Types and functions of peripheral alpha-adrenoceptors.

Both in the periphery and in the central nervous system, alpha-adrenoceptors can be subdivided into alpha 1-types. alpha 1-Receptors are sensitive to low concentrations of the agonists phenylephrine and methoxamine, and of the antagonists corynanthine and prazosin. alpha 1-Receptors are sensitive to low concentrations of the agonists clonidine and tramazoline, and of the antagonists yohimbine and rauwolscine. Although it has been shown recently that some smooth muscle tissues contain postsynaptic, contraction-mediating alpha 2-adrenoceptors, the smooth muscle of rabbit aorta and pulmonary artery possesses only alpha 1-receptors. In rabbit portal vein, practically all of the effect of exogenous agonists is mediated by alpha 1-receptors. On the other hand, prazosin resistance and rauwolscine sensitivity of the contractile response to transmural sympathetic nerve stimulation indicates that released noradrenaline may act on a small population of postsynaptic alpha 2-receptors as well.