Pathway of insulin in pancreatic tissue on its release by the B-cell.

Insulin was revealed in the extracellular space and blood capillaries of the rat pancreas by applying protein A-gold immunocytochemistry. On the discharge by the B-cell, insulin diffuses in the extracellular space and interacts with the plasma membrane of all pancreatic cells. For the B-cell, the interstitial microvillar plasma membrane domain was preferentially labeled compared with the flat domain. In contrast, the digitations and flat domains of the basal plasma membrane of the acinar cells were equally labeled. In the endothelium, the labeling was superior in fenestrated areas than in the high cytoplasmic ones, the fenestrae, the luminal and abluminal plasma membranes being labeled. In the cytoplasmic areas the plasmalemmal vesicles were significantly labeled; the intercellular junctions were not. These results indicate that upon binding to the membrane, the transfer of insulin across the capillaries occurs through both the fenestrations and the vesicular system. The labeling of insulin in the subendothelial space and blood capillaries decreased significantly from the insular to the peri-insular and further to the teleinsular regions, demonstrating that insulin levels, high in insular tissue, decrease very rapidly as insulin circulates through and out of the pancreas. The pancreatic cells are thus exposed to very high levels of insulin that vary according to the regions, probably contributing to the topographical partition of the acinar parenchyma into peri-insular and teleinsular tissues.