The α2-adrenoceptor antagonist yohimbine normalizes increased islet blood flow in GK rats: a model of type 2 diabetes

Overexpression of α2A-adrenoceptors contributes to type 2 diabetes in GK rats. We aimed to investigate if α2-adrenoceptor inhibition affected islet blood flow in these rats. Anesthetized GK and Wistar-F rats were given the α2-adrenoceptor inhibitor yohimbine (2.5 mg/kg BW) intravenously. The GK rats had higher blood glucose and serum insulin concentrations than WF rats. Yohimbine affected neither of these values in WF rats, but decreased blood glucose and increased serum insulin concentrations in GK rats. Total pancreatic and islet blood flows, measured with microspheres, were increased in GK when compared to WF rats. Yohimbine affected none of the blood flow values in WF rats, whereas islet blood flow in GK rats was reduced to values similar to those seen in WF rats. Overexpression of α2-adrenoceptors may contribute to the increased islet blood flow seen in GK rats, and may be eligible for pharmacologic intervention.

Markedly decreased blood perfusion of pancreatic islets transplanted intraportally into the liver: disruption of islet integrity necessary for islet revascularization

Experimental studies indicate low revascularization of intraportally transplanted islets. This study aimed to quantify, for the first time, the blood perfusion of intrahepatically transplanted islets and elucidate necessary factors for proper islet graft revascularization at this site. Yellow chameleon protein 3.0 islets expressing fluorescent protein in all cells were transplanted. Graft blood perfusion was determined by microspheres. The vascular density and relative contribution of donor blood vessels in revascularization was evaluated using islets expressing green fluorescent protein under the Tie-2 promoter. Blood perfusion of intrahepatic islets was as a mean only 5% of that of native islets at 1-month posttransplantation. However, there was a marked heterogeneity where blood perfusion was less decreased in islets transplanted without prior culture and in many cases restored in islets with disrupted integrity. Analysis of vascular density showed that distorted islets were well revascularized, whereas islets still intact at 1-month posttransplantation were almost avascular. Few donor endothelial cells were observed in the new islet vasculature. The very low blood perfusion of intraportally transplanted islets is likely to predispose for ischemia and hamper islet function. Since donor endothelial cells do not expand posttransplantation, disruption of islet integrity is necessary for revascularization to occur by recipient blood vessels.

Endothelin-1 markedly decreases the blood perfusion of transplanted pancreatic islets in rats

BACKGROUND

Transplantation of insulin-producing β-cells is the only available curative treatment for type 1 diabetes. However, graft function declines within the first years after transplantation, which may reflect inadequate vascular engraftment. Endothelin-1 (ET-1) is a potent vasoconstrictor whose production is regulated by both hypoxia and inflammation. Moreover, the plasma concentration of ET-1 is elevated in patients with type 1 diabetes. The aim of this study was to investigate the gene expression and effects of ET-1 and its 2 receptor antagonists, BQ123 and BQ788, on blood flow in syngeneic rat islet transplants.

METHODS

Pancreatic islets from Wistar Furth rats were isolated and transplanted syngeneically under the kidney capsule. Transplant and kidney cortex blood flow was measured using laser Doppler flowmetry after administration of ET-1 via topical application, or after administration of BQ123 and BQ788 intravenously. The grafts and isolated islets were analyzed for mRNA expression of ET-1, ET(A) receptor, ET(B) receptor, and endothelin-converting enzyme 1 using by reverse-transcription polymerase chain reaction.

RESULTS

ET-1 markedly decreased transplant blood flow (77.5 ± 4.4% 1 minute after administration; n = 6), whereas neither BQ123 nor BQ788 had vascular effects. No differences in relative gene expression between the grafts and freshly isolated control islets were seen for ET-1 (0.65 ± 0.14 [n = 8] vs 0.79 ± 0.24 [n = 5]), ET(A) receptor (0.37 ± 0.14 [n = 8] vs 0.25 ± 0.04 [n =5]), ET(B) receptor (4.78 ± 1.43 [n = 8] vs 1.94 ± 0.32 [n = 5]), or endothelin converting enzyme 1 (7.25 ± 1.88 [n = 8] vs 11.83 ± 0.95 [n = 5]) when expressed as 2(-ΔCt).

CONCLUSION

Exogenous ET-1 strongly affects the blood perfusion of transplanted islets, and endogenous levels can, if up-regulated, contribute to graft failure.

Increased recruitment but impaired function of leukocytes during inflammation in mouse models of type 1 and type 2 diabetes

Background

Patients suffering from diabetes show defective bacterial clearance. This study investigates the effects of elevated plasma glucose levels during diabetes on leukocyte recruitment and function in established models of inflammation.

