PubMed Health⌕ Search

Biomedical subjects

M B Wheeler

Publications and source records attributed to M B Wheeler.

At least 37 records · Page 2Linked to original sources
PubMed2001

Decreased CRH mRNA expression in the fetal guinea pig hypothalamus following maternal nutrient restriction.

The regulation of corticotropin-releasing hormone (CRH) mRNA expression following maternal nutrient restriction was examined in the fetal hypothalamus. Pregnant guinea pigs were food restricted for 48 h or fed normally during late gestation. After nutrient restriction, CRH mRNA levels in the hypothalamic paraventricular nucleus of the fetus were determined using in situ hybridization and were found to be significantly decreased (P<0.0001) compared to controls. In conclusion, we have successfully sequenced the coding sequence of the guinea pig CRH gene, and have shown that a short period (48 h) of maternal nutrient restriction inhibits CRH mRNA expression in the fetal hypothalamus.

Animals↗
PubMed2001

Biological activities of glucagon-like peptide-1 analogues in vitro and in vivo.

Studies support a role for glucagon-like peptide 1 (GLP-1) as a potential treatment for diabetes. However, since GLP-1 is rapidly degraded in the circulation by cleavage at Ala(2), its clinical application is limited. Hence, understanding the structure-activity of GLP-1 may lead to the development of more stable and potent analogues. In this study, we investigated GLP-1 analogues including those with N-, C-, and midchain modifications and a series of secretin-class chimeric peptides. Peptides were analyzed in CHO cells expressing the hGLP-1 receptor (R7 cells), and in vivo oral glucose tolerance tests (OGTTs) were performed after injection of the peptides in normal and diabetic (db/db) mice. [D-Ala(2)]GLP-1 and [Gly(2)]GLP-1 showed normal or relatively lower receptor binding and cAMP activation but exerted markedly enhanced abilities to reduce the glycemic response to an OGTT in vivo. Improved biological effectiveness of [D-Ala(2)]GLP-1 was also observed in diabetic db/db mice. Similarly, improved biological activity of acetyl- and hexenoic-His(1)-GLP-1, glucagon((1-5)-, glucagon((1-10))-, PACAP(1-5)-, VIP(1-5)-, and secretin((1-10))-GLP-1 was observed, despite normal or lower receptor binding and activation in vitro. [Ala(8/11/12/16)] substitutions also increased biological activity in vivo over wtGLP-1, while C-terminal truncation of 4-12 amino acids abolished receptor binding and biological activity. All other modified peptides examined showed normal or decreased activity in vitro and in vivo. These results indicate that specific N- and midchain modifications to GLP-1 can increase its potency in vivo. Specifically, linkage of acyl-chains to the alpha-amino group of His(1) and replacement of Ala(2) result in significantly increased biological effects of GLP-1 in vivo, likely due to decreased degradation rather than enhanced receptor interactions. Replacement of certain residues in the midchain of GLP-1 also augment biological activity.

Amino Acid Sequence↗
PubMed2001

Generation and exploration of a dense genetic map in a region of a QTL affecting corpora lutea in a Meishan x Yorkshire cross.

Previously genomic scans revealed quantitative trait loci (QTL) on porcine Chromosome 8 (SSC8) as significantly affecting the number of corpora lutea (CL) in swine. In one study, statistical evidence for the putative QTL was found in the chromosomal region defined by the microsatellites (MS) SW205, SW444, SW206, and SW29. A Yeast Artificial Chromosome library was screened by using the corresponding primers for clones containing these MS by PCR. From five positive YAC clones, 10 additional MS were isolated and mapped to SSC8 with the INRA-University of Minnesota porcine Radiation Hybrid (IMpRH) panel. The genetic map position of the QTL has been refined by addition of these 10 markers. The QTL evaluation included pedigrees of F2-intercross Meishan x Yorkshire design, with phenotypic data of 108 F2 female offspring and genotypic data for 29 MS markers on SSC8. The analysis was performed by using the least squares regression method. The calculated QTL effect for CL obtained by the multilocus least squares method showed a maximum test statistic (F value = 13.98) at position 99 cM between three MS derived from YACs containing SW205 and SW1843 spanning an interval of 7.1 cM. The point-wise (nominal) P-value was 5.21 x 10-6 corresponding to a genome-wide P-value of 0.009. The additive QTL effect explained 17.4% of the phenotypic variance.

Animals↗
PubMed2001

Evolution of receptors for proglucagon-derived peptides: isolation of frog glucagon receptors.

