Is alpha1D-adrenoceptor protein detectable in rat tissues?

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.