +/- 1.07 nM; EC50 = 17.1 +/- 3.5 nM). A chimera which included part of TM-3 (CH-4) demonstrated binding and signalling (IC50 = 8.33 +/- 0.14 nM; EC50 = 467.5 +/- 173.6 pM) similar to the wild type receptor (IC50 = 1.33 +/- 0.19 nM; EC50 = 497.9 +/- 211.7 pM). Surprisingly constructs CH-2 and CH-3, while devoid of detectable 125I-GLP-1 binding, were capable of eliciting GLP-1-specific cAMP production (EC50s CH-2 = 81.4 +/- 19.6 nM; CH-3 = 5.99 +/- 0.68 nM) suggesting that receptor activation is not completely dependent on high affinity receptor binding. These data clearly demonstrate that the NT domain of the GIP receptor acts as the ligand-specific binding domain and that the first transmembrane domain is important for receptor activation.

GIP(6-30amide) contains the high affinity binding region of GIP and is a potent inhibitor of GIP1-42 action in vitro

GIP (Glucose-dependent Insulinotropic Polypeptide) is an important regulator of insulin secretion. The effects of truncated forms of the peptide, GIP(10-30), GIP(6-30amide) and GIP(7-30), on binding of 125I-GIP(1-42) to GIP receptors in transfected CHO-KI cells, and on cyclic AMP responses to GIP(1-42), have been studied with a view to defining further the receptor binding region of GIP, and to establish whether such truncated peptides exhibit agonist or antagonist activity. All three peptides were found to be receptor antagonists, however GIP(6-30amide) exhibited receptor binding affinity equivalent to that of GIP(1-42) in competitive binding studies (IC50 = 3.08+/-0.57 nM). GIP(6-30amide) inhibited GIP(1-42)-induced cAMP production by 58% at a concentration of 100 nM, whereas GIP(10-30) and GIP(7-30), inhibited only in the microM range. GIP(6-30amide) therefore contains the high affinity binding region of GIP and is a potent inhibitor of GIP(1-42) action in vitro.

The enteroinsular axis in dipeptidyl peptidase IV-negative rats

Evidence has accumulated that the incretins glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1(7-36) amide) are degraded and rendered biologically inactive in plasma by the enzyme dipeptidyl peptidase IV (DPIV). A strain of Fischer rats lacking the DPIV enzyme were used in the current investigation as a model for examining the enteroinsular axis under conditions in which normal inactivation of GIP and GLP-1(7-36) does not occur. This was assessed by comparing GIP and GLP-1(7-36) responses following oral glucose in normal versus DPIV-deficient Fischer rats, and by comparing the insulinotropic potency of both peptides in the perfused pancreas of both groups. The insulin response to an oral glucose challenge was decreased slightly in DPIV-negative rats compared with control animals. Of the two incretins, the GIP response to oral glucose was reduced by 50% compared with controls, whereas GLP-1(7-36) release in response to glucose was unchanged. A decrease of 30% in the sensitivity of the perfused pancreas of DPIV-negative rats to GIP was observed, whereas the insulin response to GLP(7-36) was identical in both groups. Incubation of both peptides in plasma from DPIV-positive and -negative rats was performed to determine the effect of the presence or absence of DPIV on the insulinotropic activity of GLP-1(7-36) and GIP in the isolated perfused rat pancreas. Incubation in plasma from DPIV-positive rats resulted in a 65% decrease in insulinotropic activity of both incretins compared with incubation in plasma from DPIV-deficient rats. It was hypothesized that the reduced GIP response and decreased sensitivity of the pancreas to GIP are compensatory mechanisms that maintain insulin and glucose levels within a normal range despite abnormal degradation of GIP. An explanation of the lack of effect of the absence of DPIV on the GLP-1(7-36) response to oral glucose and insulinotropic action of this peptide must await further study.

Investigation of glucose-dependent insulinotropic polypeptide-(1-42) and glucagon-like peptide-1-(7-36) degradation in vitro by dipeptidyl peptidase IV using matrix-assisted laser desorption/ionization-time of flight mass spectrometry. A novel kinetic approach

The incretins glucose-dependent insulinotropic polypeptide (GIP1-42) and glucagon-like peptide-1-(7-36)-amide (GLP-17-36), hormones that potentiate glucose-induced insulin secretion from the endocrine pancreas, are substrates of the circulating exopeptidase dipeptidyl peptidase IV and are rendered biologically inactive upon cleavage of their N-terminal dipeptides. This study was designed to determine if matrix-assisted laser desorption/ionization-time of flight mass spectrometry is a useful analytical tool to study the hydrolysis of these hormones by dipeptidyl peptidase IV, including kinetic analysis. Spectra indicated that serum-incubated peptides were cleaved by this enzyme with only minor secondary degradation due to other serum protease activity. Quantification of the mass spectrometric signals allowed kinetic constants for both porcine kidney- and human serum dipeptidyl peptidase IV-catalyzed incretin hydrolysis to be calculated. The binding constants (Km) of these incretins to purified porcine kidney-derived enzyme were 1.8 +/- 0.3 and 3.8 +/- 0.3 microM, whereas the binding constants observed in human serum were 39 +/- 29 and 13 +/- 9 microM for glucose-dependent-insulinotropic polypeptide and glucagon-like peptide-1-(7-36)-amide respectively. The large range of Km values found in human serum suggests a heterogeneous pool of enzyme. The close correlation between the reported kinetic constants and those previously described validates this novel approach to kinetic analysis.

