Luminal constrictions due to endothelial cells in capillaries of mouse exocrine pancreas

During our recent studies of the capillaries in exocrine pancreas of mouse, numerous local constrictions which reduced the luminal diameter were observed both by scanning electron microscopy of corrosion casts and by in vivo microscopy. In the present study we have identified the features responsible for the constrictions and compared the diameters of vessels and constrictions measured using the two methods. A simple theoretical model was used to predict the effects of such constrictions on blood flow in the acinar capillaries of the pancreas. Intravital observations revealed that bulging endothelial cells were primarily responsible for the constrictions. For samples of 100 measurements, good agreement was found between the mean capillary diameters from casts (6.3 microns +/- 0.50 SD) and in vivo (6.2 microns +/- 0.53 SD), but the mean diameter measurement at constrictions was greater (P < 0.01) in casts (3.9 microns +/- 0.84 SD) than in vivo (3.5 microns +/- 1.05 SD). Topical application of norepinephrine caused endothelial nuclear regions to bulge into the capillary lumen, decreasing the mean diameter at these locations to 3.3 microns +/- 0.9 (SD, n = 21). Based on the 100 in vivo measurements, the theoretical model predicted that, on average, the constrictions would reduce flows to 51% of those in fully open vessels. It is unlikely, however, that the constrictions observed in acinar capillaries of the pancreas of mouse would result in significant blockage of the vessels by red blood cells.