Evaluation of alpha1-adrenoceptors in the rabbit iris: pharmacological characterization and expression of mRNA

Subtypes of alpha1-adrenoceptor in rabbit iris have been examined in functional, binding and molecular biological experiments. In functional studies, exogenous and endogenous noradrenaline produced contractions of the iris dilator muscle. The contractile responses to noradrenaline were competitively antagonized by a range of alpha1-adrenoceptor antagonists (pA2 values): prazosin (8.1), WB4101 (8.2), BMY7378 (5.9), YM617 (9.5), JTH-601 (8.8), HV723 (7.8) and KMD-3213 (9.8). The same order of inhibitory potency was seen in the adrenergic responses to electrical stimulation. This affinity profile corresponds well to that of the putative alpha1L-adrenoceptor, which has been proposed in lower urinary tract tissues. In binding studies on rabbit iris membrane however, prazosin, KMD-3213 and WB4101 displayed high affinity (pKd or pKi: 9.6, 10.3, 9.6, respectively), and BMY7378 displayed low affinity (pKi: 6.9). These results show that the binding sites typically correspond to alpha1A-adrenoceptor subtype in character, and we could not detect the significant amount of alpha1L-adrenoceptor subtype. The expression of the three distinct mRNAs that encode proteins of alpha1a-, alpha1b- and alpha1d-adrenoceptors was studied using reverse transcription-polymerase chain reaction (RT-PCR). RT-PCR demonstrated the strongest expression of the alpha1a-adrenoceptor, weak expression of the alpha1b-adrenoceptor and undetectable expression of the alpha1d-adrenoceptor. The present study suggests that alpha1A-adrenoceptor is a major subtype detectable in binding and RT-PCR studies in rabbit iris, but that the adrenergic contractions of iris dilator muscle are mediated via activation of alpha1-adrenoceptor subtype having low affinity for prazosin and WB4101.