The evolution of vascular changes in the spontaneously hypertensive rat.

A longitudinal study on the development of vascular lesions was carried out in the spontaneously hypertensive rat (SHR) of the Aoki-Okamoto strain. The aorta and intrarenal arterial vessels were examined at different ages, from 5 to 48 weeks, by light and electron microscopy. Endothelial permeability to injected horseradish peroxidase (HRP) was evaluated in 20-week-old animals. Morphologic differences between vessels of SHRs and age-matched normotensive controls (Wistar-Kyoto strain) were first noted at 10 weeks of age and became more pronounced with time. Vascular pathology involved both intima and media. Medial thickening was seen in both aorta and peripheral arteries and, in the latter, was associated with decreased luminal diameters. These medial changes may contribute to the maintenance of the elevated blood pressure. Intimal lesions affected predominantly the aorta and were characterized by an expansion of the subendothelial space with deposition of acid mucopolysaccharides. There was increased accumulation of tracer HRP in the expanded subendothelium, which suggested enhanced permeability and/or retention of the tracer. In animal species susceptible to atherosclerosis, these intimal changes could serve as the structural basis for the higher propensity for atheromatous lesions in hypertensive individuals. In the SHR, despite stabilization of systolic blood pressure at about 20 weeks of age, both intimal and medial lesions continue to progress and become more extensive and severe; this suggests that not only the severity of hypertension but also its duration are significant determinants of the degree of vascular damage.