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 ß-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.

Blood lipids affect rat islet blood flow regulation through ß3-adrenoceptors.

Pancreatic islet blood perfusion varies according to the needs for insulin secretion. We examined the effects of blood lipids on pancreatic islet blood flow in anesthetized rats. Acute administration of Intralipid to anesthetized rats increased both triglycerides and free fatty acids, associated with a simultaneous increase in total pancreatic and islet blood flow. A preceding abdominal vagotomy markedly potentiated this and led acutely to a 10-fold increase in islet blood flow associated with a similar increase in serum insulin concentrations. The islet blood flow and serum insulin response could be largely prevented by pretreatment with propranolol and the selective ß3-adrenergic inhibitor SR-59230A. The nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester prevented the blood flow increase but was less effective in reducing serum insulin. Increased islet blood flow after Intralipid administration was also seen in islet and whole pancreas transplanted rats, i.e., models with different degrees of chronic islet denervation, but the effect was not as pronounced. In isolated vascularly perfused single islets Intralipid dilated islet arterioles, but this was not affected by SR-59230A. Both the sympathetic and parasympathetic nervous system are important for the coordination of islet blood flow and insulin release during hyperlipidemia, with a previously unknown role for ß3-adrenoceptors.

Effects of Mn-DPDP and manganese chloride on hemodynamics and glucose tolerance in anesthetized rats.

BACKGROUND: Previous studies have demonstrated that magnetic resonance imaging may be a method of choice to visualize transplanted pancreatic islets. However, contrast agents may interfere with microcirculation and affect graft function. PURPOSE: To evaluate the effects manganese-containing contrast media on regional blood flow and glucose tolerance. MATERIAL AND METHODS: Anesthetized rats were injected intravenously with MnCl2 (10 µM/kg body weight) or Mn-DPDP (Teslascan™; 5 µM/kg body weight). Blood flow measurements were made with a microsphere technique 10 min later. In separate animals vascular arteriolar reactivity in isolated, perfused islets was examined. Furthermore, an intraperitoneal glucose tolerance test was performed in separate rats. RESULTS: Glucose tolerance was unaffected by both agents. No changes in regional blood flow were seen after administration of Mn-DPDP, except for an increase in arterial liver blood flow. MnCl2 increased all blood flow values except that of the kidney. MnCl2, but not Mn-DPDP, caused a vasoconstriction in isolated rat islet arterioles but only at very high doses. CONCLUSION: Mn-DPDP administration does not affect glucose tolerance or regional blood flow, besides an increase in arterial hepatic blood flow, and may therefore be suitable for visualization of islets.

Blood flow in endogenous and transplanted pancreatic islets in anesthetized rats: effects of lactate and pyruvate.

OBJECTIVE: The objective of this study was to evaluate the effects of exogenously administered lactate and pyruvate on blood perfusion in endogenous and transplanted islets. METHODS: Anesthetized Wistar-Furth rats were given lactate or pyruvate intravenously, and regional blood perfusion was studied 3 or 30 minutes later with a microsphere technique. Separate rats received a 30-minute infusion of pyruvate or lactate into the portal vein before blood flow measurements. We also administered these substances to islet-implanted rats 4 weeks after transplantation and measured graft blood flow with laser Doppler flowmetry. The expression of monocarboxylate transporter 1 and lactate dehydrogenase A was analyzed. RESULTS: The expression of monocarboxylate transporter 1 and lactate dehydrogenase A was markedly up-regulated in transplanted as compared with endogenous islets. Administration of pyruvate, but not lactate, increased mesenteric blood flow after 3 minutes. Pyruvate decreased mesenteric blood flow after 30 minutes, whereas lactate decreased only islet blood flow. These responses were absent in transplanted animals. A continuous intraportal infusion of lactate or pyruvate increased selectively islet blood flow but did not affect blood perfusion of transplanted islets. CONCLUSIONS: Lactate and pyruvate affect islet blood flow through effects mediated by interactions between the liver and the nervous system. Such a response can help adjust the release of islet hormones during excess substrate concentrations.

