MULTIPLE ALPHA(2) ADRENERGIC-RECEPTOR SUBTYPES .1. COMPARISON OF [H-3] RX821002-LABELED RAT R(ALPHA-2A) ADRENERGIC-RECEPTORS IN CEREBRAL-CORTEX TO HUMAN H-ALPHA2A ADRENERGIC-RECEPTOR AND OTHER POPULATIONS OF ALPHA-2-ADRENERGIC SUBTYPES

In the present study, we examined the binding of the alpha-2 adrenergi c receptor (AR) antagonist ]-(2-(2-methoxy-1,4-benzodioxan-2yl)-2-imid azoline ([H-3]RX821002) to alpha-2 AR in rat cerebral cortex (CC) and compared the properties of these sites to those of rat alpha-2A (R(alp ha-2A)) AR in submaxillary gland (SMG), human alpha-2A (H-alpha-2a) AR in human platelets and alpha-2B AR in neonatal rat lung. In the prese nce of guanadinium phosphate, [H-3]RX821002 bound with high affinity t o a large and homogeneous population of sites in CC (K-d = 0.30 +/- 0. 03 nM and B-max = 271 +/- 7 fmol/mg of protein), SMG (K-d = 0.7 and B- max = 274), human platelets (K-d = 0.6 nM and B-max = 189) and neonata l rat lung (K-d = 0.9 and B-max = 161). A total of 34 chemically diver se AR ligands monophasically inhibited the binding of [H-3]RX821002 fr om each site with, for the CC, the most potent ligand being atipamezol e (K-i = 0.2 nM). For all ligands, and at each site, Hill coefficients did not differ significantly from unity. Although the profiles of inh ibition of [H-3]RX821002 were virtually identical in rat CC and SMG, t hese populations revealed several marked differences to human platelet s; the alkaloids, rauwolscine and yohimbine, as well as the benzodioxa ne, {2-(2, methoxyphenoxyethyl)-aminomethyl-1,4-benzodioxane} (WB 4101 ), displayed about IO-fold lower affinity for R(alpha-2A), as compared to H-alpha-2A sites, whereas the benzopyrrolidines, fluparoxan and de s-fluorofluparoxan, showed about 10-fold greater affinity for R(alpha- 2A) sites. Further, whereas the calculation of potency ratios for sele cted pairs of ligands, as well as of correlation coefficients, reveale d virtual identity between R(alpha-2A) AR in CC and SMG, these analyse s revealed that each of these populations of R(alpha-2A) AR clearly di ffered to H-alpha-2A AR in human platelets. In addition, both R(alpha- 2A) AR in rat CC and SMG as well as H-alpha-2A AR in human platelets m arkedly differed to alpha28 AR in neonatal rat lung; thus, they showed 20-fold higher affinity for ,3-dihydroisoindole)methyl)4,5-dihydroimi dazoline} (BRL 44408), oxymetazoline, guanfacine'and guanabenz yet 10- to 100-fold lower affinity for ethyl)-4,4-dimethyl-1,3-(2H,4H)-isoqui nolinedione} (ARC 239) prazosin, chlorpromazine and corynanthine. Simi lar differences in R(alpha-2A) and H-alpha-2A sites to alpha-2C sites were apparent upon analysis of literature data. In conclusion, [H-3]RX 821002 binds to a single population of R(alpha-2A) AR in CC indistingu ishable from R(alpha-2A) AR in rat SMG. These populations of R(alpha-2 A) AR clearly differ to H-alpha-2A AR as concerns the affinities of se veral chemical classes of alpha-2 AR antagonist. Further, many ligands can differentiate R(alpha-2A) AR from rat alpha-2B (and alpha-2C) AR. These data provide a framework for an evaluation of the functional ro les of individual alpha-2 AR subtypes (see companion paper).