LUMINAL CONSTRICTIONS DUE TO ENDOTHELIAL-CELLS IN CAPILLARIES OF MOUSE EXOCRINE PANCREAS

During our recent studies of the capillaries in exocrine pancreas of m ouse, 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 constrictio ns on blood how in the acinar capillaries of the pancreas. Intravital observations revealed that bulging endothelial cells were primarily re sponsible for the constrictions. For samples of 100 measurements, good agreement was found between the mean capillary diameters from casts ( 6.3 mu m +/- 0.50 SD) and in vivo (6.2 mu m +/- 0.53 SD), but the mean diameter measurement at constrictions was greater (P < 0.01) in casts (3.9 mu m +/- 0.84 SD) than in vivo (3.5 mu m +/- 1.05 SD). Topical a pplication of norepinephrine caused endothelial nuclear regions to bul ge into the capillary lumen, decreasing the mean diameter at these loc ations to 3.3 mu m +/- 0.9 (SD, n = 21). Based on the 100 in vivo meas urements, the theoretical model predicted that, on average, the constr ictions would reduce flows to 51% of those in fully open vessels. It i s unlikely, however, that the constrictions observed in acinar capilla ries of the pancreas of mouse would result in significant blockage of the vessels by red blood cells. (C) 1995 Academic Press, Inc.