Abstract
Polycythemia has marked effects on blood flow through large arteries, but there has been little study of microvascular flow properties, particularly in the hypertensive state. We have previously shown that skin blood flow in the rat had properties very similar to those in man. We used laser Doppler techniques to determine the effect of erythropoietin-induced polycythemia on skin blood flow in 12 Spontaneously Hypertensive Rats (SHR) as compared with 12 normotensive Wistar-Kyoto (WKY) rats. We contrasted flow values at the back and base of tail, nutritively (NUTR) perfused skin sites, with those at the plantar surface of the paw, a site with substantial arteriovenous anastomotic (AVA) perfusion. Erythropoietin treatment produced a substantially greater hematocrit response in the SHR rats, rising from 47 ± 1 to 64 ± 1, than in the WKY animals, who showed an increase from 46 ± 1 to 55 ± 1. Blood pressure was higher in the EPO-treated WKY and SHR rats: 157 ± 4 mm Hg as compared to 148 ± 2 mmg Hg in the WKY animals and 228 ± 4 mm Hg as compared to 201 ± 3 mm Hg, respectively (both p <0.03). Erythropoietin treatment had very little effect on basal skin blood flow in either WKY or SHR rats. However, there was significantly lower heat-stimulated flow at the plantar surface of the paw in the EPO treated rats (WKY: 27.3 ± 1.9 ml/min/100 gm vs 35.5 ± 1.5 ml/min/100 gm in the untreated state; p <0.02; SHR: 32.7 ± 2.6 ml/min/100 gm vs 45.4 ± 2.3 ml/min/100 gm in the untreated state; p <0.01). In contrast, there was little difference at the back or tail. We conclude that erythropoietin-induced polycythemia affects flow in AVA rather than NUTR microvessels. Although raised vascular pressure has produced microvascular flow in our previous studies, the rise in pressure associated with high hematocrit appears to result from an increase in microvascular resistance, decreasing flow. Our study helps clarify clinical observations on the hemodynamic consequences of hematocrit elevation.