Methodology/Principal Findings

Diabetes was induced in C57Bl/6 mice by intravenous alloxan (causing severe hyperglycemia), or by high fat diet (moderate hyperglycemia). Leukocyte recruitment was studied in anaesthetized mice using intravital microscopy of exposed cremaster muscles, where numbers of rolling, adherent and emigrated leukocytes were quantified before and during exposure to the inflammatory chemokine MIP-2 (0.5 nM). During basal conditions, prior to addition of chemokine, the adherent and emigrated leukocytes were increased in both alloxan- (62±18% and 85±21%, respectively) and high fat diet-induced (77±25% and 86±17%, respectively) diabetes compared to control mice. MIP-2 induced leukocyte emigration in all groups, albeit significantly more cells emigrated in alloxan-treated mice (15.3±1.0) compared to control (8.0±1.1) mice. Bacterial clearance was followed for 10 days after subcutaneous injection of bioluminescent S. aureus using non-invasive IVIS imaging, and the inflammatory response was assessed by Myeloperoxidase-ELISA and confocal imaging. The phagocytic ability of leukocytes was assessed using LPS-coated fluorescent beads and flow cytometry. Despite efficient leukocyte recruitment, alloxan-treated mice demonstrated an impaired ability to clear bacterial infection, which we found correlated to a 50% decreased phagocytic ability of leukocytes in diabetic mice.

Conclusions/Significance

These results indicate that reduced ability to clear bacterial infections observed during experimentally induced diabetes is not due to reduced leukocyte recruitment since sustained hyperglycemia results in increased levels of adherent and emigrated leukocytes in mouse models of type 1 and type 2 diabetes. Instead, decreased phagocytic ability observed for leukocytes isolated from diabetic mice might account for the impaired bacterial clearance.

Clinical and experimental pancreatic islet transplantation to striated muscle: establishment of a vascular system similar to that in native islets

OBJECTIVE

Curing type 1 diabetes by transplanting pancreatic islets into the liver is associated with poor long-term outcome and graft failure at least partly due to inadequate graft revascularization. The aim of the current study was to evaluate striated muscle as a potential angiogenic site for islet transplantation.

RESEARCH DESIGN AND METHODS

The current study presents a new experimental model that is found to be applicable to clinical islet transplantation. Islets were implanted into striated muscle and intraislet vascular density and blood flow were visualized with intravital and confocal microscopy in mice and by magnetic resonance imaging in three autotransplanted pancreatectomized patients. Mice were rendered neutropenic by repeated injections of Gr-1 antibody, and diabetes was induced by alloxan treatment.

RESULTS

Contrary to liver-engrafted islets, islets transplanted to mouse muscle were revascularized with vessel densities and blood flow entirely comparable with those of islets within intact pancreas. Initiation of islet revascularization at the muscular site was dependent on neutrophils, and the function of islets transplanted to muscle was proven by curing diabetic mice. The experimental data were confirmed in autotransplanted patients where higher plasma volumes were measured in islets engrafted in forearm muscle compared with adjacent muscle tissue through high-resolution magnetic resonance imaging.

CONCLUSIONS

This study presents a novel paradigm in islet transplantation whereby recruited neutrophils are crucial for the functionally restored intraislet blood perfusion following transplantation to striated muscle under experimental and clinical situations.

Reversal of high pancreatic islet and white adipose tissue blood flow in type 2 diabetic GK rats by administration of the beta3-adrenoceptor inhibitor SR-59230A

Previous studies have shown that the Goto-Kakizaki (GK) rat, a nonobese type 2 diabetes model, has an increased white adipose tissue (WAT) and islet blood flow when compared with control rats. The aim of the study was to examine if these increased blood flow values in GK rats could be affected by the beta(3)-adrenoceptor antagonist SR-59230A. We measured organ blood flow with a microsphere technique 10 min after administration of SR-59230A (1 mg/kg body wt), or the corresponding volume of 0.9% NaCl solution (1 ml/kg body wt) in rats anaesthetized with thiobutabarbital. The GK rat had an increased blood flow in all intra-abdominal adipose tissue depots except for the sternal fat pad compared with Wistar-Furth (WF) rats. However, no differences were seen in the blood perfusion of subcutaneous white or brown adipose tissue. The blood flow was also increased in both the pancreas and in the islets in the GK rat compared with WF rats. SR-59230A treatment affected neither WAT nor pancreatic blood flow in WF rats. In GK rats, on the other hand, SR-59230A decreased both WAT and islet blood flow values to values similar to those seen in control WF rats. The whole pancreatic blood flow was not affected by SR-59230A administration in GK rats. Interestingly, the brown adipose tissue blood flow in GK rats increased after SR-59230A administration. These results suggest that beta(3)-adrenoceptors are involved in regulation of blood flow both in islet and in adipose tissue.