The mammalian proglucagon gene encodes three glucagon-like sequences, glucagon, glucagon-like peptide 1 (GLP-1) and glucagon-like peptide 2 (GLP-2). Each of these three functionally distinct proglucagon-derived peptides has a unique, but related, receptor. To better understand the origin of the unique physiological functions of each proglucagon-derived glucagon-like sequence we have cloned glucagon-like receptors from two species of frogs, Xenopus laevis and Rana pipiens. The cloned glucagon-like receptor sequences were found to be most closely related to glucagon receptors. To determine whether the evolutionary history of the receptors for proglucagon-derived peptides was the same as that inferred for the peptide hormones, we conducted a phylogenetic analysis using both parsimony and distance methods. We show that the evolutionary history of the receptors for glucagon-like sequences differ from the history of the glucagon-like sequences. The phylogeny of receptors for proglucagon-derived peptides is not monophyletic (i.e. they are not each other's closest relatives), as the receptor for the hormone glucose-dependent insulinotropic peptide (GIP) is more closely related to the glucagon receptor than either the GLP-1 or GLP-2 receptors. In contrast to the evolutionary origin of glucagon-like sequences, where glucagon is of most ancient origin, we found that the GLP-2 receptor has the most ancient origin. These observations suggest that the diversification of the glucagon-like sequences encoded by the proglucagon gene and of the receptors for these peptides occurred independently, and that either these hormones or their receptors have been recruited for new functions.

Amino Acid Sequence↗
PubMed2001

Handling individual mammalian embryos using microfluidics.

We have designed, built, and tested microfluidic systems capable of transporting individual, preimplantation mouse embryos (100-microm to 150-microm diameter) through a network of channels. Typical channels are 160 to 200 microm deep, 250 to 400 microm wide at the top, and narrower at the bottom (0 to 250 microm wide) due to the fabrication process. In these channels, a pressure gradient of 1 Pa/mm causes the medium to flow on the order of 10(-10) m3/s (100 nl/s), with an average speed of 1 to 2 mm/s. Under these flow conditions the embryos roll along the bottoms of the channels, traveling at 1/2 the speed of the fluid. By manipulating the pressure at the wells connected to the ends of the channels, the embryos can be transported to (and retained at) specific locations including culture compartments and retrieval wells.

Animals↗
PubMed2001

Ca(2+) influx and cAMP elevation overcame botulinum toxin A but not tetanus toxin inhibition of insulin exocytosis.

Previous reports showed that cleavage of vesicle-associated membrane protein-2 (VAMP-2) and synaptosomal-associated protein of 25 kDa (SNAP-25) by clostridial neurotoxins in permeabilized insulin-secreting beta-cells inhibited Ca(2+)-evoked insulin secretion. In these reports, the soluble N-ethylmaleimide-sensitive factor attachment protein target receptor proteins might have formed complexes, which preclude full accessibility of the putative sites for neurotoxin cleavage. In this work, VAMP-2 and SNAP-25 were effectively cleaved before they formed toxin-insensitive complexes by transient transfection of insulinoma HIT or INS-1 cells with tetanus toxin (TeTx) or botulinum neurotoxin A (BoNT/A), as shown by immunoblotting and immunofluorescence microscopy. This resulted in an inhibition of Ca(2+) (glucose or KCl)-evoked insulin release proportionate to the transfection efficiency (40-50%) and an accumulation of insulin granules. With the use of patch-clamp capacitance measurements, Ca(2+)-evoked exocytosis by membrane depolarization to -10 mV was abolished by TeTx (6% of control) but only moderately inhibited by BoNT/A (30% of control). Depolarization to 0 mV to maximize Ca(2+) influx partially overcame BoNT/A (50% of control) but not TeTx inhibition. Of note, cAMP activation potentiated Ca(2+)-evoked secretion by 129% in control cells but only 55% in BoNT/A-transfected cells and had negligible effects in TeTx-transfected cells. These results indicate that, whereas VAMP-2 is absolutely necessary for insulin exocytosis, the effects of SNAP-25 depletion on exocytosis, perhaps on insulin granule pool priming or mobilization steps, could be partially reversed by higher levels of Ca(2+) or cAMP potentiation.

Animals↗
PubMed2001

Members of the Kv1 and Kv2 voltage-dependent K(+) channel families regulate insulin secretion.

