Identification of α₁ adrenoceptor subtypes in the rabbit prostate

1 The alpha(1)-adrenoceptor subtypes of rabbit prostate were characterized in binding and functional experiments. 2 In saturation experiments, [H-3]-prazosin bound to two distinct affinity sites in the rabbit prostate (pK(D) = 11.20 +/- 0.22 and 8.39 +/- 0.11, B-max = 15.3 and 736 fmol mg protein(-1)). 3 In the displacement experiments, the binding was inhibited with shallow displacement curves by unlabelled prazosin, WB4101, and 5-methylurapidil, suggesting the presence of two distinct affinity sites for prazosin, WB4101, or 5-methylurapidil. On the other hand, HV723 displaced the [H-3]-prazosin binding monophasically with a low affinity. From the results, the presence of two distinct alpha(1)-adrenoceptor subtypes was suggested; presumably one is the classical alpha(1A) (cloned alpha(1C)) subtype with high affinity for prazosin, WB4101 and 5-methylurapidil but not for HV723 and the other corresponds to the alpha(1L) subtype, which shows low affinity for the four antagonists. 4 In the functional experiments, prazosin, WB4101, HV723 and 5-methylurapidil competitively antagonized the contractile response to noradrenaline with low affinities close to those for the alpha(1L) subtype determined in binding experiments. These results suggest that contractile response to noradrenaline in the rabbit prostate is predominantly mediated through the alpha(1L) subtype.