Flow distribution during infusion of UW and HTK solution in anaesthetised rats.

PURPOSE: Organ transplantation necessitates the use of preservation solutions to maintain the integrity of the organs during retrieval. The aim of this study was to investigate the flow distribution in abdominal organs in rats during acute infusion of preservation solution. METHODS: Microspheres were used to estimate the distribution of flow in the pancreas, duodenum, ileum, colon, liver, kidneys and lungs in untreated Wistar-Furth rats and in animals with an opened abdominal cavity and catheterised aorta. Some animals were infused by free flow of 5 ml of UW, HTK or Ringer solution containing microspheres at a pressure of 100 cm H(2)O through an intra-aortic catheter. RESULTS: Opening of the abdominal cavity did not affect any of the organ blood flow values. However, the fraction of total pancreatic blood flow diverted through the islets increased. During infusion of microsphere-containing UW, HTK or Ringer solution, splanchnic and renal organ flow values, represented by microsphere contents, were similar. The fraction of microspheres found in the islets was lower in UW-infused rats. The number of microspheres present in the lungs or liver was very low, suggesting that shunting was negligible. CONCLUSIONS: Infusion of HTK and UW solution into anaesthetised rats results in a flow distribution which is similar to that in normal animals in most abdominal organs, but there is a reduction in islet blood perfusion by UW but not HTK solution.

Glucose-induced time-dependent potentiation of insulin release, but not islet blood perfusion, in anesthetized rats.

BACKGROUND: Repeated administration of glucose in vivo leads to a time-dependent potentiation of insulin release. Glucose is also known to stimulate pancreatic islet blood flow, but whether this is associated with a time-dependent potentiation is unknown. We therefore repeatedly administered glucose to anesthetized rats and evaluated effects on insulin release and islet blood flow. METHODS: Male Wistar-Furth rats, anesthetized with thiobutabarbital, were injected intravenously with 1 ml of saline or glucose at times 0, 30 and 60 min. The combinations used were saline + saline + saline (SSS), glucose + saline + saline (GSS), saline + saline + glucose (SSG) and glucose + glucose + glucose (GGG). Regional organ blood flow values were measured 3 min after the final injection with a microsphere technique, and at this time also serum insulin concentrations were determined with ELISA. RESULTS: Serum insulin concentrations as well as total pancreatic, pancreatic islet and duodenal blood flow were higher in SSG and GGG-treated rats when compared to those given SSS and GSS. However, only insulin concentrations, not blood flow values, were higher in GGG rats when compared to SSG animals. CONCLUSIONS: Glucose-induced time-dependent potentiation of insulin release occurs in vivo in thiobutabarbital-anesthetized rats, but is not associated with a further increase in islet blood flow.

Moderate hypothermia induces a preferential increase in pancreatic islet blood flow in anesthetized rats.

The aim of the study was to characterize the effects of induced moderate hypothermia on splanchnic blood flow, with particular reference to that of the pancreas and the islets of Langerhans. We also investigated how interference with the autonomic nervous system at different levels influenced the blood perfusion during hypothermia. For this purpose, hypothermia (body temperature of 28 degrees C) was induced by external cooling, whereas normothermic (37.5 degrees C) anesthetized Sprague-Dawley rats were used as controls. Some rats were pretreated with either propranolol, yohimbine, atropine, hexamethonium, or a bilateral abdominal vagotomy. Our findings suggest that moderate hypothermia elicits complex, organ-specific circulatory changes, with increased perfusion noted in the pylorus, as well as the whole pancreas and the pancreatic islets. The pancreatic islets maintain their high blood perfusion through mechanisms involving both sympathetic and parasympathetic mediators, whereas the increased pyloric blood flow is mediated through parasympathetic mechanisms. Renal blood flow was decreased, and this can be prevented by ganglionic blockade and is also influenced by beta-adrenoceptors.