Effects of low and high dose administration of bis(maltolato)oxovanadium(IV) on fa/fa Zucker rats

The fatty Zucker rat, characterized by obesity, hyperinsulinemia, hyperlipidemia, and mild hyperglycemia, has been suggested as an animal model of non-insulin-dependent diabetes mellitus. The present study examined the chronic dose-dependent effect of bis(maltolato)oxovanadium(IV), a potent insulin mimetic, in this animal model of diabetes. Chronic (6 weeks) oral administration of bis(maltolato)oxovanadium(IV) (0.06 mmol.kg-1.day-1, low dose study) was effective in reducing the hyperinsulinemia associated with the fatty Zucker rat model (termination insulin: lean, 82.8 +/- 21.6; fatty, 732 +/- 89.4; fatty treated, 336 +/- 126.6 pmol/L; p < 0.05). Pancreatic perfusion data indicated a significant improvement in insulin secretory function in the fatty rats. The dose dependency of this relationship was observed in the high dose study (0.128 mmol.kg-1.day-1 for 14 weeks), wherein bis(maltolato)oxovanadium(IV) treatment restored plasma insulin levels in the fatty rats to lean levels (termination insulin: lean, 199.2 +/- 17.4; fatty 660.6 +/- 12.6; fatty treated, 153.6 +/- 9.6 pmol/L; p < 0.05) and significantly improved insulin response to a glucose challenge. In addition, bis(maltolato)oxovanadium(IV) treatment (high dose study) ameliorated the age-dependent increase in blood pressure observed in fatty Zucker rats (systolic blood pressure: lean, 127 +/- 10; fatty, 176 +/- 5; fatty treated, 156 +/- 9 mmHg (1 mmHg = 133.3 Pa)). These data indicate that chronic oral administration of bis(maltolato)oxovanadium(IV) in the drinking water was effective in reducing hyperinsulinemia, insulin resistance, glucose intolerance, and hypertension in the fatty Zucker rat.

Isolation of a murine glucose-dependent insulinotropic polypeptide (GIP) cDNA from a tumor cell line (STC6-14) and quantification of glucose-induced increases in GIP mRNA

A 537 base pair cDNA clone for murine GIP has been isolated. The elucidated sequence encodes an open reading frame of 432 base pairs which codes for a 144 amino acid precursor. Murine GIP is predicted to differ from the human hormone by three amino acid substitutions: arginine for histidine at position 18, arginine for lysine at position 30 and serine for lysine at position 34. GIP mRNA levels in STC6-14 cells incubated in the presence of varying glucose concentrations was investigated using a competitive-PCR method. In the presence of a 5-mM glucose stimulus, 1 x 10(5) GIP cells were found to contain 3.9 +/- 0.59 amol of GIP mRNA while the same number of cells contained 11.6 +/- 1.4 amol when subjected to a high (25 mM) glucose stimulus.

The insulinotropic region of gastric inhibitory polypeptide; fragment analysis suggests the bioactive site lies between residues 19 and 30

Glucose-dependent insulinotropic polypeptide or gastric inhibitory polypeptide (GIP) is a 42 amino acid intestinal hormone, which exhibits several direct and indirect effects on fat and glucose metabolism. To determine the bioactive region(s) of the molecule, synthetic and proteolytic fragments of the hormone were generated and tested for their ability to induce a biological response in the isolated, perfused rat pancreas and stomach. A synthetic fragment corresponding to porcine GIP residues 1-30 retained strong insulinotropic activity in the isolated, perfused rat pancreas but greatly reduced somatostatinotropic activity in the isolated perfused rat stomach. A synthetic fragment corresponding to porcine GIP residues 15 to 30 was biologically inactive. However, enterokinase treatment of the synthetic 15-30 fragment restored partial insulinotropic activity in the isolated, perfused rat pancreas. The hypothesis that the restoration of biological activity was due to the enzymatic removal of the amino-terminal dipeptide (Asp-Lys) of GIP15-30 was supported by the observation that a synthetic fragment lacking these two residues was also insulinotropic. Further fractionation of the molecule generated a biologically active 19-30 fragment, suggesting that the residues necessary for the insulin response are contained within this region.

