Identification of Distinct Luminal Domains for Macromolecules, Erythrocytes, and Leukocytes Within Mammalian Capillaries

TLDR

The glycocalyx and associated plasma proteins form a thick endothelial surface coat that limits the functional capillary volume available to plasma macromolecules and blood cells. The study aimed to compare the anatomic and functional diameters available for macromolecules, red blood cells, and white blood cells in hamster cremaster muscle capillaries. Functional diameters were estimated by measuring the widths of RBCs, WBCs, and the intracapillary distribution of systemically injected FITC‑dextran 70. WBCs occupy the full anatomic capillary diameter, whereas RBCs and FITC‑dextran occupy smaller widths; light‑dye treatment expands RBC and dextran widths and increases capillary hematocrit, supporting a 0.4–0.5 µm endothelial surface coat as the active blood‑wall interface.

Abstract

A thick endothelial surface coat consisting of the glycocalyx and associated plasma proteins has been hypothesized to reduce functional capillary volume available for flowing plasma macromolecules and blood cells. The purpose of this study was to compare anatomic and functional capillary diameters available for macromolecules, RBCs, and WBCs in hamster cremaster muscle capillaries. Bright-field and fluorescence microscopy provided similar estimates (mean±SE) of the anatomic capillary diameter: 5.1±0.1 μm (bright field, 39 capillaries in 10 animals) and 5.1±0.2 μm (membrane dye PKH26, 18 capillaries in 2 animals). Estimates of functional diameters were obtained by measuring the width of RBCs and WBCs and the intracapillary distribution of systemically injected fluorescein isothiocyanate (FITC)–dextran 70. WBCs (5.1±0.2 μm) fully occupied the anatomic capillary cross section. In contrast, the widths of RBCs (3.9±0.2 μm, 21 capillaries in 8 animals) and FITC-dextran (4.3±0.2 μm, 21 capillaries in 8 animals) were significantly smaller than the anatomic capillary diameter. Continuous (1- to 5-minute) excitation of fluorochromes in the capillary lumen (light-dye treatment) increased the width of RBCs passing the treated site from 3.6±0.3 to 4.4±0.3 μm (6 capillaries in 4 animals) and the width of the FITC-dextran column from 4.1±0.2 to 4.6±0.3 μm (10 capillaries in 7 animals). Furthermore, light-dye treatment increased capillary tube hematocrit by 60% in 40-μm-long capillary segments compared 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-μm-thick endothelial surface coat, which may represent the true active interface between blood and the capillary wall.