A comparison of the cutaneous microvascular properties of the Spontaneously Hypertensive rat and the Wistar-Kyoto rat

The Spontaneously Hypertensive rat (SHR) and its non-hypertensive companion strain, the Wistar-Kyoto (WKY) rat, provide an excellent comparative model to permit study of the differential properties of cutaneous microvascular beds. We explored the possibility that chronically elevated vascular pressu res in the SHR rat might affect the microvascular constitution of the skin. We measured skin blood flow at the back and at the paw of a group of 20-we ek-old WKY rats and a contrast group of SHR rats. We then performed skin bi opsies at these two locations and used the NIH Image program to count and m easure the size of capillaries, arterioles, and venules. We also determined microvascular density as percentage of total tissue area. At basal tempera ture, skin blood flow was similar in the two rat strains at both the back a nd paw. Heat induced vasodilatation resulted in a 50% increase in blood flo w at the back, reaching the same level in the two rat groups. However, at t he paw site, thermal stimulation resulted in significantly greater flow (39 .3 +/- 3.1 ml/100 gm tissue per min) in the SHR rats than the WKY rats (28. 6 +/- 1.9 ml/100 gm tissue per min, P < 0.05). The ratio of systemic arteri al pressure to skin blood flow was computed as an index of vascular resista nce to flow. At basal temperature, this index was 50% greater for the SHR r ats at both skin sites. At 44 degrees C, the resistance index decreased at both sites in both rat groups but was still similar to 50% higher at the ba ck of the SHR than the WKY rats. In contrast, the resistance index at 44 de grees C at the paw site fell to the same level in both the SHR and WKY rats . There were twice as many capillaries at the back of the WKY rats than at the back of the SHR rats (9.2 +/- 2.0 per mm(2) vs. 4.7 +/- 1.2 per mm(2), P < 0.05). Expressed as a percentage of total tissue area, the capillary de nsity at the back in the WKY rats was 0.064 +/- 0.010% as compared to 0.034 +/- 0.008% in the SHR rats (P < 0.05). There were five times more arteriol es at the paw compared to the back in both rat groups with no significant d ifference between the groups. We measured the diameter of the lumen and the thickness of the wall of each arteriole and computed their ratio as an ind ex of possible media hypertrophy. There were minimal differences seen in th ese parameters between the two rat groups at the back and paw sites. The ve nular density was significantly higher at the paw than at the back in both rat groups with no significant difference between them. Reduced capillary d ensity at the back of the SHR rats may be a developmental adaptation to hig h blood pressure. Such a reduction in the pathways of blood flow may help a ccount for increased flow resistance at that site, independent of arteriola r vasoconstriction. (C) 1999 Elsevier Science Inc. All rights reserved.