Functional expression of the rat pancreatic islet glucose-dependent insulinotropic polypeptide receptor: ligand binding and intracellular signaling properties

Incretins are endogenous peptides released from the gastrointestinal tract into the circulation during a meal that potentiate glucose-stimulated insulin secretion. At present, there are two established incretins: glucose-dependent insulinotropic polypeptide (GIP) and the truncated glucagon-like peptides (tGLPs), which are now being investigated for use in the treatment of diabetes mellitus. In the present study we cloned a rat islet GIP receptor complementary DNA (GIP-R1) to answer several important questions regarding the ligand-binding and intracellular signaling properties of the GP receptor. GIP-R1, when expressed transiently in monkey kidney (COS-7) or stably in Chinese hamster ovary (CHO-K1) cells, demonstrated comparable high affinity binding for either synthetic porcine (sp) GIP or synthetic human (sh) GIP. The IC50 values for displacement of [125I]spGIP in CHO-K1 cells were 2.6 +/- 0.8 and 3.1 +/- 0.9 nM for two different preparations of shGIP, and 3.7 +/- 1.5 and 3.6 +/- 0.4 nM for two preparations of spGIP. Saturation isotherms obtained with both intact cells and membranes gave monophasic binding curves with apparent Kd values of 204 +/- 17 and 334 +/- 94 pM, respectively. Cells expressed 12-15 x 10(3) receptors/cell. In COS-7 cells, spGIP and shGIP also exhibited similar IC50 values (7.6 +/- 1.2 and 8.9 +/- 1.8 nM, respectively). The receptor in CHO-K1 cells bound GIP-(1-30) with lower affinity (IC50 = 39 +/- 17 nM), whereas the fragments GIP-(19-30), GIP-(18-28), and GIP-(21-26) showed no apparent binding. The specificity of the receptor was further examined using several structurally related peptides. Surprisingly, exendin-(9-39) [Ex-(9-39)], a GLP-1 receptor antagonist, and Ex-4-(1-39), a GLP-1 receptor agonist, demonstrated some affinity for the GIP receptor, with 39% and 21% displacement of [125I]spGIP, respectively, at 1 microM. Other members of the secretin/vasoactive intestinal peptide family of peptides tested showed no interaction. GIP-R1 receptor binding correlated with activation of the adenylyl cyclase system, whereby spGIP and shGIP evoked concentration-dependent increases in cAMP accumulation with EC50 values of 8.7 +/- 1.5 x 10(-10)M and 8.1 +/- 1.6 x 10(-10)M for spGIP and shGIP, respectively. Increases in cAMP in the presence of 10 nM spGIP were not dependent on the ambient glucose concentration, with 22- and 18-fold increases in cAMP accumulation at 0.1 and 5.5 mM glucose, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Gastric inhibitory polypeptide release from a tumor-derived cell line

A cell line derived from intestinal tumors of transgenic mice (STC-1) was subcloned to produce a stable line with approximately 30% immunoreactive gastric inhibitory polypeptide (irGIP)-containing cells (STC 6-14). High-performance liquid chromatography (HPLC) of STC 6-14 extracts indicated that the tumor cell-derived irGIP had the same retention time as synthetic porcine GIP-(1-42) (pGIP). Approximately 30% of the cells also contained immunoreactive somatostatin (irSS), which eluted as a single peak on HPLC, corresponding with SS-(1-14). On average, each well of extracted cells (5.0 x 10(5) cultured 4 days) contained 33.3 +/- 1.4 ng irGIP and 18.4 +/- 1.5 ng irSS. Basal release of irGIP in the presence of 5 mM glucose was 733 +/- 58 pg.ml cells-1.2h-1 (2.20 +/- 0.17% of total cell content; TCC) and doubled at 20 mM glucose (4.20 +/- 0.42% TCC). The response to glucose was augmented by addition of a SS neutralizing antibody (SOMA-10) and suppressed by 10 nM SS. Basal release of irSS in 5 mM glucose was 377 +/- 35 pg.ml cells-1.2h-1 (2.05 +/- 0.19% TCC) and was increased by glucose (> or = 15 mM) and the addition of pGIP (> or = 1 nM). The STC 6-14 cell line represents a model to study the synthesis, storage, and release of GIP and SS in a controlled environment.

Degradation of glucose-dependent insulinotropic polypeptide and truncated glucagon-like peptide 1 in vitro and in vivo by dipeptidyl peptidase IV

The combined actions of glucose-dependent insulinotropic polypeptide (GIP) and truncated glucagon-like peptide-1 (tGLP-1) may fully account for the incretin effect. These hormones are released from the small intestine in response to oral glucose and stimulate insulin release. Recently, evidence has been provided demonstrating the degradation of GIP-(1-42) and GLP-1-(7-36)NH2 by the serum enzyme dipeptidyl peptidase IV (DPP IV) into the biologically inactive products GIP-(3-42) and GLP-1-(9-36)NH2. The objective of the current investigation was to develop a method to monitor the degradation of these hormones in vivo. Synthetic peptides were radiolabeled and purified by HPLC. Subsequent degradation of the peptides under various conditions was then monitored by further HPLC analysis. Incubation of [125I]GIP-(1-42) or [125I]GLP-1-(7-36)NH2 with Wistar rat serum or purified DPP IV resulted in the major N-terminal-truncated products [125I]GIP-(3-42) and [125I]GLP-1-(9-36)NH2. These products were significantly reduced when the specific DPP IV inhibitor diprotin A was included in the incubation mixture and were absent when serum from DPP IV-deficient rats was used. When the labeled peptides were infused into rats at hormone levels within the physiological range, over 50% was metabolized to the truncated forms within 2 min. These products were absent when the tracers were infused into DPP IV-deficient animals. It is concluded that DPP IV may be a primary inactivating enzyme of both GIP and tGLP-1 in vivo. As the N-terminal-truncated products of the DPP IV cleavage may not be distinguished from the biologically active hormone by currently employed assays, reports of circulating hormone levels should be reconsidered. The method described in this manuscript may be useful for investigating the durations of action of GIP and tGLP-1 in normal and pathophysiological conditions.

