Structural and functional alterations of mesenteric vascular beds in spontaneously hypertensive rats.

The morphology and reactivity of mesenteric arteries from spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar Kyoto rats (WKY) were investigated. Isolated, perfused mesenteric vascular beds were prepared from 6-, 11- and 18-week-old SHR and WKY. At these ages, the walls and media of large mesenteric arteries were significantly thicker in SHR than in WKY. The number of smooth muscle cell layers in the media was significantly larger in SHR than in WKY. This difference between SHR and WKY increased as rats grew older, in parallel with differences in the blood pressure. Flow rate-perfusion pressure curves indicated that the vascular basal resistance to flow increased more profoundly in SHR preparations than in WKY preparations as rats grew older. This may be related to the structural alterations of the resistance vessel wall in SHR. The pressor responses to KCl were greater in SHR preparations than in WKY preparations as rats grew older. This may be caused partly by the increase of the number of smooth muscle cell layers in the media of SHR resistance vessels. The pressor response to norepinephrine (NE) was significantly higher in SHR preparations than in WKY preparations at all ages investigated. In marked contrast to the vascular basal resistance and the pressor response to KCl, the pressor response to NE was extremely exaggerated in SHR at the age of 6 weeks. This extremely high NE response in younger SHR may not be caused by the structural alteration in resistance vessels. It may be caused by a functional change, which is regulated by the signal transduction process in smooth muscle cells of resistance vessels. These results suggest that the development of hypertension in SHR may be caused by genetic structural and functional abnormalities of resistance vessels. Both abnormalities may be caused by the hyperreactivity to NE through an altered signal transduction process in smooth muscle cells of resistance vessels in SHR.