A increase in the vascular resistance of virtually every organ system is a wellknown characteristic of hypertension, as modeled by spontaneously hypertensive rats (SHR) and renovascular hypertensive rats. However, the most important issue may be where in the macrovasculature and microvasculature of a given organ the greatest increase in resistance occurs. The relevance of this issue can be considered from two perspectives. First, if a limited region of the vasculature in each organ is

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... e for the bulk of the increased resistance, determining which region is predominantly involved would allow definitive evaluation of the abnormal morphological and behavioral characteristics. Second, if the overall arterial vasculature is relatively uniformly involved in elevating the vascular resistance, a general cellular or control system abnormality of the vessel wall potentially can be identified. In effect, the question to be answered is whether an isolated or generalized vascular problem contributes to the development and maintenance of hypertension. The process of determining the mechanics of increased vascular resistance at the microvascular level during hypertension has taken a variety of approaches. The anatomical approach has been to evaluate morphological characteristics of the vessel wall that might predispose the vasculature to vasoconstriction. In vivo observation of almost every major vascular bed has been used to determine if and where in the vasculature there is an unusual amount of actively or passively induced vasoconstriction. An approach that has received a great deal of attention since described by Hutchins and Darnell 1 is the temporary and permanent closure or rarefaction of arterioles as a means of increasing vascular resistance. All of these approaches have yielded and will continue to yield valuable insight into the hypertensive process. However, unless the percentage of the total resistance occurring at a given level of the microvasculature is known, the importance of a particular morphological or control abnormality is difficult to relate to the increase in vascular resistance. To assist in the problem of relating vessel morphology and behavior to vascular resistance, measurements of pressures within the microvasculature have been made. This type of data is useful to determine the physical forces placed on the vessel-wall as well as the vascular sections associated with an unusually high drop in pressure, which is indicative of a high resistance. Microvascular pressures in the kidney, 2 various skeletal muscles, 3 " 5 small intestine, 5 and cerebral cortex 7 have been measured in SHR. In each of the organs studied, microvascular pressures generally are thought to be grossly elevated in all but the precapillary arterioles, capillaries, and venules. In these latter vascular sections, the microvascular pressures are either close to normal or slightly elevated. In skeletal muscle, 3 " 5 small intestine, 6 and cerebral tissue, 7 the fall in pressure from the largest arterioles to the smallest arterioles is typically two to three times greater than normal in SHR. Meininger et al. 8 have also reported somewhat comparable circumstances of near normal pressures in the small arterioles of skeletal muscle in rats with renovascular and deoxycorticosterone-salt hypertension. Although a higher than normal resistance in the small arterioles may help to minimize increased capillary pressure during hypertension, these small arterioles are not 181 by guest on July 18, 2018 http://hyper.ahajournals.org/ Downloaded from