Longitudinal capillary heterogeneity: Implications for active hyperaemia

Active hyperaemia is a process where blood flow is increased to metabolically active cells within a tissue. In skeletal muscle, capillaries are critical in sensing skeletal muscle activity and coordinating the vascular response to direct blood flow to contracting muscle fibres during active hyperaemia. There is evidence to suggest that capillary endothelial cells (ECs) are not a homogenous set of cells and that their characteristics may change along the length of a capillary (Mrazkova et al, 1986). This longitudinal heterogeneity may have implications on capillary function with respect to their role in active hyperemia. Therefore, we sought to determine whether EC characteristics differed along the length of a capillary. We perfused 5 female and 6 male CD-1 mice with fluorescently labelled Wheat Germ Agglutinin (WGA) and GS-1 Isolectin (ISO), two lectin stains that target different components of the EC glycocalyx, WGA binding to N-acetylglucosamine and N-acetylneuraminic acid and ISO targeting a-D-galactosyl residues. We used fluorescence microscopy to view microvascular networks in whole mounts of gluteus maximus (GM), diaphragm (DIA), soleus (SOL), extensor digitorum longus (EDL), and cremaster (CRE) muscles. All muscles were excised and pinned at optimal length and the fluorescence intensity of each stain along the first 200µm of capillaries was measured. We observed that WGA stained the entire vascular network including capillaries across their entire length in all muscles and both females and males. ISO only stained arterioles and very early capillaries and staining was not visible in late capillaries or veins. The brightness of both WGA and ISO decreased significantly along the capillaries, with ISO becoming significantly dimmer than WGA. WGA fluorescence intensity decreased across the first 200um of the capillary by 30.9 ± 2.5% in GM, 11.5 ± 4.0% in DIA, 7.6 ± 2.9% in EDL and 13.1 ± 4.0% in SOL in females while fluorescence intensity decreased by 22.9 ± 3.5% in GM, 16.3 ± 3.3% in DIA, 17.5 ± 3.5% in EDL, 12.0 ± 4.9 in SOL and 18.5 ± 5.1% in CRE in males. ISO fluorescence intensity decreased by 52.8 ± 3.2% in GM, 52.3 ± 3.4% in DIA, 24.9 ± 6.1% in EDL and 41.2 ± 4.2% in SOL respectively in females while it decreased 56.8 ± 3.4% in GM, 57.2 ± 4.0% in DIA, 45.3 ± 3.5% in EDL, 35.0 ± 3.7% in SOL and 53.1 ± 4.1% in CRE in males. There were no differences in the brightness of WGA between muscles in males, however, there were significant differences between muscles in the brightness of ISO in males. Further, there were significant differences in the decrease in brightness between muscles in both WGA and ISO in females. There were also sex differences between females and males. GM, DIA, and SOL all maintained consistent brightness between males and females for both WGA and ISO while EDL brightness was significantly different for both. These data show that the capillary glycocalyx changes along the length of a capillary. More broadly, these data support the suggestion that capillary ECs are heterogeneous along the length of a capillary. This longitudinal heterogeneity could have important implications for how capillaries sense skeletal muscle activity and coordinate vascular responses during active hyperaemia. NSERC This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.