1. We have previously shown that among alpha 1-adrenoceptor antagonists used or investigated for the treatment of benign prostatic hyperplasia, tamsulosin discriminates alpha 1-adrenoceptor subtypes in rat tissues whereas alfuzosin and naftopidil do not. We now expand these studies to additional drugs (doxazosin, terazosin) being used and/or investigated for this purpose, and have evaluated all of these drugs at cloned subtypes and in human prostate. 2. Competition binding studies were performed with [3H]-prazosin in membrane samples from rat spleen, kidney and cerebral cortex and human prostate and with cloned alpha 1-adrenoceptors expressed in COS cells. Doxazosin and terazosin did not discriminate alpha 1-adrenoceptor subtypes in rat kidney and cerebral cortex. In contrast, the subtypes present in the tissues were well discriminated by the alpha 1A-adrenoceptor-selective reference drug WB 4101. 3. Alfuzosin, doxazosin, naftopidil and terazosin did not discriminate cloned alpha 1-adrenoceptor subtypes transiently expressed in COS cells whereas tamsulosin and WB 4101 did. 4. In human prostate, alfuzosin, doxazosin, naftopidil and terazosin did not discriminate the alpha 1-adrenoceptor subtypes present in this tissue whereas tamsulosin and the alpha 1A-adrenoceptor-selective reference drugs WB 4101, phentolamine and 5-methylurapidil did. Based on data with the alpha 1A-adrenoceptor-selective drugs, human prostate contains alpha 1A- and alpha 1B-adrenoceptors in an approximate 70:30% ratio. 5. We conclude that tamsulosin, in common with WB 4101, but in contrast to alfuzosin, doxazosin, naftopidil, and terazosin is selective for alpha 1A-adrenoceptors which appear to dominate in the human prostate; the therapeutic relevance of this selectivity remains to be assessed in clinical studies.

We have used the alpha1D-adrenoceptor selective antagonist, BMY 7378, the alpha1D-selective agonists, adrenaline and phenylephrine, the alpha1A-selective antagonists, (+)-niguldipine, SB 216469 and WB4101, and the non-subtype-selective alpha1-adrenoceptor antagonist, nemonapride, to investigate the presence of alpha1D-adrenoceptors in rat tissues at the protein level. Radioligand binding studies using [3H]prazosin as the radioligand were performed in three tissues containing alpha1D-adrenoceptor mRNA, spleen, cerebral cortex and kidney, and in comparison in one tissue not containing alpha1D-adrenoceptor mRNA, liver. Cerebral cortex and kidney were also studied upon alpha1B-adrenoceptor inactivation by chloroethylclonidine treatment (10 microM, 30 min, 37 degrees C). Experiments with cloned rat alpha1-adrenoceptor subtypes transiently expressed in COS cells confirmed the known selectivity of the investigated drugs for alpha1-adrenoceptor subtypes or the lack thereof of nemonapride. Accordingly nemonapride had steep and monophasic competition curves in all native and chloroethylclonidine-treated tissues. BMY 7378 also had steep and monophasic competition curves and low affinity in all native tissues. In contrast, adrenaline and phenylephrine (in the presence of 100 microM GTP) had monophasic competition curves of low affinity in liver and spleen but biphasic competition curves in cerebral cortex and kidney. Following chloroethylclonidine treatment competition curves for adrenaline, phenylephrine, (+)-niguldipine, SB 216469 and WB 4101 remained biphasic in cerebral cortex and kidney while those for nemonapride remained monophasic. We conclude that alpha1D-adrenoceptors are not readily detectable at the protein level in a variety of rat tissues where their mRNA is expressed. The biphasic competition curves of some agonists and antagonists in chloroethylclonidine-treated rat tissues do not represent alpha1D-adrenoceptors and are not readily explained by the present alpha1A/alpha1B/alpha1D-adrenoceptor classification.

1 The affinity of the ax-adrenoceptor antagonist SB 216469 (also known as REC 15/2739) has been determined at native and cloned ao-adrenoceptor subtypes by radioligand binding and at functional acadrenoceptor subtypes in isolated tissues. 2 In radioligand binding studies with [3H]-prazosin, SB 216469 had a high affinity at the aLAadrenoceptors of the rat cerebral cortex and kidney (9.5-9.8) but a lower affinity at the XLBadrenoceptors of the rat spleen and liver (7.7-8.2). 3 At cloned rat al-adrenoceptor subtypes transiently expressed in COS-1 cells and also at cloned human oxl-adrenoceptor subtypes stably transfected in Rat-i cells, SB 216469 exhibited a high affinity at the ala-adrenoceptors (9.6-10.4) with a significantly lower affinity at the alb-adrenoceptor (8.0-8.4) and an intermediate affinity at the ald-adrenoceptor (8.7-9.2).4 At functional ax-adrenoceptors, SB 216469 had a similar pharmacological profile, with a high affinity at the XlA-adrenoceptors of the rat vas deferens and anococcygeus muscle (pA2 = 9.5-10.0), a low affinity at the alB-adrenoceptors of the rat spleen (6.7) and guinea-pig aorta (8.0), and an intermediate affinity at the ClD-adrenoceptors of the rat aorta (8.8). 5 Several recent studies have concluded that the a1-adrenoceptor present in the human prostate has the pharmacological characteristics of the axA-adrenoceptor subtype. However, the affinity of SB 216469 at human prostatic ax-adrenoceptors (pA2=8.1) determined in isolated tissue strips, was significantly lower than the values obtained at either the cloned axa-adrenoceptors (human, rat, bovine) or the native LaIAadrenoceptors in radioligand binding and functional studies in the rat. 6 Our results with SB 216469, therefore, suggest that the ax-adrenoceptor mediating contractile responses of the human prostate has properties which distinguish it from the cloned xia-adrenoceptor or native OxlA-adrenoceptor. Since it has previously been shown that the receptor is not the CXlB-or aIDadrenoceptor, the functional a1-adrenoceptor of the human prostate may represent a novel receptor with properties which differ from any of the a,-adrenoceptors currently defined by pharmacological means.