Arginase increases total pancreatic and islet blood flow in anaesthetized mice.

BACKGROUND: Previous studies have demonstrated that the high basal pancreatic islet blood perfusion is crucially dependent on nitric oxide formation. Arginase can interfere with the formation of nitric oxide by limiting substrate availability. The aim of the present study was to evaluate the influence of arginase on islet blood perfusion in anaesthetized mice. METHODS: The blood perfusion of the pancreatic islets was measured with a microsphere technique in anaesthetized NMRI mice after administration of arginase. RESULTS: Arginase administration increased both total pancreatic and islet blood flow to the same degree. Also adrenal blood flow was increased, whereas other organ blood flow values were unaffected. CONCLUSION: Arginase induces a paradoxical increase in pancreatic and islet blood flow, the reasons for which are still unknown.

Endothelin-1 and pancreatic islet vasculature: studies in vivo and on isolated, vascularly perfused pancreatic islets.

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 (ET(A)) receptor (BQ-123) nor endothelin-B (ET(B)) 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 ET(A) receptors.

Duct ligation and pancreatic islet blood flow in rats: physiological growth of islets does not affect islet blood perfusion.

OBJECTIVES: The aim of this study was to evaluate islet blood-flow changes during stimulated growth of the islet organ without any associated functional impairment of islet function. DESIGN: A duct ligation encompassing the distal two-thirds of the pancreas was performed in adult, male Sprague-Dawley rats. METHODS: Pancreatic islet blood flow was measured in duct-ligated and sham-operated rats 1, 2 or 4 weeks after surgery. In some animals studied 4 weeks after surgery, islet blood flow was also measured also during hyperglycaemic conditions. RESULTS: A marked atrophy of the exocrine pancreas was seen in all duct-ligated rats. Blood glucose and serum insulin concentrations were normal. An increased islet mass was only seen 4 weeks after surgery. No differences in islet blood perfusion were noted at any time point after duct ligation. In both sham-operated and duct-ligated rats islet blood flow was increased during hyperglycaemia; the response was, however, slightly more pronounced in the duct-ligated part of the gland. CONCLUSIONS: Normal, physiological islet growth does not cause any major changes in the islet blood perfusion or its regulation. This is in contrast to findings during increased functional demands on the islets or during deteriorated islet function, when increased islet blood flow is consistently seen.

Marked increase in white adipose tissue blood perfusion in the type 2 diabetic GK rat.

The aim of the present study was to evaluate and cor-relate 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 obese-normoglycemic and type 2 diabetic rats opens the intriguing possibility that changes in this blood perfusion may influence and/or modulate the beta-cell dysfunction in type 2 diabetes.

Changes in graft blood flow early after syngeneic rat pancreas-duodenum transplantation.

Organ transplantation is associated with changes in graft blood flow, both acutely caused by reperfusion associated phenomena, and chronically due to e.g. denervation. The aim of the study was to investigate regional blood flow early after implantation of a syngeneic pancreas-duodenum transplant in rats, i.e. during reperfusion. Warm ischemia time was 1-2 min and cold ischemia 90 min. Blood flow values were measured with coloured microspheres both 10 and 30 min after implantation in transplanted rats, and at one time point in control rats. A marked decrease in the blood perfusion of the transplanted duodenum compared to the endogenous intestine was seen at both 10 and 30 min. Total graft pancreatic blood flow was increased both 10 and 30 min after implantation, whilst islet blood flow remained unchanged compared to the endogenous gland. We conclude that the blood perfusion of the graft is markedly changed in the immediate post-transplantation period, presumably due to reperfusion. However, islet blood perfusion remains constant, suggesting that islet vasculature is less sensitive to changes induced by the implantation.

Lack of compensatory increase in islet blood flow and islet mass in GK rats following 60% partial pancreatectomy.

