Effects of tonin on the response to norepinephrine by the aortic strip of the hypertensive rat

The response to norepinephrine (NE) of arterial smooth muscle from two types of experimental hypertensive rats was investigated. Aortic strips from one-kidney, one-clip hypertensive animals were less responsive to NE than those from their normotensive controls but strips from one-kidney, one-clip hypertensive animals showed no difference from their corresponding controls. The contractility in response to NE was the same in all groups. These results suggest that the mechanisms responsible for lesser reactivity in the one-kidney hypertensive group are not a consequence of elevated blood pressure itself but may be related to changes in the intrinsic sensitivity of aortic smooth muscle. Tonin potentiated the contraction induced by NE in aortic strips from hypertensive and normotensive rats. This effect was more pronounced in the one-kidney, one-clip hypertensive animals, so that although the aortic smooth muscle from these animals is less reactive to NE, the decreased reactivity can be more than compensated by the presence of tonin. The mechanism of potentiation is not yet clear but the fact that Saralasin did not inhibit it suggests that angiotensin II is not generated in situ.