Biphasic constrictor response of the rabbit ear artery.

The perfused rabbit ear artery shows a biphasic contractile response to intraluminal norepinephrine and sympathetic nerve stimulation. The peak of the first phase occurs after approximately 10 seconds of exposure to NE, and the second, after IK to 2 minutes. The magnitudes of the two responses are similar. The time course of the second phase of contraction is similar to the rate of saturation of the vessel's extracellular space as measured by tissue uptake of tritiated norepinephrine. On the other hand, the first phase of contraction occurs when norepinephrine has penetrated only partly into the media. It is proposed that the two phases of contraction are due to different mechanisms of excitation and that the first phase is associated with excitation of the surface layer of smooth muscle cells with subsequent myogenic propagation of excitation through the thickness of the vessel wall. The second may be related to the local concentration of norepinephrine in the extracellular space of the vessel.