Structural changes of three categories of mesenteric arteries (representing elastic, muscular and arteriolar vessels) from 10- to 12-week-old and 28-week-old spontaneously hypertensive rats (SHR) were studied morphometrically at the light microscope level, and the results compared with age-matched Wistar-Kyoto normotensive rats. In 10- to 12-week-old SHR, hypertrophy of the vessel wall occurred only in the muscular and arteriolar vessels. At 28 weeks, further thickening of the vessel wall occurred in the muscular and arteriolar vessels, and the superior mesenteric artery (elastic vessel) was also thickened in the SHR. There was no evidence that the wall of the relaxed hypertrophied vessels encroached upon the lumen of the vessel. The structural basis for the increase in the vessel wall thickness varied with vessel type. In the superior mesenteric artery, increase in the media at 28 weeks of age would be consistent with hypertrophy of the smooth muscle cells. In the large muscular arteries, at 10–12 weeks of age, increase in medial mass occurred with increase in the number of the smooth muscle cell layers whereas at 28 weeks further increase in media could be due to hypertrophy of the smooth muscle cells. In the small arteriolar vessels, medial enlargement was due at all ages to an increase in the number of smooth muscle layers. Our results show that in the SHR hypertrophy of the media occurs not only in the small arteriolar vessels, but also in large elastic and muscular arteries.

Abstract. Morphometric measurements on different arteries at the light-microscopic level and ultrastructural studies of the mesenteric arteries were carried out in salt-sensitive (DS) and salt-resistant (DR) Dahl rats given a high-salt (8%) or low-salt (0.4%) diet for 6-7 weeks. Hypertension was produced in DS rats given high-salt diet (DS-H), while only moderate hypertension was produced in DS rats given low-salt diet (DS-L). Blood pressure in DR rats given high salt (DR-H) and low salt (DR-L). however, was normal. Cross-sectional area of the media was increased significantly in the superior mesenteric artery (an elastic artery), large mesenteric arteries (muscular arteries) and small mesenteric arteries (small muscular arteries or arterioles) from DS-H rats. In all the vessel types, this increase was positively correlated with the increase in blood pressure. In the superior mesenteric artery, medial wall increase was probably due to an increase in intercellular space, and/or hypertrophy of the smooth muscle cells. Similarly, increase in the media of small mesenteric arteries was probably due to hypertrophy of the smooth muscle cells. In contrast, increase in the media of large mes enteric arteries was related to hyperplasia of the smooth muscle cells. Damage to endothelial cells was noted in the 3 vessel types from DS-H. Intimal lesions composed of myointimal cells were found in the superior mesenteric arteries of all the rat groups. Our results showed that the incidence of these lesion formations was higher in the following order: DS-H> DS-L> DR-H> DR-L, suggesting that the degree of hypertension (DS vs. DR rats) and the amount of salt in the diet (DR-H vs. DR-L) may be some of the factors contributing to the development of these lesions. We conclude that hyperreactivity of the arteries due to increase in medial smooth muscle mass (e.g. muscular arteries), and/or probably impaired relaxation capability f the arteries in the DS-H rats due to endothelial cell damage, may contribute to the elevation of BP in the Dahl model of genetic hypertension.