Direct action of norepinephrine on aortic baroreceptors of rat adventitia

The adventitia of the rat aortic arch, with the depressor nerve intact, was separated from the underlying media to obtain a preparation in which the direct effects of norepinephrine on baroreceptor discharge could be examined free of smooth muscle effects. Cyclic stretch was applied to the adventitia to evoke discharge in single aortic baroreceptor fibers. Norepinephrine and phenylephrine in concentrations of 10(-7) to 10(-5) M increased baroreceptor discharge. No effect on discharge was seen with isoproterenol. Phentolamine but not propranolol blocked the increase in discharge produced by norepinephrine. An alpha-adrenergic modulation of baroreceptor discharge that is not dependent on smooth muscle tension has been demonstrated.