The pancreatic islets of Langerhans are highly vascularized micro-organs that play a key role in the regulation of blood glucose homeostasis. The specific arrangement of endocrine cell types in islets suggests a coupling between morphology and function within the islet. Here, we established a line-scanning confocal microscopy approach to examine the relationship between blood flow and islet cell type arrangement by real-time in vivo imaging of intra-islet blood flow in mice. These data were used to reconstruct the in vivo 3D architecture of the islet and time-resolved blood flow patterns throughout the islet vascular bed. The results revealed 2 predominant blood flow patterns in mouse islets: inner-to-outer, in which blood perfuses the core of β cells before the islet perimeter of non-β cells, and top-to-bottom, in which blood perfuses the islet from one side to the other regardless of cell type. Our approach included both millisecond temporal resolution and submicron spatial resolution, allowing for real-time imaging of islet blood flow within the living mouse, which has not to our knowledge been attainable by other methods.

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Pancreatic islets adapt to insulin resistance through a complex set of changes, including β-cell hyperplasia and hypertrophy. To determine if islet vascularization changes in response to insulin resistance, we investigated three independent models of insulin resistance: ob/ob, GLUT4+/−, and mice with high-fat diet–induced obesity. Intravital blood vessel labeling and immunocytochemistry revealed a vascular plasticity in which islet vessel area was significantly increased, but intraislet vessel density was decreased as the result of insulin resistance. These vascular changes were independent of islet size and were only observed within the β-cell core but not in the islet periphery. Intraislet endothelial cell fenestration, proliferation, and islet angiogenic factor/receptor expression were unchanged in insulin-resistant compared with control mice, indicating that islet capillary expansion is mediated by dilation of preexisting vessels and not by angiogenesis. We propose that the islet capillary dilation is modulated by endothelial nitric oxide synthase via complementary signals derived from β-cells, parasympathetic nerves, and increased islet blood flow. These compensatory changes in islet vascularization may influence whether β-cells can adequately respond to insulin resistance and prevent the development of diabetes.

Pancreatic islets are highly vascularized and arranged so that regions containing ␤-cells are distinct from those containing other cell types. Although islet blood flow has been studied extensively, little is known about the dynamics of islet blood flow during hypoglycemia or hyperglycemia. To investigate changes in islet blood flow as a function of blood glucose level, we clamped blood glucose sequentially at hyperglycemic (ϳ300 mg/dl or 16.8 mM) and hypoglycemic (ϳ50 mg/dl or 2.8 mM) levels while simultaneously imaging intraislet blood flow in mouse models that express green fluorescent protein in the ␤-cells or yellow fluorescent protein in the ␣-cells. Using line scanning confocal microscopy, in vivo blood flow was assayed after intravenous injection of fluorescent dextran or sulforhodamine-labeled red blood cells. Regardless of the sequence of hypoglycemia and hyperglycemia, islet blood flow is faster during hyperglycemia, and apparent blood volume is greater during hyperglycemia than during hypoglycemia. However, there is no change in the order of perfusion of different islet endocrine cell types in hypoglycemia compared with hyperglycemia, with the islet core of ␤-cells usually perfused first. In contrast to the results in islets, there was no significant difference in flow rate in the exocrine pancreas during hyperglycemia compared with hypoglycemia. These results indicate that glucose differentially regulates blood flow in the pancreatic islet vasculature independently of blood flow in the rest of the pancreas.THE ISLETS OF LANGERHANS are highly vascularized miniorgans within the pancreas. Researchers have long observed an intimate relationship between pancreatic islets and their distinguished vasculature (7). The vessels within the islet are more numerous and more tortuous than vessels in the surrounding exocrine tissue (2, 4). These characteristics of islet vasculature are likely related to the islets' function of sensing and responding to changes in blood glucose. The morphology of the pancreatic islet is notable in that regions containing ␤-cells are distinct from those containing ␣-and ␦-cells (19). However, investigations of the physiological importance of this distinct cellular organization of the islet and its vasculature are limited (19,26,27).Prior studies have shown two predominant islet blood flow patterns in murine islets (6,18,23,25). The most common pattern is where the blood initially perfuses the islet core and then the islet perimeter (3,18,22,25). In the second pattern, blood flows from one side of the islet to the other side, and thus perfuses core and perimeter areas simultaneously (16,18). The majority of these experimental data, however, have been gathered irrespective of blood glucose levels. Thus, little is known about islet blood flow during different metabolic states or changes in blood glucose levels. Since it has been suggested that glucose and/or insulin levels affect blood flow (10), we examined the islet blood flow dynamics as a function of blood glucose level.To examine ...