Influence of a perpetual-daylight Arctic environment on periodicity in human cholesterol synthesis

To identify factors associated with control of human cholesterol synthesis, periodicity in cholesterogenesis, hormonal levels and food consumption behavior were examined in 5 healthy individuals at the beginning and end of 18 d in a perpetual daylight Arctic environment devoid of time cues. At d 2 (phase I) and d 16 (phase II), cholesterol fractional synthesis rate was determined at 6 h intervals over 30 h as deuterium incorporation into plasma free cholesterol. Total plasma cholesterol, insulin and glucose-dependent insulinotropic polypeptide were also measured at each timepoint. Food intake and sleeping patterns were recorded prior to and during each phase. Cholesterol fractional synthesis rate (FSR) exhibited periodicity in all subjects on each phase, but did not differ between phase I (FSR rate = 0.038 +/- 0.038 pools.d-1) and phase II (FSR rate = 0.037 +/- 0.072 pools.d-1) phases. Phase II FSR period length was associated with both the duration between first and last meals (r2 = 0.81, p = 0.037) and total hours spent awake (r2 = 0.99, p = 0.001). Insulin and glucose-dependent insulinotropic polypeptide levels were not associated with FSR periodicity. These results suggest that meal timing and sleep/wake cycles are more important factors than insulin and glucose-dependent insulinotropic polypeptide in controlling the rhythms of whole body cholesterol synthesis.

Altered glucose dependence of glucagon-like peptide I(7-36)-induced insulin secretion from the Zucker (fa/fa) rat pancreas

In previous studies on the enteroinsular axis in Zucker rats, it was found that glucose-dependent insulinotropic polypeptide (GIP) levels were normal in obese animals, but the glucose threshold for the insulinotropic action of GIP in the perfused rat pancreas was reduced. Glucagon-like peptide I (GLP-I)(7-36) is also an important incretin, and in the current study, glucose, insulin, and immunoreactive (IR)-COOH-terminal GLP-I responses to oral glucose were compared in lean (Fa/?) and obese (fa/fa) rats. In addition, the concentration thresholds for stimulation and glucose dependence of perfused pancreases to GLP-I(7-36) were examined. Glucose responses to oral glucose were similar in fa/fa and Fa/? rats. Obese animals were hyperinsulinemic when fasting and after oral glucose. Significant increases in IR-GLP-I levels in response to glucose were only observed in fa/fa rats. Perfused pancreases from fa/fa rats hypersecreted insulin at all glucose concentrations. In the presence of 4.4 mmol/l glucose, GLP-I(7-36) increased insulin secretion in fa/fa pancreases approximately 25-fold, whereas there was only a 5-fold increase in Fa/? pancreases. Pancreases from fa/fa rats, perfused with a glucose gradient (2.8-11 mmol/l) in the presence of GLP-I(7-36), responded with an immediate increase in insulin secretion, i.e., at a glucose concentration of 2.8 mmol/l, whereas Fa/? pancreases required a minimum of 4.22 mmol/l glucose for stimulation. With high glucose (16.7 mmol/l), both fa/fa and Fa/? rat pancreases exhibited similar responsiveness to GLP-I(7-36), having thresholds of < 50 pmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of glucose-dependent insulinotropic polypeptide and glucagon-like peptide-I-(7-36) on insulin secretion

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide (GLP)-I-(7-36) are probably the most important "incretins," but there is controversy as to their relative insulinotropic activities. The effects of natural (np) and synthetic porcine (sp) GIP, synthetic human (sh) GIP, and GLP-I-(7-36) on insulin secretion from the perfused rat pancreas were compared using gradient perfusion. Insulin secretion was increased by both spGIP and GLP-I-(7-36) at concentrations of approximately 16 pM. Maximal responses to GLP-I-(7-36) in the presence of 16.7 mM glucose were slightly greater than with npGIP or spGIP, but with 10 mM glucose spGIP and GLP-I-(7-36) exerted equivalent effects. Responses to shGIP were greatly reduced compared with spGIP. In the presence of 50 pM spGIP or GLP-I-(7-36) the glucose threshold was 4.5 +/- 0.11 mM. The data indicate that GLP-I-(7-36) and porcine GIP are equally insulinotropic and share the same glucose threshold for activity, whereas shGIP is less active. At the concentrations found postprandially, however, GIP is likely to be the more important incretin.