In pancreatic beta-cells, voltage-dependent K(+) (Kv) channels are potential mediators of repolarization, closure of Ca(2+) channels, and limitation of insulin secretion. The specific Kv channels expressed in beta-cells and their contribution to the delayed rectifier current and regulation of insulin secretion in these cells are unclear. High-level protein expression and mRNA transcripts for Kv1.4, 1.6, and 2.1 were detected in rat islets and insulinoma cells. Inhibition of these channels with tetraethylammonium decreased I(DR) by approximately 85% and enhanced glucose-stimulated insulin secretion by 2- to 4-fold. Adenovirus-mediated expression of a C-terminal truncated Kv2.1 subunit, specifically eliminating Kv2 family currents, reduced delayed rectifier currents in these cells by 60-70% and enhanced glucose-stimulated insulin secretion from rat islets by 60%. Expression of a C-terminal truncated Kv1.4 subunit, abolishing Kv1 channel family currents, reduced delayed rectifier currents by approximately 25% and enhanced glucose-stimulated insulin secretion from rat islets by 40%. This study establishes that Kv2 and 1 channel homologs mediate the majority of repolarizing delayed rectifier current in rat beta-cells and that antagonism of Kv2.1 may prove to be a novel glucose-dependent therapeutic treatment for type 2 diabetes.

1-Methyl-3-isobutylxanthine↗
PubMed2001

Increased uncoupling protein-2 levels in beta-cells are associated with impaired glucose-stimulated insulin secretion: mechanism of action.

In pancreatic beta-cells, glucose metabolism signals insulin secretion by altering the cellular array of messenger molecules. ATP is particularly important, given its role in regulating cation channel activity, exocytosis, and events dependent upon its hydrolysis. Uncoupling protein (UCP)-2 is proposed to catalyze a mitochondrial inner-membrane H(+) leak that bypasses ATP synthase, thereby reducing cellular ATP content. Previously, we showed that overexpression of UCP-2 suppressed glucose-stimulated insulin secretion (GSIS) in isolated islets (1). The aim of this study was to identify downstream consequences of UCP-2 overexpression and to determine whether insufficient insulin secretion in a diabetic model was correlated with increased endogenous UCP-2 expression. In isolated islets from normal rats, the degree to which GSIS was suppressed was inversely correlated with the amount of UCP-2 expression induced. Depolarizing the islets with KCl or inhibiting ATP-dependent K(+) (K(ATP)) channels with glybenclamide elicited similar insulin secretion in control and UCP-2-overexpressing islets. The glucose-stimulated mitochondrial membrane ((m)) hyperpolarization was reduced in beta-cells overexpressing UCP-2. ATP content of UCP-2-induced islets was reduced by 50%, and there was no change in the efflux of Rb(+) at high versus low glucose concentrations, suggesting that low ATP led to reduced glucose-induced depolarization, thereby causing reduced insulin secretion. Sprague-Dawley rats fed a diet with 40% fat for 3 weeks were glucose intolerant, and in vitro insulin secretion at high glucose was only increased 8.5-fold over basal, compared with 28-fold in control rats. Islet UCP-2 mRNA expression was increased twofold. These studies provide further strong evidence that UCP-2 is an important negative regulator of beta-cell insulin secretion and demonstrate that reduced (m) and increased activity of K(ATP) channels are mechanisms by which UCP-2-mediated effects are mediated. These studies also raise the possibility that a pathological upregulation of UCP-2 expression in the prediabetic state could contribute to the loss of glucose responsiveness observed in obesity-related type 2 diabetes in humans.

Adenosine Triphosphate↗
PubMed2001

Short communication: effects of increased expression of alpha-lactalbumin in transgenic mice on milk yield and pup growth.

Lactose synthase (a complex of beta1,4-galactosyltransferase and alpha-lactalbumin) forms lactose in the Golgi complex of mammary epithelial cells. To determine whether alpha-lactalbumin is a limiting component in this complex, transgenic mice that expressed bovine alpha-lactalbumin were studied. Transgenic mice produced 0.5 to 1.5 mg/ml of bovine alpha-lactalbumin in their milk, 5- to 15-fold more alpha-lactalbumin than in milk of control mice. Transgenic and control mice produced milk with the same concentrations of lactose, cream, and total solids, and showed similar mammary gland growth, morphology, and histology. Milk from transgenic mice had 0.6% less protein than milk from control mice (P < 0.05). The in vitro lactose synthase activity in mammary gland homogenates from alpha-lactalbumin transgenic mice was increased (P < 0.05), demonstrating that bovine alpha-lactalbumin could interact with murine beta1,4-galactosyltransferase. Pups reared by lactating transgenic mice showed a 4% increase in growth on d 10 of lactation, suggesting that milk production was increased (P = 0.06). Milk volume, estimated using the weigh-suckle-weigh technique, tended to be higher (although not significantly) in transgenic mice (P = 0.11). These results suggest that augmenting alpha-lactalbumin expression in the dam increases the growth of suckling offspring.

