Evidence of alpha 2-adrenoceptors in the nasal blood vessels of the dog.

alpha-Adrenoceptor subtypes were demonstrated in an in vitro preparation of the vascular smooth muscle of the canine nasal mucosa. alpha 1-Receptors were located on the surface of vascular smooth muscle (postsynaptic). alpha 2 Subtypes were situated both at postsynaptic and presynaptic sites on adrenergic nerve terminals. Presynaptic alpha 2-adrenoceptor stimulation yielded an inhibition of excessive norepinephrine release from nerve terminals. Stimulation of postsynaptic receptors, either alpha 1 or alpha 2, produced vasoconstriction. Although there was good evidence of the presence of postsynaptic alpha 2-receptors, they did not appear to be dominant among the postsynaptic receptors.