Stationary red blood cells induce a negative charge on mucosal capillary endothelium

Previous studies on Wistar-Furth rat intestinal mucosal capillaries show a variation in extent of binding of cationized ferritin (CF) to the endothelial surface. Three possible causes of this variation were investigated: 1) location of capillaries along the intestine, 2) time after feeding, and 3) effect of short-term hemostasis. In studies 1 (5 rats) and 2 (22 rats), the intestinal circulation was perfused with CF and perfusion fixed for electron microscopy. Observation of capillaries demonstrated that variation in CF binding could not be explained by factors 1 or 2. In study 3, capillaries on the mucosal surface were viewed using epifluorescent microscopy and were perfused with fluorescein isothiocyanate (FITC)-labeled CF. Initial perfusion produced no binding, but perfusion after 2 or 10 min of stasis gave extensive binding (19 rats). Stasis with red blood cells in saline (6 rats) or with hemoglobin solution (6 rats) gave similar results, but stasis with saline, platelet-rich plasma, or red cell ghosts in saline did not produce binding (8, 7, and 5 rats, respectively). Hemoglobin injected into the circulation without stasis also caused binding of FITC-CF, but not if the nitric oxide (NO) donor, SIN-1, was injected simultaneously. Stasis with saline containing the NO inhibitor, NG-nitro-L-arginine methyl ester produced some binding of FITC-CF (5 rats). We conclude that intestinal mucosal capillaries do not express a net negative surface charge under brisk flow conditions, but charge is quickly generated after blood stasis. It is possible that when red blood cells are stationary, hemoglobin may trap NO that otherwise might neutralize the negatively charged groups.