Meal frequency influences circulating hormone levels but not lipogenesis rates in humans

To determine whether human lipogenesis is influenced by the frequency of meal consumption, 12 subjects were divided into two groups and fed isocaloric nutritionally adequate liquid diets over 3 days, either as three larger diurnal (n = 6) or as six small, evenly spaced (n = 6) meals per day. On day 2 (08:00 h) of each diet period, 0.7 g deuterium (D) oxide/kg body water was administered and blood was collected every 4 hours over 48 hours for measurement of plasma insulin and glucose-dependent insulinotropic polypeptide (GIP) levels. At each time point, the incorporation of D into plasma triglyceride fatty acid (TG-FA) was also determined by isotope ratio mass spectrometry after TG-FA extraction and combustion/reduction. Insulin and GIP levels were elevated over daytime periods in subjects fed three versus six meals per day. Contribution of de novo synthesis to total TG-FA production was not significantly different for days 2 and 3 in subjects consuming three (6.56% +/- 1.32% and 6.64% +/- 2.08%, respectively) and six (7.67% +/- 2.29% and 7.88% +/- 1.46%, respectively) meals per day. Net TG-FA synthesis rates over days 2 and 3 were 1.47 +/- 0.33 and 1.55 +/- 0.53 g/d, respectively, for subjects fed three meals per day, and 1.64 +/- 0.47 and 1.69 +/- 0.30 g/d for subjects fed six meals per day. These findings suggest that consuming fewer but larger daily meals is not accompanied by increases in TG-FA synthesis, despite the observation of hormonal peaks.

Release of gastric inhibitory polypeptide from cultured canine endocrine cells

Canine intestinal duodenal and jejunal epithelial cell preparations enriched for endocrine cells were obtained by sequential collagenase digestion and centrifugal elutriation and maintained in culture for a 40-h period. Adherent cells contained a total cell content (TCC) of 11.5 +/- 2.5 ng (mean +/- SE) immunoreactive gastric inhibitory peptide (IRGIP)/well and 1.4 +/- 0.2 ng immunoreactive somatostatin (IRS)/well. Release experiments were performed by incubation of the cells with various stimuli over a 2-h period. Basal release of IRGIP in 5 mM glucose-5 mM K+ was 2.7 +/- 0.4% TCC. Incubation with concentrations of K+ > 20 mM or glucose > 15 mM significantly increased IRGIP release, as did the addition of a somatostatin immunoneutralizing antibody to the basal media. The addition of the Ca2+ ionophore, A-23187 (10 microM), or the adenylate cyclase activator, forskolin (100 microM), resulted in an IRGIP output greater than four times basal. Porcine gastrin-releasing peptide (GRP), at 1-100 nM, significantly stimulated IRGIP release in a concentration-dependent fashion. IRS release was increased significantly by 55 mM K+, 20 mM glucose, 10 microM A-23187, 100 nM GRP, or 100 microM forskolin.

Gastric inhibitory polypeptide and glucagon-like peptide-1(7-36) amide exert similar effects on somatostatin secretion but opposite effects on gastrin secretion from the rat stomach

Previous studies on the isolated perfused stomach have shown that gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1(7-36) amide (GLP-1(7-36) amide) stimulate release of somatostatin (somatostatin-like immunoreactivity, SLI). GIP produced a paradoxical increase in gastrin secretion, whereas GLP-1(7-36) was inhibitory. In the current study, the actions of synthetic (sp) and native (np) porcine and synthetic human (sh) GIP, GLP-1(7-36), and GLP-1(7-37) on SLI and gastrin secretion were compared using a gradient perfusion of peptide. All peptides increased SLI secretion at a threshold concentration of approximately 50 pmol/L (p < 0.05). The initial rate of increase in response to spGIP (119 +/- 39 pg/min) was greater than with other forms of GIP or GLP-1. Maximal increases obtained with the two porcine peptides did not differ. Gastrin secretion was increased by concentrations of spGIP and npGIP similar to those increasing SLI secretion, but the maximal response to shGIP was lower. In contrast to GIP-induced increases, both GLP-1(7-36) and GLP-1(7-37) suppressed gastrin secretion. It is concluded that human and porcine GIP, GLP-1(7-36), and GLP-1(7-37) all stimulate SLI secretion but with different maximal effects, and GIP stimulates gastrin secretion whereas both forms of GLP-1 inhibit gastrin secretion.

Feedback regulation of glucose-dependent insulinotropic polypeptide (GIP) secretion by insulin in conscious rats

Feedback regulation of glucose-dependent insulinotropic polypeptide (GIP) was studied in a conscious rat model. Male Wistar rats were preconditioned to partial restraint, the tail artery and vein were cannulated under local anesthesia. All animals received 1 g/kg oral glucose by gavage and were divided into 3 groups: One group ('euglycemic hyperinsulinemic', EH) underwent rapid induction of hyperinsulinemia by i.v. insulin infusion from 5 min prior to oral glucose until 120 min after (mean plasma insulin = 2285 +/- 108 pM +/- S.E.M., n = 5); a second group ('hyperglycemic hyperinsulinemic', HH) underwent rapid induction of both hyperinsulinemia and hyperglycemia (mean serum glucose = 12.9 +/- 0.4 mM +/- S.E.M., mean plasma insulin 3160 +/- 109 pM, n = 5). A third group ('control') underwent saline infusion alone (n = 5). Arterial blood was collected for GIP estimation at -10, 0, 10, 20, 30, 50, 70, 90 and 120 min after oral glucose. In the control group GIP rose by 96% from a mean basal concentration of 114 +/- 12 pM to a peak of 224 +/- 14 pM by 20 min, and returned to baseline within 70 min. In EH, the GIP rise was blunted and the peak (146 +/- 31 pM) occurred at 10 min, while in HH GIP peaked at 192 +/- 32 pM 10 min after oral glucose (a 92% increase over basal). Compared to controls, total area under the curve for GIP was less for EH (598 +/- 112 versus 971 +/- 94 pmol/l/h +/- S.E.M., P < 0.034). GIP response in HH was similar to the control group at 853 +/- 134 pmol/l/h.(ABSTRACT TRUNCATED AT 250 WORDS)

