Flow distribution between the endocrine and exocrine parts of the isolated rat pancreas during perfusion in vitro with different glucose concentrations

Pancreatic islet blood flow and its measurement

Pancreatic islets are richly vascularized, and islet blood vessels are uniquely adapted to maintain and support the internal milieu of the islets favoring normal endocrine function. Islet blood flow is normally very high compared with that to the exocrine pancreas and is autonomously regulated through complex interactions between the nervous system, metabolites from insulin secreting β-cells, endothelium-derived mediators, and hormones. The islet blood flow is normally coupled to the needs for insulin release and is usually disturbed during glucose intolerance and overt diabetes. The present review provides a brief background on islet vascular function and especially focuses on available techniques to measure islet blood perfusion. The gold standard for islet blood flow measurements in experimental animals is the microsphere technique, and its advantages and disadvantages will be discussed. In humans there are still no methods to measure islet blood flow selectively, but new developments in radiological techniques hold great hopes for the future.

Glucose intolerance and reduced islet blood flow in transgenic mice expressing the FRK tyrosine kinase under the control of the rat insulin promoter

The FRK tyrosine kinase has previously been shown to transduce beta-cell cytotoxic signals in response to cytokines and streptozotocin and to promote beta-cell proliferation and an increased beta-cell mass. We therefore aimed to further evaluate the effects of overexpression of FRK tyrosine kinase in beta-cells. A transgenic mouse expressing kinase-active FRK under control of the insulin promoter (RIP-FRK) was studied with regard to islet endocrine function and vascular morphology. Mild glucose intolerance develops in RIP-FRK male mice of at least 4 mo of age. This effect is accompanied by reduced glucose-stimulated insulin secretion in vivo and reduced second-phase insulin secretion in response to glucose and arginine upon pancreas perfusion. Islets isolated from the FRK transgenic mice display a glucose-induced insulin secretory response in vitro similar to that of control islets. However, islet blood flow per islet volume is decreased in the FRK transgenic mice. These mice also exhibit a reduced islet capillary lumen diameter as shown by electron microscopy. Total body weight and pancreas weight are not significantly affected, but the beta-cell mass is increased. The data suggest that long-term expression of active FRK in beta-cells causes an in vivo insulin-secretory defect, which may be the consequence of islet vascular abnormalities that yield a decreased islet blood flow.

Characterization of the Physiological Spaces and Distribution of Tolbutamide in the Perfused Rat Pancreas

PURPOSE

To set up and validate a viable perfused rat pancreas model suitable for pharmacokinetic studies.

MATERIALS AND METHODS

We setup and conducted multiple indicator dilution studies in the single pass perfused rat pancreas. The distribution of the reference markers [99mTc]-red blood cells (RBC), [14C]-sucrose, and [3H]-water, and tolbutamide were analysed using both non-parametric and parametric methods.

RESULTS

The perfusion preparation was observed to be viable by oxygen consumption, outflow perfusate pH, lactate release and insulin release in response to glucose. Parametric analysis of the outflow profiles suggested that the transport of water and tolbutamide from the vascular space was permeability limited. Parametric and nonparametric estimates of Vd for RBC and sucrose were similar and were 0.14+/-0.01, 0.15 0.005 and 0.35+/-0.01 ml/g. The parametric estimate for water, 1.04+/-0.05 ml/g was greater than the nonparametric estimate, 0.89+/-0.02 ml/g. The multiple indicator dilution method Vd of tolbutamide of 0.75+0.08 ml/g was similar to the reported value of 0.73+/-0.04 ml/g estimated by tissue partitioning studies.

CONCLUSIONS

A viable single pass pancreas perfusion model was established and applied to define distribution spaces of reference markers and the distribution kinetics of tolbutamide.

A perfusion protocol for highly efficient transduction of intact pancreatic islets of Langerhans

AIMS/HYPOTHESIS

Successful gene transfer to pancreatic islets might be a powerful tool for dissecting the biological pathways involved in the functional impairment and destruction of beta cells in type 1 diabetes. In the long run, such an approach may also prove useful for promoting islet graft survival after transplantation in diabetic patients. However, efficient genetic modification of primary insulin-producing cells is limited by the specific compact structure of the pancreatic islet. We present here a whole-pancreas perfusion-based transduction procedure for genetic modification of intact pancreatic islets.

