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To investigate the influence of hypertension and of treatment with the vasodilator hydralazine or with the dihydropyridine calcium antagonist isradipine on the morphology of the thoracic aorta in spontaneously hypertensive rats (SHR).The systolic blood pressure (SBP), body weight and morphology of the thoracic aorta were evaluated. The ultrastructure of the smooth muscle component of the aorta and of the tunica intima were analysed by transmission and scanning electron microscopy.The SHR were divided into three groups: a control group, which was left untreated, and two treatment groups, one with 1 mg/kg per day hydralazine and the other with 0.1 mg/kg per day isradipine. Three age-matched groups of Wistar-Kyoto (WKY) rats were included in the study: one group was left untreated and was used as a normotensive reference group, the other two groups were treated with 1 mg/kg per day hydralazine or with 0.1 mg/kg per day isradipine.The SBP did not change in the WKY rats treated with hydralazine, with isradipine or untreated, but was significantly increased in the SHR as a function of age. Both hydralazine and isradipine significantly reduced the SBP in the SHR after the second week of treatment. Light microscopy analysis of the thoracic aorta revealed thickening of the wall of the tunica media as well as an increase in the wall: lumen ratio in the SHR. Treatment with hydralazine had no effect on the morphometric parameters evaluated, whereas isradipine administration significantly reduced the thickening of both the wall and the tunica media of the aorta, and reduced the wall: lumen ratio. No significant modifications in the structure of the thoracic aorta were noticed in the hydralazine- or isradipine-treated WKY rats compared with untreated WKY rats. Transmission and scanning electron microscopy analysis demonstrated in control SHR hypertrophy of smooth muscle cells of the tunica media, an increased size and impairment of the internal elastic lamina, and a widening of the subendothelial space. Hypertrophy of the endothelium was also noticeable in the SHR. Treatment with isradipine reduced the hypertrophy of smooth muscle cells of the tunica media and the structural impairment of the tunica intima. No effect of isradipine treatment on the morphology of the aorta was noticed in the WKY rats.The present results show that the effect of isradipine was different from that of hydralazine. Both compounds lowered the SBP, but only isradipine countered the structural changes of the aorta in the SHR. The effect of isradipine administration is particularly pronounced on the hypertension-dependent changes of endothelium. This suggests that isradipine may have a protective effect on the endothelium.