Abstract

In spinal and pentobarbital-anesthetized cats, we studied the effect of angiotensin on responses of the nictitating membrane to injected norepinephrine. Angiotensin alone, injected i.a. toward the membrane, caused a slow, small contraction, rarely greater than 5% of maximum. Yet it produced consistently a dose- and time-dependent inhibition of the contractile effect of norepinephrine injected subsequently either also i.a. or i.v. For inhibiting a fixed dose of norepinephrine (ED 30-35%), the threshold dose of angiotensin was 0.01 µg/kg, and the optimum interval between the injection of angiotensin and that of norepinephrine was 20 seconds. Tachyphylaxis to the inhibitory effect of anglotensin was minimal. Vasopressin injected in place of angiotensin exerted a similar inhibitory effect on norepinephrine. Angiotensin also inhibited the response of the nictitating membrane to tyramine. Injected i.a. toward the membrane during, instead of just before, a contractile response to norepinephrine, angiotensin caused the contracted membrane to relax relatively rapidly. This manifestation of angiotensin's inhibitory effect on norepinephrine was mimicked by clamping the ipsilateral external carotid artery and reversed by injecting histamine or bradykinin i.a. toward the membrane. These results show that in vivo on the cat nictitating membrane, the interactions between angiotensin and bloodborne norepinephrine are negative ones due largely, if not entirely, to vasoconstriction produced within the membrane by the peptide.