MATERIALS AND METHODS

We used flow cytometry analysis and confocal microscopy to evaluate the efficiency of in vitro and perfusion-based transduction protocols that use adenoviral and lentiviral vectors expressing green fluorescent protein. Islet cell viability was assessed by fluorescence microscopy and beta cell function was determined via glucose-stimulated insulin secretion.

RESULTS

In intact rat and human pancreatic islets, adenoviral and lentiviral vectors mediated gene transfer to about 30% of cells, but they did not reach the inner cellular mass within the islet core. Using the whole-pancreas perfusion protocol, we demonstrate that at least in rodent models the centrally located insulin-producing cells can be transduced with high efficiency, while preserving the structural integrity of the islet. Moreover, islet cell viability and function are not impaired by this procedure.

CONCLUSIONS/INTERPRETATION

These results support the view that perfusion-based transduction protocols may significantly improve the yield of successfully engineered primary insulin-producing cells for diabetes research.

Islets transplanted intraportally into the liver are stimulated to insulin and glucagon release exclusively through the hepatic artery

Not much is known about the physiology of intraportally transplanted islets. One reason for this is that it is difficult to study such islets, since they are scattered throughout the liver. We employed a perfusion technique to characterize the functional properties of syngeneic intrahepatic 1-month-old islet grafts, and compared them to islets transplanted beneath the kidney capsule, as well as native islets. The cellular composition of the islet grafts was also examined. Glucose and arginine administered through the hepatic artery, but not through the portal vein, induced insulin release from the intraportally implanted islets. Moreover, arginine, only when administered through the hepatic artery, induced glucagon release from the same islets. The first phase of glucose-stimulated insulin release from both islets transplanted to the liver and kidney was delayed, and less prominent when compared to the pancreas. Intraportally transplanted islets contained fewer glucagon-positive cells than islets transplanted to the kidney and native islets. Our findings demonstrate that intraportally transplanted islets respond with insulin and glucagon to secretagogues, but only when stimulated through the hepatic artery. Whether intrahepatic islets may sense other substances than glucose or arginine occurring in high concentrations in the portal vein following intestinal uptake remains to be studied.

Washout kinetics of blood cells from the perfused pancreas of normoglycemic and diabetic rats

The aim of the present study was to evaluate vascular compartments within the rat pancreas with compartmental analysis of the outflow of blood cells from the perfused gland in situ. The presence of two vascular compartments requiring approximately 15 and 30 min for emptying, was noted in normoglycemic rats. The pancreas from diabetic rats, in which the islet beta-cells had been destroyed by intravenous injection of streptozotocin 1 or 6 weeks earlier, demonstrated the same outflow characteristics. It is therefore likely that these observations reflect the presence of two vascular compartments within the rat pancreas, possibly representing the islet-acinar vasculature and the ductal vasculature.

Alloxan, but not streptozotocin, increases blood perfusion of pancreatic islets in rats

It has recently been shown that selective B-cell toxins alloxan and streptozotocin (STZ) possess marked effects also on the vascular system. To evaluate to what extent changes in blood perfusion of islets induced by alloxan or STZ could be of importance for diabetogenic action of these compounds, we first investigated acute effects of alloxan (75 mg/kg body wt iv) and STZ (40 mg/kg body wt iv) on both whole pancreatic blood flow (PBF) and islet blood flow (IBF) in adult rats. Alloxan caused a marked increase in IBF, which was most pronounced 3 min after administration and remained for 30 min. PBF, however, was decreased 3 min after alloxan administration but was similar to that of control animals from 10 min and onward. These two opposite effects on IBF and PBF caused the fraction of whole PBF diverted through islets to increase from approximately 10 to 50%. Pretreatment with glucose (2 g/kg body wt iv), indomethacin (3.5 mg/kg body wt iv), dimethyl sulfoxide (10 ml/kg body wt ip of a 33% solution), superoxide dismutase (SOD, 1,000 kU/kg body wt iv), NG-methyl-L-arginine (30 mg/kg body wt iv), theophylline (7 mg/kg body wt iv), or terbutaline (1 mg/kg body wt iv) failed to affect stimulation of IBF by alloxan observed at 3 min. SOD was found to exert a marked stimulation of IBF both when given alone and together with alloxan. Alloxan increased IBF and decreased PBF also in a syngeneic pancreaticoduodenal graft in rats but did not affect flow distribution in a perfused pancreas-duodenum preparation.(ABSTRACT TRUNCATED AT 250 WORDS)