Alpha adrenoceptor sites in vascular smooth muscle: Differentiation by selective antagonist binding

Abstract

The properties of alpha adrenoceptors in rat-tail artery membranes were studied using tritiated ligands that are selective for the α 1 and α 2 subtypes. High-affinity saturable binding was obtained for the α 1 antagonist prazosin yielding a B max of 144 ± 31.6 fmol/mg protein (mean ± SEM, N = 3) and a K d of 0.17 ± 0.04 nM, and also for the α 2 antagonist rauwolscine which yielded a B max of 141.3 ± 19.3 mol/ mg protein and a K d of 1.57 ± 0.32 nM. The [ 3H]prazosin-labelled sites displayed a pharmacological profile characteristic of an α 1 adrenoceptor, whereas the [ 3H]rauwolscine-labelled sites exhibited the expected α 2 adrenoceptor profile. Agonist affinity for [ 3H]rauwolscine sites was reduced by Gpp(NH)p and Na +, and the effects appeared synergistic for adrenaline, but non-interactive for UK-14304. Agonist interaction with [ 3H]prazosin sites in the rat-tail artery was also regulated by Gpp(NH)p and Na +, although clearly in a qualitatively and quantitatively different manner from the [ 3H]rauwolscine sites. These results suggest that distinct binding sites for [ 3H]prazosin and [ 3H]rauwolscine could be differentiated with antagonist ligands. These distinct antagonist recognition sites demonstrate the pharmacological profile expected for α 1 and α 2 adrenoceptors, and the quantitatively differing abilities of Na + and Gpp(NH)p to regulate agonist interactions with these sites are suggestive, but do not necessarily prove, that different G proteins may be involved in this regulation.