1 The pharmacological characteristics of cloned mammalian alpha1A/D-, alpha1B- and alpha1C-adrenoceptor subtypes expressed in rat 1 fibroblasts were determined in comparison to the binding and functional properties of these subtypes in rat tissues. 2 Analysis of [H-3]-prazosin binding to membrane homogenates from rat 1 fibroblast cells expressing each of the alpha1-subtypes indicated high affinity binding to a single population of binding sites. Binding affinities were similar for alpha1A/D-, alpha1B-, and alpha1C-subtypes (K(d)s: 0.13, 0.10 and 0.15 nm respectively) although a higher density of alpha1B- and alpha1C-receptors (B(max): 4068 and 10,323 fmol mg-1 protein respectively) were expressed in comparison to alpha1A/D (838 fmol mg-1). 3 Displacement of [H-3]-prazosin from membranes expressing cloned alpha1-adrenoceptor subtypes revealed that 5-methyl-urapidil, WB4101, benoxathian and phentolamine displayed high affinity and selectivity for alpha1A/D- over alpha1B-subtypes. These compounds also had high affinity and selectivity for alpha1C- over alpha1B-subtypes. 5-Methyl-urapidil showed selectivity for alpha1C (K(i) 0.60 +/- 0.16 nM) over both alpha1A/D (K(i), 9.8 +/- 2.8 nm) and alpha1B (K(i) 57.2 +/- 12 nm) subtypes. Prazosin and doxazosin were not subtype selective. 4 In comparison to [H-3]-prazosin a similar pharmacological profile was obtained with [I-125]-HEAT using cloned alpha1/D-, alpha1B- and alpha1C-adrenoceptors expressed in rat 1 fibroblasts. 5 The affinities of prazosin, WB 4101, 5-methyl-urapidil, phentolamine and benoxathian at cloned alpha1A/D-receptors were consistent with alpha1A affinities determined with chlorethylclonidine-treated rat cortical membranes. Affinities at cloned alpha1B-receptors were consistent with alpha1B affinities determined with rat liver membranes. 6 Using the epididymal rat vas deferens as a functional measure of alpha1A affinity, prazosin (pA2 9.23 +/- 0.28), WB 4101 (pA2 9.58 +/- 0.12), phentolamine (pK(B) 7.90 +/- 0.16), benoxathian (pK(B) 9.21 +/- 0.21) and 5-methyl-urapadil (pK(B) 8.51 +/- 0.16) were potent antagonists of noradrenaline-induced contractions. 7 At present, evidence from cloning studies suggests the existence of at least three alpha1-adrenoceptor subtypes. In contrast to the recent proposal for alpha1-adrenoceptor classification, the pharmacology of the cloned alpha1A/D (or alpha1D)-adrenoceptor is more consistent with that of an alpha1A-adrenoceptor characterized in rat cerebral cortex and vas deferens.
