Differential sensitivity of the two phases of ear artery contraction to intimal and adventitial norepinephrine.

The biphasic contraction of the rabbit ear artery to norepinephrine (NE) was investigated in the normal (adventitial stimulation) and the everted (intimal stimulation) segment of ear artery. The 2nd phase response showed an intimal ED50 of 8.2 X 10(-8) M which was significantly (p less than 0.05) lower than the adventitial ED50 of 42.6 X 10(-8) M. This difference was abolished by inhibition of neuronal and extraneuronal uptake for NE. The 1st phase response also showed an ED50 for the intimal stimulation (6.9 X 10(-8) M) which was significantly (p less than 0.05) lower than adventitial (65.5 X 10(-8) M). This difference was reduced but not abolished by NE uptake inhibition. This suggests that some feature of the adrenergic neuroeffector apparatus is asymmetrically arranged to favor fast responses to blood borne NE.