1. The response of arterial smooth muscle to noradrenaline was studied in one-clip hypertensive rats with or without the contralateral/kidney and in normotensive rabbits.

2. Strips of aorta from one-kidney, one-clip hypertensive animals were less responsive to noradrenaline than normotensive control rats. The contractile response of strips from two-kidney, one-clip hypertensive animals was not different from the control group. These results suggest that the mechanisms responsible for the lesser reactivity in the one-kidney hypertensive group are not a consequence of elevated blood pressure itself, but may be related to the intrinsic contractility of aortic smooth muscle.

3. Tonin potentiated the contraction induced by noradrenaline in aortic strips from hypertensive and normotensive rats. However, this effect was more important in the one-kidney, one-clip hypertensive animals. In the aortic and mesenteric strips from normal rabbits, tonin produced not only potentiation to noradrenaline but direct contraction.

4. The potentiation to noradrenaline and the direct effect of tonin were not affected by a variety of antagonists but were blocked by a calcium ion antagonist, verapamil, suggesting that tonin may act directly on vascular smooth muscle through mechanisms which might be mediated by calcium ions.

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