Meal-frequency effects on plasma hormone concentrations and cholesterol synthesis in humans

To examine meal-frequency effects on circulating hormone concentrations and cholesterol synthesis, male subjects consumed liquid diets given as either six evenly spaced (ES) or three diurnal (DI) meals over 3 d. Deuterium oxide was given on day 2 and blood sampled every 4 h over days 2 and 3 to measure plasma cholesterol, glucose, insulin, and glucose-dependent insulinotropic polypeptide (GIP) concentrations and cholesterol synthesis. Cholesterol synthesis was determined from deuterium incorporation into plasma free cholesterol by using constrained and unconstrained curve-fit models. Plasma total cholesterol concentrations decreased in both ES and DI groups (P < 0.05). The ES group had lower insulin (P < 0.05) and GIP (P < 0.001) concentrations compared with the DI group. Cholesterol synthesis was reduced (P < 0.01) in the ES vs the DI group when determined by using constrained (0.050 +/- 0.002 vs 0.075 +/- 0.005 pools/d, respectively) and unconstrained (0.072 +/- 0.005 vs 0.119 +/- 0.011 pools/d, respectively) models. These data suggest meal frequency-dependent control of cholesterogenesis via hormonally mediated mechanisms.

Long-term effectiveness of oral vanadyl sulphate in streptozotocin-diabetic rats

Recent studies have demonstrated the insulin-like effects of oral vanadyl sulphate in the streptozotocin-diabetic rat, including the amelioration of hyperglycaemia and the prevention of diabetes-related cardiac and adipose tissue dysfunction. However, the possibility that vanadyl treatment, routinely initiated at 3 days after the induction of diabetes, had prevented the full cytotoxic destruction of the beta cell, and thus accounted for the apparent anti-diabetic properties of vanadyl was questioned. Hence in the present study, we examined the effectiveness of vanadyl sulphate as a glucose-lowering and anti-diabetic agent when administration was delayed from the time of induction of diabetes. Male Wistar rats were injected with a single intravenous dose of streptozotocin (55 mg/kg). Vanadyl sulphate was administered in the drinking water at a concentration of 0.75 mg/ml from 3, 10 and 17 days after the streptozotocin injection and treatment was then maintained for 5 months. Vanadyl intake was accompanied by lowered serum levels of triglyceride and cholesterol with no associated enhancement in circulating insulin. Vanadyl-treated diabetic animals showed improved glucose tolerance while insulin release in vivo was still markedly lower than in non-diabetic rats. Adipose tissue function, as expressed by basal and epinephrine-stimulated lipolysis in isolated adipose tissue, was also normalized in vanadyl-treated diabetic animals. These responses were all observed whether vanadyl treatment was initiated 3, 10 or 17 days after induction of diabetes. Finally, prolonged treatment with vanadyl sulphate (in this case up to 5 months) did not cause any apparent hepatic toxicity as assessed histologically. Diabetes-induced morphological changes in the kidney were also prevented by vanadyl treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphological and functional alterations to a sub-group of regulatory peptides in human pancreas and intestine after jejuno-ileal bypass

The effect of ileo-gastrostomy on insulin and cholecystokinin secretion and the endocrine cells containing cholecystokinin, somatostatin, motilin, secretin, neurotensin and enteroglucagon was studied. Fasting and postprandial plasma samples collected pre- and post-operation demonstrated significant changes in circulating insulin and cholecystokinin. Fasting and postprandial insulin levels decreased three months after ileo-gastrostomy (postprandial pre-operation 40 +/- 8 microU/ml compared to 15 +/- 24 microU/ml post-operation, P < 0.02). The postprandial levels of cholecystokinin were significantly increased (pre-operation 7.1 +/- 1.1 pg/ml compared to 12.2 +/- 1.7 pg/ml post-operation, P < 0.02). Quantification of the endocrine cell populations in the jejunum in-continuity three months after ileo-gastrostomy demonstrated an hyperplasia of cholecystokinin-, secretin-, gastric inhibitory polypeptide-, motilin- and somatostatin-containing cells. In samples of the ileum taken from within the bypass loop the neurotensin- and somatostatin-containing cells were unaffected while the enteroglucagon-containing cells were significantly increased in number. Ileo-gastrostomy resulted in significant alterations to the abundance of regulatory peptide-containing endocrine cells and to circulating levels of insulin and cholecystokinin. These changes are implicated in the dramatic weight loss associated with the operation.

