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.

This study investigated the mechanisms for glucose-induced islet blood flow increase in rats. The effects of adenosine, adenosine receptor antagonists, and vagotomy on islet blood flow were evaluated with a microsphere technique. Vagotomy prevented the islet blood flow increase expected 3, 10, and 20 min after injection of glucose, whereas theophylline (a nonspecific adenosine receptor antagonist) prevented the islet blood flow increase from occurring 10 and 20 min after glucose administration. Administration of selective adenosine receptor antagonists suggested that the response to theophylline was mediated by A1receptors. Exogenous administration of adenosine did not affect islet blood flow, but local accumulation of adenosine, induced by the adenosine uptake inhibitor dipyridamole, caused a doubling of islet blood flow. In conclusion, the increased islet blood flow seen 3 min after induction of hyperglycemia is caused by the vagal nerve, whereas the increase in islet blood perfusion seen at 10 and 20 min after glucose administration is caused by both the vagal nerve and adenosine.

We investigated if coloured microspheres could be used for repeated measurements of pancreatic islet blood flow in rats. An initial injection of 1.0-1.5 10 5 microspheres (black colour), with a size of 10 or 15 m, was made into the ascending aorta, while an arterial reference sample was collected from the femoral artery. Twelve min later, 1 ml of saline or 30% D-glucose was injected intravenously. Three min after this injection a second injection of 10-or 15-m microspheres (green colour) was given. The animals were then killed, and the pancreas and adrenals were removed and samples (150-200 mg) were secured from the duodenum, ileum, colon, right kidney and liver. The microsphere contents were determined with the aid of a freeze-thawing technique and blood flow values were calculated. Our results suggest that 10-m microspheres, but not 15-m microspheres, provide reproducible islet and total pancreatic blood flow measurements when repeatedly injected. Values for the blood flow to the intestines, kidney and liver were less sensitive to the size of the microspheres. We conclude that repeated administration of 15-m microspheres induces a high risk for erroneous islet and total pancreatic blood flow measurements, whereas two such measurements can be performed if 10-m microspheres are used.

The aim of the present study was to evaluate and correlate islet to brown and white adipose tissue (WAT) blood perfusion in one obese rat and one nonobese rat with type 2 diabetes (obese Zucker [OZ] and GK rats, respectively). We measured blood perfusion with a microsphere technique in anesthetized animals and subsequently estimated the blood flow to seven different WAT depots and brown adipose tissue, in addition to the whole pancreas and pancreatic islets. Both GK and OZ rats had higher islet blood perfusion than their respective control strains. Adipose tissue blood flow (ATBF) was similar to or lower than that of controls in the normoglycemic OZ rats. GK rats, however, had 5-10 times higher blood perfusion than control Wistar rats in most WAT depots. Vascular density and macrophage numbers in WAT did not differ between the different strains. The discrepancy in ATBF between the obesenormoglycemic and type 2 diabetic rats opens the intriguing possibility that changes in this blood perfusion may influence and/or modulate the ␤-cell dysfunction in type 2 diabetes. Diabetes 54

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 insulinproducing 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.

Lai EY, Persson AE, Bodin B, Källskog Ö , Andersson A, Pettersson U, Hansell P, Jansson L. Endothelin-1 and pancreatic islet vasculature: studies in vivo and on isolated, vascularly perfused pancreatic islets. Am J Physiol Endocrinol Metab 292: E1616 -E1623, 2007. First published February 6, 2006; doi:10.1152/ajpendo.00640.2006.-Endothelin-1 (ET-1) is a potent endothelium-derived vasoconstrictor, which also stimulates insulin release. The aim of the present study was to evaluate whether exogenously administered ET-1 affected pancreatic islet blood flow in vivo in rats and the islet arteriolar reactivity in vitro in mice. Furthermore, we aimed to determine the ET-receptor subtype that was involved in such responses. When applying a microsphere technique for measurements of islet blood perfusion in vivo, we found that ET-1 (5 nmol/kg) consistently and markedly decreased total pancreatic and especially islet blood flow, despite having only minor effects on blood pressure. Neither endothelin A (ETA) receptor (BQ-123) nor endothelin-B (ETB) receptor (BQ-788) antagonists, alone or in combination, could prevent this reduction in blood flow. To avoid confounding interactions in vivo, we also examined the arteriolar vascular reactivity in isolated, perfused mouse islets. In the latter preparation, we demonstrated a dose-dependent constriction in response to ET-1. Administration of BQ-123 prevented this, whereas BQ-788 induced a right shift in the response. In conclusion, the pancreatic islet vasculature is highly sensitive to exogenous ET-1, which mediates its effect mainly through ETA receptors. endocrine pancreas; endothelium; blood flow regulation THE BLOOD PERFUSION OF THE PANCREATIC ISLETS is normally adapted to the metabolic needs of the tissue (5, 23). A prerequisite for an adequate endocrine function is the ability of the islets to rapidly react to changes in nutrient concentrations with an adequate release and dispersal of hormones. This necessitates the means to regulate the islet blood flow so that this dispersal can be achieved. To obtain this, the blood perfusion is regulated through metabolic, nervous, hormonal, and paracrine mechanisms (5,7,23). Recently, the importance of endothelium-derived factors for the islet blood flow responses has been highlighted, especially with regard to nitric oxide (NO) (35,46) and ANG II (6).Endothelin (ET) is one endothelium-derived substance that is important, not only for maintenance of normal blood pressure (17) but also for blood flow regulation in general (11,15,20,33). ET exists in three isoforms in humans (ET-1, ET-2, and ET-3) (21), with ET-1 being the most well characterized. The primary target of ET-1 is the vasculature, and increased plasma concentrations are encountered during most diseases involving the cardiovascular system (31, 34, 44), including diabetes (20). ET-1 binds to two types of G-protein-coupled receptors, namely, ET A receptors, which mediate constrictive effects on vascular smooth muscle cells (VSMC), and ET B receptors, which mediate vasodilation ...

Serum insulin concentration per se does not affect islet blood flow, whereas the ambient blood glucose concentration is of major importance in this context.

Nitric oxide derived from the activity of nNOS contributes to the blood perfusion in the upper portions of the gastrointestinal tract, viz. the parts supplied by the cranial mesenteric artery, and the kidneys, whilst no effects are seen on colonic or adrenal blood flow. Pancreatic islet blood flow was unaffected by nNOS inhibition, thereby suggesting that NO derived from the other isoforms of NOS maintains the high basal islet blood perfusion.