The relationship of laser-Doppler skin blood flow measurements to the cutaneous microvascular anatomy

The hairless plantar paw surface of the rat shows high skin blood flow with a substantial response to thermal stimulation. This contrasts with hair- covered areas such as the back, where there is much lower basal flow and thermal response. These properties are similar to the differences seen in humans between skin sites which have a high density of arterioles and venules (AV areas) and sites with predominantly nutritive (NUTR) capillary perfusion. However, there has been no previous study of the microvascular anatomy of rodent skin. We used NIH Image, a quantitative imaging program, to count the capillaries, arterioles, and venules in the skin of the plantar paw surface and the back of 14 Wistar-Kyoto rats. We also used laser-Doppler techniques to determine skin blood flow at these sites. We found significantly more vessels per unit area at the paw. There were twice as many capillaries in the paw (19.6 ± 2.4 per mm ²) compared to the back (9 ± 1.5 per mm ²) (P 0.001). Similarly, there were three times as many venules (11.8 ± 1.2 per mm ² vs 3.48 ± 0.45 per mm ²; P <0.001). The largest difference was in the number of arterioles (7.76 ± 0.74 per mm ² vs 0.79 ± 0.13 per mm ² at the back; P <0.001). The greater microvascular density at the paw was reflected in a threefold higher basal blood flow (6.6 ± 0.44 ml/min/100 g) compared to that in the back (1.99 ± 0.07 ml/min/100 g) (P <0.001). Microvascular volume at the back was 0.14 ± 0.01 x 10 ⁶ RBC/ml in the basal state compared to 0.31 ± 0.01 X 10 ⁶ RBC/ml at the paw. Thus, the increased number of vessels at the paw resulted in a twofold increase in microvascular volume. The plantar paw surface has considerably more vessels than the back. As might be expected, there is a higher proportion of arterioles and venules compared to capillaries at the paw than at the back. Thus, the plantar paw surface is an AV site compared to the back, which is a NUTR site. Although our prior studies have largely assumed that we could use the paw and back as contrast sites comparable to AV and NUTR sites in humans, we have now for the first time conclusively established this fact. The increased microvascular density at the paw results in higher skin blood flow at this site.