Insulin response of cultured islets from diabetic and nondiabetic BB rats

This study examines the insulin response of pancreatic islets isolated from diabetic BB rats (BBD), nondiabetic BB rats (BBN), and Wistar rats to in vitro stimulation. After a 48-hour culture period, insulin release in response to glucose (17.8 mmol/L) either alone, with glucose-dependent insulinotropic polypeptide (GIP) +/- somatostatin (SS), or with Arg +/- SS was measured. A static incubation system was used. Insulin secretion from islets cultured in 4.4 mmol/L glucose (basal) did not differ between BBN and BBD rats (0.50% +/- 0.08%, 0.67% +/- 0.25% of total islet cell content [TCC], respectively). High glucose concentrations (17.8 mmol/L) stimulated a modest increase in insulin release from BBD and BBN islets (1.8% +/- 0.48% and 2.1% +/- 0.19% TCC, respectively). The addition of GIP (1 nmol/L) enhanced glucose-stimulated insulin secretion from BBN rat islets (2.9% +/- 0.42% TCC), but had no effect on BBD islets (2.04% +/- 0.57% TCC). Somatostatin (1 mumol/L) completely reversed the glucose- and/or GIP-stimulated insulin secretion from both BBN and BBD rat islets to basal levels (0.42% +/- 0.043%, 0.42% +/- 0.09% TCC, respectively). Arg (1 mmol/L) enhanced glucose-stimulated insulin secretion in both groups, although the greatest response was elicited from BBD rat islets (8.4-fold v 3.2-fold). Experiments comparing BB rats with Wistar rats demonstrated significant differences in the glucose-stimulated (17.8 mmol/L) insulin response of the islets. Islets taken from BBN and BBD were less responsive to glucose than those from Wistar rats. However, islets from BBD rats were hyperresponsive to Arg when compared with islets from Wistar rats.(ABSTRACT TRUNCATED AT 250 WORDS)

A rapid, sensitive, and easy-to-perform solid phase insulin radioimmunoassay

Accurate measurement of basal insulin release in perifusion, perfusion and low-density beta-cell preparations has been difficult with present assays. A simple, competitive, equilibrium, 15-hour insulin assay using 125I-insulin with microtiter plate immobilized antibody, has been developed. This method, a Solid-phase-RadioImmunoAssay (SPRIA), is very sensitive and has a broad useful range (1 - 64 microU/ml). For a test series of 4 standard curves, interassay variation between controls of 1, 4, 16, and 64 microU/ml was +/- 5.2% (SEM) and intra-assay variation over the range of standards between 0.5 to 64 microU/ml was 5.1% (SEM). Nonspecific binding was not significantly different from empty borosilicate culture tubes; 4.0 +/- 0.4 and 3.5 +/- 0.5 counts/minute (mean +/- SEM; n = 54), respectively. This SPRIA can be used with existing gamma-counters, while reducing the radioactive and glass waste presently produced by RIA (test-tubes can be reused). The radioactivity of unused test-tubes was compared against test-tubes used for greater than 10 assays, values were 3.5 +/- 0.5 and 4.4 +/- 0.6 counts/minute mean +/- SEM; n = 54), respectively. Results of an oral glucose tolerance test (oGTT) performed on four male Wistar Furth rats showed a close correlation between SPRIA and RIA insulin values (linear regression, r2 = 0.990). This SPRIA measured plasma insulin levels from a human oGTT with a variation of less than or equal to 3.7% (SEM) between sample triplicates. Standard curves from three commonly measured insulin isoforms (human, rat and porcine) showed a high correlation (multiple linear regression, r2 = 0.998, n = 5 standard curves). In order to determine SPRIA's ability to measure acid extracts, insulin recovery from 2N acetic acid was compared against insulin recovery from Dulbecco's Modified Eagles medium (DME). The insulin recovery from 2N acetic acid was greater than 90% of that achieved with DME. In conclusion, an easy-to-perform assay which is ideal for the rapid quantification of insulin from isolated islets of Langerhans, isolated beta-cells, acetic acid extracts or plasma with greater sensitivity, and less waste than the conventional RIA has been developed.

Hiroshima-like neutrons from A-bomb replica: physical basis for their use in biological experiments

The lineal energy distribution and several other dosimetric parameters were measured for the neutrons emitted from a replica of the Hiroshima bomb to determine their usefulness in biological experiments designed to estimate the effectiveness of actual Hiroshima neutrons. The "Little-Boy" replica (LBR) was constructed at the Los Alamos National Laboratory in support of the recent atomic-bomb dose reevaluation and was made of identical materials and had nearly identical dimensions and geometry as the Hiroshima bomb. However, the LBR was operated as a steady-state nuclear reactor, which permitted measurements under controlled conditions. Detailed dosimetric measurements and calculations were made at distances of up to 2.1 m from the center of the LBR uranium core. At these distances, the in-air kerma was approximately 97% from neutrons and kerma rates were shown to be particularly useful for biological experiments (up to approximately 7 Gy/h was possible). Quantitative intercomparisons of neutron energy spectra, lineal energy distributions, and measured cytogenetic results for several fission-neutron sources indicate that Hiroshima and LBR neutrons should be of similar biological effectiveness. Based on these evaluations, and cytogenetic results for LBR neutrons reported in a companion paper (this issue), it is estimated that Hiroshima neutrons were 20 to 30% more effective than the fission neutrons commonly used in radiobiology.