Resistin increases islet blood flow and decreases subcutaneous adipose tissue blood flow in anaesthetized rats

AIM

Resistin is an adipokine which has been suggested to participate in the induction of insulin resistance associated with type 2 diabetes. The aim of the present study was to investigate whether acute administration of resistin influences tissue blood perfusion in rats.

METHODS

Resistin was administered as an intravenous infusion of 7.5 microg h(-1) (1.5 mL h(-1)) for 30 min to rats anaesthetized with thiobutabarbital. A microsphere technique was used to estimate the blood flow to six different depots of white adipose tissue (WAT), brown adipose tissue (BAT), as well as to the pancreas, islets, duodenum, colon, kidneys, adrenal glands and liver.

RESULTS

Resistin administration led to an increased blood flow to the pancreas and islets and a decrease in subcutaneous WAT and BAT. Intra-abdominal white adipose tissue blood flow and that to other organs were not affected.

CONCLUSION

Acute administration of resistin markedly affects the blood perfusion of both the pancreas and subcutaneous white adipose tissue depots. At present it is unknown whether resistin exerts a direct effect on the vasculature, or works through local or systemic activation of endothelial cells and/or macrophages. The extent to which this might contribute to the insulin resistance caused by resistin is yet unknown.

The gastric mucus layers: constituents and regulation of accumulation

The mucus layer continuously covering the gastric mucosa consists of a loosely adherent layer that can be easily removed by suction, leaving a firmly adherent mucus layer attached to the epithelium. These two layers exhibit different gastroprotective roles; therefore, individual regulation of thickness and mucin composition were studied. Mucus thickness was measured in vivo with micropipettes in anesthetized mice [isoflurane; C57BL/6, Muc1-/-, inducible nitric oxide synthase (iNOS)-/-, and neuronal NOS (nNOS)-/-] and rats (inactin) after surgical exposure of the gastric mucosa. The two mucus layers covering the gastric mucosa were differently regulated. Luminal administration of PGE(2) increased the thickness of both layers, whereas luminal NO stimulated only firmly adherent mucus accumulation. A new gastroprotective role for iNOS was indicated since iNOS-deficient mice had thinner firmly adherent mucus layers and a lower mucus accumulation rate, whereas nNOS did not appear to be involved in mucus secretion. Downregulation of gastric mucus accumulation was observed in Muc1-/- mice. Both the firmly and loosely adherent mucus layers consisted of Muc5ac mucins. In conclusion, this study showed that, even though both the two mucus layers covering the gastric mucosa consist of Muc5ac, they are differently regulated by luminal PGE(2) and NO. A new gastroprotective role for iNOS was indicated since iNOS-/- mice had a thinner firmly adherent mucus layer. In addition, a regulatory role of Muc1 was demonstrated since downregulation of gastric mucus accumulation was observed in Muc1-/- mice.

Impaired mucus-bicarbonate barrier in Helicobacter pylori-infected mice

To resist the harsh intrinsic milieu, several lines of defense exist in the stomach. The aim of this study was to investigate the effect of the gastric pathogen Helicobacter pylori on these mechanisms in vivo. We used FVB/N mice expressing human alpha-1,3/4-fucosyl transferase (producing Lewis b epitopes) and inoculated with H. pylori 1. Mice were anesthetized with isoflurane or Hypnorm-midazolam, the stomach was exteriorized, and the surface of the corpus mucosa was exposed. Mucus thickness was measured with micropipettes, juxtamucosal pH (pH(jm)) was measured with pH-sensitive microelectrodes, blood flow was measured with laser-Doppler flowmetry, and mRNA levels of the bicarbonate transporter SLC26A9 were quantified with real-time PCR. The increase in mucosal blood flow seen in response to luminal acid (pH 1.5) in control animals (140 +/- 9% of control) was abolished in infected mice. The firmly adherent mucus layer was significantly thinner in infected mice (31 +/- 2 microm) than in control mice (46 +/- 5 microm), and no mucus accumulation occurred in infected mice. pH(jm) decreased significantly more on exposure to luminal acid in infected mice (luminal pH 1.5, pH(jm) 2.4 +/- 0.7) than in control mice (pH(jm) 6.4 +/- 0.5). Despite reduced pH(jm), SLC26A9 mRNA expression was significantly, by increased 1.9-fold, in infected mice. The reduction in pH(jm) by infection with H. pylori might be due to a reduced firmly adherent mucus layer, increased mucus permeability to H(+), and/or inhibition of bicarbonate transport. The upregulation of SLC26A9 in H. pylori-infected epithelium might be a result of continuous inhibition of the transporter, e.g., by ammonium, a H. pylori product, which has been previously shown to inhibit SLC26A9.

