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The role of certain resistance vessel characteristics in the etiology of hypertension has been investigated using F2-generation hypertensive/normotensive rats. The F2 populations were bred from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) as well as from stroke-prone SHR (SHRSP) and outbred Wistar-Kyoto rats (WKYO), and are denoted SHR/WKY and SHRSP/WKYO, respectively. Mesenteric resistance vessels (lumen diameter ca 200 μm) were taken from rats in the highest and lowest blood pressure quartiles of 14-week SHR/WKY and 18-week SHRSP/WKYO, as well as from 15-week pure SHR and WKY and from 18-week pure SHRSP and WKYO. The vessels were mounted on a myograph for determination of their structural and contractile characteristics. The structure of the vessels, as expressed for example by their media : lumen ratio, was greatest in the vessels from the SHR and the SHRSP compared to those from the WKY and WKYO. Increased structure was seen in the vessels from the rats in the high blood pressure quartiles of each group of F2 populations, compared to the vessels from the rats in the low blood pressure quartiles, although in the SHR/WKY the difference was smaller than expected if structure were a major determinant of blood pressure. Vascular sensitivity to calcium and the effect of cocaine on noradrenaline sensitivity were increased in the vessels from the SHR and SHRSP, but were the same in the high and low blood pressure quartiles of the F2 populations. However, when activated with noradrenaline, the vessels from the SHR and the SHR/WKY in the high blood pressure quartile showed greater oscillatory activity (rhythmic variations in tension) than the vessels from the WKY and SHR/WKY in the low blood pressure quartile, with similar tendencies as regards the SHRSP and SHRSP/WYKO. It is concluded that increased calcium sensitivity and effects of cocaine in mesenteric resistance vessels are not associated with major causes of increased blood pressure in SHR or SHRSP, but that this could be the case for oscillatory activity of the vessels from the SHR. The increased structure of mesenteric resistance vessels in SHR may not be a major determinant of the increased blood pressure.