The effects of a 60% partial pancreatectomy were studied in hyperglycemic GK (Goto-Kakizaki) rats. Partial pancreatectomy or a sham operation was performed on 12-week-old female Wistar rats, GK rats or hybrids between male GK rats and female Wistar rats. Measurements of pancreatic blood flow and islet blood flow were performed by a microsphere technique 2 weeks after surgery. Glucose tolerance was decreased in hybrid compared with Wistar rats, and in GK rats compared with both hybrid and Wistar rats before surgery. Partial pancreatectomy induced minor changes in glucose tolerance. Wistar rats had a decreased islet mass following partial pancreatectomy. Both hybrid and GK rats showed a significant decrease in relative islet volume, but only GK rats in total islet mass, compared with Wistar rats 2 weeks after surgery. Pancreatic blood flow and islet blood flow did not significantly differ between sham-operated Wistar, hybrid or GK rats. After partial pancreatectomy, islet blood flow in relation to islet mass increased 3-fold in Wistar rats and 2-fold in hybrid rats. In contrast, GK rats showed no increase in islet blood flow following partial pancreatectomy. It is concluded that compensatory mechanisms after partial pancreatectomy are operating less efficiently in hybrid and GK rats.

Acute effects of a 50% partial pancreatectomy on total pancreatic and islet blood flow in rats.

OBJECTIVES: The aim of the study was to investigate how an acute increase in functional demand for insulin release affected islet blood perfusion in anesthetized rats. METHODS: We measured total pancreatic and islet blood flow with differently colored microspheres before and 30 minutes after a 50% partial pancreatectomy. RESULTS: The blood glucose concentrations increased in the animals subjected to partial pancreatectomy. The fact that serum insulin concentrations remained unaffected implies that the islets in fact doubled their output of insulin to maintain the same degree of insulinemia. Still, pancreatic islet blood flow was the same as in the sham-operated animals. Likewise, the number of perfused pancreatic islets and the flow distribution between individual islets were not influenced by the partial pancreatectomy. CONCLUSIONS: We conclude that the acute demand for insulin secretion induced by a 50% partial pancreatectomy is not necessarily associated with an acute increase in islet blood perfusion. These findings suggest that basal islet blood flow is high enough to allow for short-term changes in hormone release without simultaneous changes in blood perfusion.

K(ATP) channels and pancreatic islet blood flow in anesthetized rats: increased blood flow induced by potassium channel openers.

K(ATP) channels are important for insulin secretion and depolarization of vascular smooth muscle. In view of the importance of drugs affecting K(ATP) channels in the treatment of diabetes, we investigated the effects of these channels on splanchnic blood perfusion in general and pancreatic islet blood flow in particular. We treated anesthetized Sprague-Dawley rats with the K(ATP) channel openers diazoxide or NNC 55-0118 or the K(ATP) channel closer glipizide. Both diazoxide and NNC 55-0118 dose-dependently increased total pancreatic and islet blood flow in the presence of moderate hyperglycemia, but had no effects on the blood perfusion of other splanchnic organs. Diazoxide markedly lowered the mean arterial blood pressure and thus increased vascular conductance in all organs studied. NNC 55-0118 had much smaller effects on the blood pressure. Glipizide did not affect total pancreatic blood flow, but decreased islet blood flow by 50% in the presence of hypoglycemia. We conclude that K(ATP) channels actively participate in the blood flow regulation of the pancreatic islets and that substances affecting such channels may also influence islet blood flow.

Glucose-induced islet blood flow increase in rats: interaction between nervous and metabolic mediators.

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 A1 receptors. 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.

Multiple injections of coloured microspheres for islet blood flow measurements in anaesthetised rats: influence of microsphere size.

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 x 10(5) microspheres (black colour), with a size of 10 or 15 microm, 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-microm 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-microm microspheres, but not 15-microm 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-microm microspheres induces a high risk for erroneous islet and total pancreatic blood flow measurements, whereas two such measurements can be performed if 10-microm microspheres are used.