Insulin secretion and islet endocrine cell content at onset and during the early stages of diabetes in the BB rat: effect of the level of glycemic control

Although it is agreed that autoimmune destruction of pancreatic islets in diabetic BB rats is rapid, reports of endocrine cell content of islets from BB diabetic rats at the time of onset of diabetes vary considerably. Because of the rapid onset of the disease (hours) and the attendant changes in islet morphology and insulin secretion, it was the aim of this study to compare islet beta-cell numbers to other islet endocrine cells as close to the time of onset of hyperglycemia as possible (within 12 h). As it has been reported that hyperglycemia renders the beta cell insensitive to glucose, the early effects of different levels of insulin therapy (well-controlled vs. poorly controlled glycemia) on islet morphology and insulin secretion were examined. When measured within 12 h of onset, insulin content of BB diabetic islets, measured by morphometric analysis or pancreatic extraction, was 60% of insulin content of control islets. Despite significant amounts of insulin remaining in the pancreas, 1-day diabetic rats exhibited fasting hyperglycemia and were glucose intolerant. The insulin response from the isolated perfused pancreas to glucose and the glucose-dependent insulinotropic hormone, gastric inhibitory polypeptide (GIP), was reduced by 95%. Islet content of other endocrine peptides, glucagon, somatostatin, and pancreatic polypeptide, was normal at onset and at 2 weeks post onset. A group of diabetic animals, maintained in a hyperglycemic state for 7 days with low doses of insulin, were compared with a group kept normoglycemic by appropriate insulin therapy. No insulin could be detected in islets of poorly controlled diabetics, while well-controlled animals had 30% of the normal islet insulin content.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the enteroinsular axis in two strains of obese rat, the fatty Zucker and the JCR:LA-corpulent

The 'fatty' Zucker and more recently the JCR:LA-cp 'corpulent' have been studied extensively as genetic models of the hyperinsulinemia, insulin resistance and abnormal fat metabolism of obesity. It has been hypothesized that an abnormal enteroinsular axis leading to hypersecretion of the insulin releasing hormone gastric inhibitory polypeptide (GIP) could contribute to the hyperinsulinemia of obesity, although this has been controversial. The present study was undertaken to compare the enteroinsular axis in fatty Zucker and JCR:LA-cp rats. The in vivo GIP and insulin responses to an oral glucose challenge, as well as glucose tolerance, were compared in lean and obese phenotypes of both strains as well as the sensitivity of the perfused pancreas to the secretagogues glucose, arginine and GIP. In addition, the effect of perfusate glucose concentration on the beta cell response to GIP was assessed in both strains. Tissue samples from the pancreas were taken for immunocytochemical analysis of comparative size and composition of pancreatic islets. Our results indicate that corpulent rats are hyperGIPemic when compared to fatty Zuckers and that hyperinsulinemia (both in vivo and in vitro) is more severe in the JCR:LA-cp than in the fatty Zucker, as is the degree of insulin resistance (as evidenced by glucose intolerance). Islets of corpulent rats were found to be larger than those of fa/fa rats as well as having a larger area occupied by beta cells. It was concluded that GIP may contribute to fasting hyperinsulinemia in the Zucker rat (as a result of a defective glucose threshold for the insulinotropic action of GIP), whereas the hyperGIPemia of the JCR:LA-cp rat may contribute to the massive nutrient-stimulated hyperinsulinemia observed in the male phenotype of this strain.

Anatomical relationship between neuropeptide-containing fibers and efferent vagal neurons projecting to the rat corpus

Injections of the retrograde tracers into the posterior surface of the stomach at the greater curvature resulted in labelling of the right half of the dorsal motor nucleus of the vagus. Whereas injections into the anterior and posterior surfaces of the corpus resulted in bilateral labelling in the medulla. Immunocytochemical staining of the labelled sections using antisera to substance P was confined to a dense network of fibers within the dorsal motor nucleus of the vagus and the nucleus tractus solitarius with no cell bodies being detected. Calcitonin gene-related peptide-immunoreactivity was detected in nerve fibers in the nucleus tractus solitarius and cell bodies of the hypoglossal nucleus. Finally, neuropeptide Y-immunoreactivity was confined to nerve fibers within the vagal complex. Of the neurons labelled by the retrograde tracers injected into the corpus all were in close spatial contact with fibers containing substance P-immunoreactivity. A smaller number were associated with neuropeptide Y-containing fibers with a few adjacent to calcitonin gene-related peptide-immunoreactive fibers. These results indicate that substance P and neuropeptide Y may directly regulate efferent neurons controlling gastric motility and acid secretion. Calcitonin gene-related peptide, however, is unlikely to directly modulate the cell bodies of the neurons in the dorsal motor nucleus but may modulate the dendrites from these neurons in the nucleus tractus solitarius.