Animals↗
PubMed2001

Transgenic alteration of sow milk to improve piglet growth and health.

There are many potential applications of transgenic methodologies for developing new and improved strains of livestock. One practical application of transgenic technology in pig production is to improve milk production or composition. The first week after parturition is the period of greatest loss for pig producers, with highest morbidity and mortality attributed to malnutrition and scours. Despite the benefits to be gained by improving lactation performance, little progress has been made in this area through genetic selection or nutrition. Transgenic technology provides an important tool for addressing the problem of low milk production and its detrimental impact on pig production. Transgenic pigs over-expressing the milk protein bovine alpha-lactalbumin were developed. alpha-Lactalbumin was selected for its role in lactose synthesis and regulation of milk volume. Sows hemizygous for the transgene produced as much as 0.9 g bovine alpha-lactalbumin l-1 pig milk. The outcomes assessed were milk composition, milk yield and piglet growth. First parity alpha-lactalbumin gilts had higher milk lactose content in early lactation and 20-50% greater milk yield on days 3-9 of lactation than did non-transgenic gilts. Weight gain of piglets suckling alpha-lactalbumin gilts was greater (days 7-21 after parturition) than that of control piglets. Thus, transgenic over-expression of milk proteins may provide a means for improving the lactation performance of pigs.

Animals↗
PubMed2000

Characterization of glucagon-like peptide-1 receptor-binding determinants.

Glucagon-like peptide 1 (GLP-1) is a potent insulinotropic hormone currently under study as a therapeutic agent for type 2 diabetes. Since an understanding of the molecular mechanisms leading to high-affinity receptor (R) binding and activation may facilitate the development of more potent GLP-1R agonists, we have localized specific regions of GLP-1R required for binding. The purified N-terminal fragment (hereafter referred to as NT) of the GLP-1R produced in either insect (Sf9) or mammalian (COS-7) cells was shown to bind GLP-1. The physical interaction of NT with GLP-1 was first demonstrated by cross-linking ((125)I-GLP-1/NT complex band at approximately 28 kDa) and secondly by attachment to Ni(2+)-NTA beads. The GLP-1R NT protein attached to beads bound GLP-1, but with lower affinity (inhibitory concentration (IC(50)): 4.5 x 10(-7) M) than wild-type (WT) GLP-1R (IC(50): 5.2 x 10(-9)M). The low affinity of GLP-1R NT suggested that other receptor domains may contribute to GLP-1 binding. This was supported by studies using chimeric glucose-dependent insulinotropic polypeptide (GIP)/GLP-1 receptors. GIP(1-151)/GLP-1R, but not GIP(1-222)/GLP-1R, exhibited specific GLP-1 binding and GLP-1-induced cAMP production, suggesting that the region encompassing transmembrane (TM) domain 1 through to TM3 was required for binding. Since it was hypothesized that certain charged or polar amino acids in this region might be involved in binding, these residues (TM2-TM3) were analyzed by substitution mutagenesis. Five mutants (K197A, D198A, K202A, D215A, R227A) displayed remarkably reduced binding affinity. These studies indicate that the NT domain of the GLP-1R is able to bind GLP-1, but charged residues concentrated at the distal TM2/extracellular loop-1 (EC1) interface (K197, D198, K202) and in EC1 (D215 and R227) probably contribute to the binding determinants of the GLP-1R.

Amino Acid Sequence↗
PubMed1999

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.

Animals↗
PubMed1999

A genomic scan of porcine reproductive traits reveals possible quantitative trait loci (QTLs) for number of corpora lutea.