X-ray microanalysis of apical fluid in cystic fibrosis airway epithelial cell lines

The ionic composition of the fluid lining the airways (airway surface liquid, ASL) in healthy subjects and patients with cystic fibrosis (CF) has been a matter of controversy. It has been attempted to resolve conflicting theories by using cell cultures, but published results show a wide variety of values for the ionic concentrations in the apical fluid in these cultures. To investigate CFTR-mediated HCO(3)(-) conductance and the role of HCO(3)(-) in regulating ASL pH we determined the pH of the fluid covering the apical surface of airway epithelial cells. A normal (16HBE14o (-)) and a CF (CFBE41o (-)) bronchial epithelial cell line were grown on membrane inserts in both a liquid-liquid interface culture system for 7 days, and in an air-liquid interface culture system for one month. The elemental composition of the fluid covering the apical surface was determined by X-ray microanalysis of frozen-hydrated specimens, or by X-ray microanalysis of Sephadex beads that had been equilibrated with the apical fluid. Analysis showed that the apical fluid had a Na(+) and Cl(-) concentration of about 80-100 mM and thus was slightly hypotonic. The ionic concentrations were somewhat higher in air-liquid interface than in liquid-liquid interface cultures. The apical fluid in CF cells had significantly higher concentrations of Na and Cl than that in control cultures. In control cultures, the concentrations of Na and Cl in the apical fluid increased if glibenclamide, an inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR) was added to the apical medium. Exposing the cells to the metabolic inhibitor NaCN also resulted in a significant increase of the Na and Cl concentrations in the apical fluid. The results agree with the notion that these cell cultures are mainly absorptive cells, and that ion absorption by the CF cells is reduced compared to that in normal cells. The pH measurements of the fluid covering the apical part of cell cultures support the notion that bicarbonate ions may be transported by CFTR, and that this can be inhibited by specific CFTR inhibitors.

An in vivo model for gastric physiological and pathophysiological studies in the mouse

AIM

In vivo models for studying gastrointestinal physiology and pathophysiology are well established in rats. Since a number of genetically modified mice are available there is a need for reliable mouse models. The aim of this project was to develop an in vivo mouse model for gastrointestinal studies.

METHODS

C57bl/6, NMRI and transgenic FVB/N (expressing human alpha-1,3/4-fucosyltransferase) mice were anaesthetized with isoflurane and the gastric mucosa exteriorized for intravital microscopy. Acid-base status and acid secretion were measured and blood pressure was continuously monitored. Gastric mucosal blood flow was recorded by laser-Doppler flowmetry. Mucus thickness and accumulation rate were measured with micropipettes.

RESULTS

We have developed an in vivo mouse model for studies of the gastric mucosa. With isoflurane anaesthesia the preparation can be studied for up to 5 h with stable blood pressure and mucosal blood flow. Acid-base status agrees with results from other laboratories. Blood flow increased in both C57bl/6 and alpha1.3/4-FT mice in response to luminal HCl, and the mucus gel could be divided into a firmly and a loosely adherent layer, all comparable with results in the rat. However, the firmly adherent mucus layer was thinner (45 +/- 2 microm), and the mucus accumulation rate lower, than in the rat. Furthermore, both basal and stimulated acid secretion showed lower outputs than in the rat.

CONCLUSIONS

This model has great potential for investigations of gastrointestinal physiology and pathophysiology and can be applied for Helicobacter pylori infection studies.

SLC26A9 is expressed in gastric surface epithelial cells, mediates Cl-/HCO3- exchange, and is inhibited by NH4+

