The effects of SB 216469, an antagonist which discriminates between the α(1A)-adrenoceptor and the human prostatic α₁-adrenoceptor

1 The affinity of the α₁-adrenoceptor antagonist SB 216469 (also known as REC 15/2739) has been determined at native and cloned α₁-adrenoceptor subtypes by radioligand binding and at functional α₁-adrenoceptor subtypes in isolated tissues. 2 In radioligand binding studies with [³H]-prazosin, SB 216469 had a high affinity at the α(1A)-adrenoceptors of the rat cerebral cortex and kidney (9.5-9.8) but a lower affinity at the α(1B)-adrenoceptors of the rat spleen and liver (7.7-8.2). 3 At cloned rat α₁-adrenoceptor subtypes transiently expressed in COS-1 cells and also at cloned human α₁-adrenoceptor subtypes stably transfected in Rat-1 cells, SB 216469 exhibited a high affinity at the α(1a)-adrenoceptors (9.6-10.4) with a significantly lower affinity at the α(1b)-adrenoceptor (8.0-8.4) and an intermediate affinity at the m-adrenoceptor (8.7-9.2). 4 At functional α₁-adrenoceptors, SB 216469 had a similar pharmacological profile, with a high affinity at the α(1A)-adrenoceptors of the rat vas deferens and anococcygeus muscle (pA₂ = 9.5-10.0), a low affinity at the α(1B)-adrenoceptors of the rat spleen (6.7) and guinea-pig aorta (8.0), and an intermediate affinity at the α(1D)-adrenoceptors of the rat aorta (8.8). 5 Several recent studies have concluded that the α₁-adrenoceptor present in the human prostate has the pharmacological characteristics of the α(1A)-adrenoceptor subtype. However, the affinity of SB 216469 at human prostatic α₁-adrenoceptors (pA₂ = 8.1) determined in isolated tissue strips, was significantly lower than the values obtained at either the cloned α(1a)-adrenoceptors (human, rat, bovine) or the native α(1A)-adrenoceptors in radioligand binding and functional studies in the rat. 6 Our results with SB 216469, therefore, suggest that the α₁-adrenoceptor mediating contractile responses of the human prostate has properties which distinguish it from the cloned α₁-adrenoceptor or native α(1A)-adrenoceptor. Since it has previously been shown that the receptor is not the α(1B)- or α(1D)- adrenoceptor, the functional α₁-adrenoceptor of the human prostate may represent a novel receptor with properties which differ from any of the α₁-adrenoceptors currently defined by pharmacological means.