Two distinct α1‐adrenoceptor subtypes in rabbit liver:a binding study

1 The characteristics of alpha(1)-adrenoceptor subtypes present on rabbit liver membranes were determined by radioligand binding and compared with the characteristics of binding in rat liver. 2 In saturation experiments using rabbit liver, [H-3]-prazosin bound to two distinct affinity sites (pK(D)=10.3+/-0.19 and 8.13+/-0.17, B-max=11.6+/-3.3 and 657.8+/-198.0 fmol mg(-1) protein, respectively). In studies using rat liver, [H-3]-prazosin bound to a single affinity site (pK(D)=9.98+/-0.27, B-max=190.5+/-38.5 fmol mg(-1) protein). 3 In competition experiments, unlabelled prazosin displaced biphasically the binding of 200 pM [H-3]- prazosin to the rabbit liver; the resulting two pK(I) values (9.85+/-0.08 and 8.01+/-0.09) were consistent with the affinity constants obtained in the saturation experiments. Two sites were also recognized by doxazosin (pK(I) 9.73+/-0.78 and 8.12+/-0.34), 2-(2,6-dimethoxy phenoxyethyl)-aminomethyl-1,4-benzodioxane (WB4101) pK(I) (9.74+/-0.32 and 7.57+/-0.34) and 5-methylurapidil (pK(I) 8.69+/-0.27 and 6.75+/-0.35), and the population of low affinity sites for the three antagonists was approximately 70%. Two distinct affinity constants (pK(I) 8.55+/-0.09 and 7.90+/-0.09) were also calculated for alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy) ethyl)-amino)-propyl)-benzeneacetonitrile fumarate (HV723). 4 By contrast, [H-3]-prazosin binding sites of rat liver membranes were detected as a single population with a high affinity for prazosin (pK(I) 10.01+/-0.08), and doxazosin (pK(I) 9.67+/-0.20) but with a low affinity for WB4101 (pK(I) 8.25+/-0.09), 5-methylurapidil (pK(I) 7.22+/-0.01) and HV723 (pK(I) 8.88+/-0.05). 5 These results indicate the presence of two distinct alpha(1)-adrenoceptor subtypes in the rabbit liver, but only a single site in rat liver. The pharmacological characteristics of prazosin-high and -low sites in rabbit liver suggest identity with alpha(1A) and putative alpha(1L) subtypes, respectively. The site in rat liver is of the alpha(1B) subtype.