Role of Islet Endothelial Cells in β-cell Function and Growth

The pancreatic islets are collections of endocrine cells, dispersed throughout the pancreas. In adult islets, endocrine cells are closely associated with capillary endothelial cells and receive a high blood perfusion. Transplanted pancreatic islets, on the other hand, have a vascular disturbance, manifested as decreased blood vessel density. Besides impaired islet blood perfusion and oxygenation, this means that the normal close proximity between endothelial cells and β-cell in adult islets is interrupted. The aim of the thesis was to investigate if, and to what extent, β-cells and islet endothelial cells can interact with one another. This hypothesis was investigated during physiological growth of pancreatic islets, following transplantation and in vitro. We observed that islet endothelial and endocrine cell replication coincided immediately after birth, as well as during pregnancy. In pregnant animals, β-cell proliferation colocalized to islets with increased endothelial cell replication, indicating that the two processes were interconnected. The pregnancy hormone prolactin favored endothelial cell replication, and these activated cells could then augment β-cell proliferation. We found that prolactin pretreatment increased blood vessel density and oxygen tension in islets after transplantation. Furthermore, prolactin pretreatment improved endocrine function in a minimal islet transplant model. Partial pancreatectomy performed in association with islet transplantation improved revascularization, oxygen tension and glucose stimulated insulin release from the graft. In conclusion, the findings suggest that endocrine and endothelial cells interact with one another to regulate growth and function in pancreatic islets. This may form the basis for interventions aiming to improve revascularization and function of transplanted islets