Reproductive traits have low heritabilities, are expressed in only one sex, and are not measurable until sexual maturity (Avalos and Smith, Anim Prod 44:153, 1987). Using traditional methods, selection for reproductive traits is relatively less effective than selecting for growth or carcass traits. Traits most affected by a small number of genes with major effects rather than many genes with small effects are most amenable to MAS. As part of our porcine genome scan to identify quantitative trait loci (QTLs) of economic importance in marker-assisted selective (MAS) breeding programs, we examined 8 reproductive and farrowing traits in the University of Illinois (UI) Meishan x Yorkshire Resource Family. Gilts were genotyped with 119 microsatellite markers (MS) with intervals averaging 24 cM over all 18 porcine autosomes. F-ratios supporting QTL location were calculated by the least squares regression method. Results suggestive of linkage at the 5% genome-wide level were observed for the number of stillborn piglets on Chromosome (Chr) 4 (SSC4) (p-value = 0.0001), corpora lutea on SSC8 (p-value = 0.00027), and gestation length on SSC9 (p-value = 0.00019). Results for additional loci relevant to litter size, number of corpora lutea on SSC15 and 7 (p-value = 0.0029 and 0.0028 at 107 and 150 cM, respectively), gestation length on SSC15 and 1 (p-value = 0.0017 and 0.0069 at 96 and 166 cM, respectively), uterine length on SSC7 and 5 (p-value = 0.0044 and 0.0075 at 148 and 1 cM, respectively) and piglets born per litter on SSC6 (p-value = 0.0075 at 102 cM), were not statistically significant at the 5% genome-wide level. Thus, the use of a linked marker to facilitate selection for reproductive traits has considerable potential. By using linked markers, selection can be applied to both sexes before sexual maturity, making genetic selection considerably more efficient and less costly.

Animals↗
PubMed1999

Effect of fetal age and method of recovery on isolation of preantral follicles from bovine ovaries.

The objective of this study was to compare enzymatic and mechanical methods, at distinct fetal ages, on isolation of different developmental stages of preantral follicles from bovine ovaries. Fetal ovaries were obtained from pregnant cattle at 150 to 270 d of gestation, and 135,521 preantral follicles at different stages of development were studied. The dissociation of ovaries with a mechanical procedure resulted in an average of 938.16 prenatral follicles. In contrast, 3,715.56 follicles were obtained when enzymatic digestion was used (P = 0.0001). Histological evaluation confirmed follicular stages and demonstrated that both mechanical and mechanical-enzymatic procedure did not affect the cellular integrity of the follicles. Granulosa cell-oocyte complexes surrounded by a basal membrane, were considered preantral follicles in this study. The ratio of different stages of isolated preantral follicles was significantly (P = 0.0001) correlated to fetal age. The earliest fetal age at which tertiary follicles were identified was at 210 d of gestation. The results confirm previous observation that follicular development and atresia are initiated during fetal development. These data provide information on methodologies to isolate intact bovine preantral follicles for investigating the control and regulation of follicular development and the growth of preantral follicles in vitro.

Animals↗
PubMed1999

Mutations to the third cytoplasmic domain of the glucagon-like peptide 1 (GLP-1) receptor can functionally uncouple GLP-1-stimulated insulin secretion in HIT-T15 cells.

Glucagon-like peptide-1 (GLP-1) is an insulinotropic hormone with powerful antidiabetogenic effects that are thought to be mediated by adenylyl cyclase (AC). Recently, we generated two GLP-1 receptor mutant isoforms (IC3-1 and DM-1) that displayed efficient ligand binding and the ability to promote Ca2+ mobilization from intracellular stores but lacked the ability to couple to AC. In the present study, the wild-type rat GLP-1 receptor (WT-GLP-1 R) or the IC3-1 and DM-1 mutant forms were expressed for the first time in the insulin-producing HIT-T15 cells. Only cells expressing WT-GLP-1 R displayed dramatically elevated GLP-1-induced cAMP responses and elevated insulin secretion. The increase in GLP-1-stimulated secretion in cells expressing WT-GLP-1 R, however, was not accompanied by differences in glucose-stimulated insulin release. Prolonged exposure to GLP-1 (10 nM, 17 h), not only led to an increase in insulin secretion but also increased insulin mRNA levels, but only in cells expressing the WT-GLP-1 R and not the mutant isoforms. Electrophysiological analyses revealed that GLP-1 application enhanced L-type voltage-dependent Ca2+ channel (VDCC) currents > 2-fold and caused a positive shift in VDCC voltage-dependent inactivation in WT-GLP-1R cells only, not control or mutant (DM-1) cells. This action on the Ca2+ current was further enhanced by the VDCC agonist, BAYK8644, suggesting GLP-1 acts via a distinct mechanism dependent on cAMP. These studies demonstrate that the GLP-1 receptor efficiently couples to AC to stimulate insulin secretion and that receptors lacking critical residues in the proximal region of the third intracellular loop can effectively uncouple the receptor from cAMP production, VDCC activity, insulin secretion, and insulin biosynthesis.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1999

Glucagon-like peptide 1 increases insulin sensitivity in depancreatized dogs.

