Alpha adrenoceptor sites in vascular smooth muscle. Differentiation by selective antagonist binding
The properties of alpha adrenoceptors in rat-tail artery membranes were studied using tritiated ligands that are selective for the alpha 1 and alpha 2 subtypes. High-affinity saturable binding was obtained for the alpha 1 antagonist prazosin yielding a Bmax of 144 +/- 31.6 fmol/mg protein (mean +/- SEM, N = 3) and a Kd of 0.17 +/- 0.04 nM, and also for the alpha 2 antagonist rauwolscine which yielded a Bmax of 141.3 +/- 19.3 fmol/mg protein and a Kd of 1.57 +/- 0.32 nM. The [3H]prazosin-labelled sites displayed a pharmacological profile characteristic of an alpha 1 adrenoceptor, whereas the [3H]rauwolscine-labelled sites exhibited the expected alpha 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 alpha 1 and alpha 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.