IDENTIFICATION OF DISTINCT LUMINAL DOMAINS FOR MACROMOLECULES, ERYTHROCYTES, AND LEUKOCYTES WITHIN MAMMALIAN CAPILLARIES

A thick endothelial surface coat consisting of the glycocalyx and asso ciated plasma proteins has been hypothesized to reduce functional capi llary volume available for flowing plasma macromolecules and blood cel ls. The purpose of this study was to compare anatomic and functional c apillary diameters available for macromolecules, RBCs, and WBCs in ham ster cremaster muscle capillaries. Bright-field and fluorescence micro scopy provided similar estimates (mean+/-SE) of the anatomic capillary diameter: 5.1+/-0.1 mu m (bright field, 39 capillaries in 10 animals) and 5.1+/-0.2 mu m (membrane dye PKH26, 18 capillaries in 2 animals). Estimates of functional diameters were obtained by measuring the widt h of RBCs and WBCs and the intracapillary distribution of systemically injected fluorescein isothiocyanate (FITC)-dextran 70. WBCs (5.1+/-0. 2 mu m) fully occupied the anatomic capillary cross section. In contra st, the widths of RBCs (3.9+/-0.2 mu m, 21 capillaries in 8 animals) a nd FITC-dextran (4.3+/-0.2 mu m, 21 capillaries in 8 animals) were sig nificantly smaller than the anatomic capillary diameter. Continuous (1 - to 5-minute) excitation of fluorochromes in the capillary lumen (lig ht-dye treatment) increased the width of RBCs passing the treated site from 3.6+/-0.3 to 4.4+/-0.3 mu m (6 capillaries in 4 animals) and the width of the FITC-dextran column from 4.1+/-0.2 to 4.6+/-0.3 mu m (10 capillaries in 7 animals). Furthermore, light-dye treatment increased capillary tube hematocrit by 60% in 40-mu m-long capillary segments c ompared with untreated sites in the same capillaries. It is concluded that the wall of skeletal muscle capillaries is decorated with a 0.4- to 0.5-mu m-thick endothelial surface coat, which may represent the tr ue active interface between blood and the capillary wall.