To determine whether glucagon-like peptide (GLP)-1 increases insulin sensitivity in addition to stimulating insulin secretion, we studied totally depancreatized dogs to eliminate GLP-1's incretin effect. Somatostatin was infused (0.8 microg x kg(-1) x min(-1)) to inhibit extrapancreatic glucagon in dogs, and basal glucagon was restored by intraportal infusion (0.65 ng x kg(-1) x min(-1)). To simulate the residual intraportal insulin secretion in type 2 diabetes, basal intraportal insulin infusion was given to obtain plasma glucose concentrations of approximately 10 mmol/l. Glucose was clamped at this level for the remainder of the experiment, which included peripheral insulin infusion (high dose, 5.4 pmol x kg(-1) x min(-1), or low dose, 0.75 pmol x kg(-1) x min(-1)) with or without GLP-1(7-36) amide (1.5 pmol x kg(-1) x min(-1)). Glucose production and utilization were measured with 3-[3H]glucose, using radiolabeled glucose infusates. In 12 paired experiments with six dogs at the high insulin dose, GLP-1 infusion resulted in higher glucose requirements than saline (60.9+/-11.0 vs. 43.6+/-8.3 micromol x kg(-1) x min(-1), P< 0.001), because of greater glucose utilization (72.6+/-11.0 vs. 56.8+/-9.7 micromol x kg(-1) x min(-1), P<0.001), whereas the suppression of glucose production was not affected by GLP-1. Free fatty acids (FFAs) were significantly lower with GLP-1 than saline (375.3+/-103.0 vs. 524.4+/-101.1 micromol/l, P<0.01), as was glycerol (77.9+/-17.5 vs. 125.6+/-51.8 micromol/l, P<0.05). GLP-1 receptor gene expression was found using reverse transcriptase-polymerase chain reaction of poly(A)-selected RNA in muscle and adipose tissue, but not in liver. Low levels of GLP-1 receptor gene expression were also found in adipose tissue using Northern blotting. In 10 paired experiments with five dogs at the low insulin dose, GLP-1 infusion did not affect glucose utilization or FFA and glycerol suppression when compared with saline, suggesting that GLP-1's effect on insulin action was dependent on the insulin dose. In conclusion, in depancreatized dogs, GLP-1 potentiates insulin-stimulated glucose utilization, an effect that might be contributed in part by GLP-1 potentiation of insulin's antilipolytic action.

Adipose Tissue↗
PubMed1999

Beta-cell hypertrophy in fa/fa rats is associated with basal glucose hypersensitivity and reduced SNARE protein expression.

In normal isolated beta-cells, the response to glucose is heterogeneous and characterized by an increasing number of secretory cells as glucose concentration rises (Pipeleers DG, Kiekens R, Ling Z, Wilikens A, Schuit F: Physiologic relevance of heterogeneity in the pancreatic beta-cell population. Diabetologia 37 (Suppl. 2):S57-S64, 1994). We hypothesized that fasting hyperinsulinemia in obesity might be explained by altered beta-cell heterogeneity of signal transduction mechanisms, possibly involving exocytotic proteins. Insulin secretion from individual beta-cells sorted according to the size of the islet donor (<125 microm, >250 microm, and intermediate diameter) was measured by reverse hemolytic plaque assay. Beta-cells from fa/fa rats were hypertrophied 25-40%, independent of donor islet size. This was accompanied by an increased proportion of secretory cells (recruitment) at 5.5-11.0 mmol/l glucose, increased secretion per cell at 2.8 mmol/l glucose, and decreased insulin content after acute glucose exposure without an increase in secretion per cell. Decreased expression of exocytotic (soluble N-ethylmaleimide-sensitive fusion protein receptor [SNARE]) proteins, vesicle-associated membrane protein isoform 2 (VAMP-2), synaptosomal protein of 25 kDa (SNAP-25), and syntaxin-1 and -2 in fa/fa beta-cells may contribute to the failure to sustain excessive plaque size at higher glucose concentrations. Fasting hyperinsulinemia may be maintained by increased recruitment and an exaggerated secretory response in all fa-derived islet populations. Glucose regulates beta-cell responsiveness in the short term, and these effects may involve altered expression of SNARE proteins.

Animals↗