HCO3- secretion by gastric mucous cells is essential for protection against acidic injury and peptic ulcer. Herein we report the identification of an apical HCO3- transporter in gastric surface epithelial cells. Northern hybridization and RT-PCR demonstrate the expression of this transporter, also known as SLC26A9, in mouse and rat stomach and trachea (but not kidney). In situ hybridization in mouse stomach showed abundant expression of SLC26A9 in surface epithelial cells with apical localization on immunofluorescence labeling. Functional studies in HEK-293 cells demonstrated that SLC26A9 mediates Cl-/HCO3- exchange and is also capable of Cl--independent HCO3- extrusion. Unlike other anion exchangers or transport proteins reported to date, SLC26A9 activity is inhibited by ammonium (NH4+). The inhibitory effect of NH4+ on gastric HCO3- secretion was also indicated by reduced gastric juxtamucosal pH (pHjm) in rat stomach in vivo. This report is the first to describe the inhibition of HCO3- transport in vitro and the reduction of pHjm in stomach in vivo by NH4+. Given its critical localization on the apical membrane of surface epithelial cells, its ability to transport HCO3-, and its inhibition by NH4+, we propose that SLC26A9 mediates HCO3- secretion in surface epithelial cells and is essential for protection against acidic injury in the stomach. Disease states that are associated with increased ammonia (NH3)/NH4+ generation (e.g., Helicobacter pylori) may impair gastric HCO3- secretion and therefore predispose patients to peptic ulcer by inhibiting SLC26A9.

Inducible nitric oxide synthase is involved in acid-induced gastric hyperemia in rats and mice

The role of different isoforms of nitric oxide synthase (NOS) in the gastric mucosal hyperemia, induced by 155 mM luminal hydrochloric acid (pH approximately 0.8) without a barrier breaker, was investigated. Rats were anesthetized with Inactin (120 mg/kg ip), and mice were anesthetized with Forene (2.2% in 40% oxygen gas at 150 ml/min); the gastric mucosa was exteriorized. Gastric mucosal blood flow was measured with laser-Doppler flowmetry (LDF) in rats treated with Nomega-nitro-l-arginine (l-NNA; unspecific NOS inhibitor), l-N6-(1-iminoethyl)lysine [l-NIL; inducible (i) NOS inhibitor], or S-methyl-l-thiocitrulline [SMTC; neuronal (n) NOS inhibitor], 10 mg/kg, followed by 3 mg. kg-1. h-1 iv, in iNOS-deficient (-/-) and nNOS(-/-) mice. mRNA was isolated from the gastric mucosa in iNOS(-/-) and wild-type (wt) mice, and real-time RT-PCR was performed. The effect of 155 mM acid on gastric mucosal permeability was determined by measuring the clearance of 51Cr-EDTA from blood to lumen. LDF increased by 48 +/- 13% during 155 mM HCl luminally, an increase that was abolished by l-NNA, SMTC, or l-NIL. In iNOS wt mice, LDF increased by 33 +/- 8% during luminal acid. The blood flow increase was attenuated substantially in iNOS(-/-) mice. RT-PCR revealed iNOS mRNA expression in the gastric mucosa in the iNOS wt groups. The blood flow increase in response to acid was not abolished in nNOS(-/-) mice (nNOS-sufficient mice, 39 +/- 18%; heterozygous mice, 25 +/- 19%; -/- mice, 19 +/- 7%). Mucosal permeability was transiently increased during 155 mM HCl. The results suggest that iNOS is constitutively expressed in the gastric mucosa and is involved in acid-induced hyperemia, suggesting a novel role for iNOS in gastric mucosal protection.

The importance of mucus layers and bicarbonate transport in preservation of gastric juxtamucosal pH

Mucus thickness is suggested to be related to mucosal protection. We therefore investigated the importance of the removable mucous layer and epithelial bicarbonate transport in preservation of the gastric juxtamucosal pH (pH(jm)) during luminal acid. Anesthetized rats were prepared for intravital microscopy of the gastric mucosa, and pH(jm) was measured with pH-sensitive microelectrodes. The mucus was either left intact (IM) or removed (MR) down to the firmly attached mucous layer, and HCl (pH 1) was applied luminally. Removal of the loosely adherent mucous layer did not influence the pH(jm) during luminal acid (pentagastrin: IM/MR 7.03 +/- 0.09/6.82 +/- 0.19; pentagastrin + indomethacin: IM/MR 6.89 +/- 0.20/6.95 +/- 0.27; ranitidine: IM/MR 2.38 +/- 0.64/2.97 +/- 0.62), unless prostaglandin synthesis and acid secretion were inhibited (ranitidine + indomethacin: IM/MR 2.03 +/- 0.37/1.66 +/- 0.18). Neutral pH(jm) is maintained during endogenous acid secretion and luminal pH 1, unless DIDS was applied luminally, which resulted in a substantially decreased pH(jm) (1.37 +/- 0.21). Neutral pH(jm) is maintained by a DIDS-sensitive bicarbonate transport over the surface epithelium. The loosely adherent mucous layer only contributes to maintaining pH(jm) during luminal pH 1 if acid secretion and prostaglandin synthesis are inhibited.