Preferential noradrenergic innervation of alpha-adrenergic receptors in vascular smooth muscle.

In the isolated perfused hindlimb preparation of the dog, pressor responses to norepinephrine (NE) are mediated by postsynaptic alpha 1- and alpha 2-adrenergic receptors. Results obtained using preferential alpha 1- and alpha 2-adrenergic receptor antagonists suggest that the alpha 1-subtype is predominantly innervated while both alpha 1- and alpha 2-adrenergic receptor subtypes in vascular smooth muscle are accessible to circulating agonists and antagonists. Preliminary studies in the isolated perfused cat spleen support these in vivo findings in the dog. In contrast to vascular smooth muscle, the cat nictitating membrane appears to contain only alpha 1-adrenergic receptors postsynaptically.