The effect of prolonged anti-hypertensive drug treatment on the blood pressure of conscious spontaneously hypertensive rats (SH-rats), and of age-matched normotensive Sprague-Dawley rats was determined during the development of hypertension in SH-rats and in the early stages of established hypertension. A comparison of the vascular reactivity to noradrenaline (NA) and 5-hydroxytryptamine (5-HT) was also made in isolated perfused mesenteric artery preparations from treated and control SH- and Sprague-Dawley rats. 2. Chronic treatment from age 4 to 16 weeks with hydrallazine alone, or a combination of hydrallazine/hydrochlorothiazide/reserpine, ad libitum in the drinking water, prevented the development of hypertension in SH-rats and also reduced the vascular reactivity to NA and 5-HT in isolated vessel preparations from treated compared to control rats. 3. Similar drug treatments started in early established hypertension reduced blood pressure in SH-rats over the 12 week treatment period (from age 8 to 20 weeks) without affecting vascular reactivity to NA and 5-HT in the isolated vessel preparation. 4. Drug treatments had little effect on blood pressure of age-matched Sprague-Dawley rats and no effect on vascular reactivity to NA and 5-HT in the isolated perfused mesenteric artery preparation from treated compared to control rats. 5. These results indicate that the development of increased vascular reactivity and of hypertension in SH-rats occurs simultaneously and, therefore, the vascular changes may be a consequence of the structural changes induced by the raised blood pressure. 6. In established hypertension, no regression of vascular changes was observed despite prolonged reduction of blood pressure. The role of an increased vascular reactivity in the maintenance of hypertension is therefore questionable.
