3H]Bunazosin, a Novel Selective Radioligand of Alpha1 Adrenoceptors in Human Prostates

Abstract

The binding properties of a new radioligand, [3H]bunazosin, were studied in membranes of human prostates with benign prostatic hypertrophy (BPH). Specific binding of [3H]bunazosin was saturable, reversible, and of high affinity (Kd = 0.55 ± 0.04nM). The density of [3H]bunazosin binding sites (Bmax) was 676 ± 33fmol/mg. protein. [3H]Bunazosin rapidly associated with its binding sites in membranes of human prostates and reached steady state by 20min. at 25C. The rate constants for association and dissociation of [3H]bunazosin binding were calculated to be 0.11 ± 0.01/nM/min. and 0.05 ± 0.02/min. (n = 4), respectively. Seven alphai adrenoceptor antagonists competed with [3H]bunazosin for the binding sites in the rank order: R-(—)-YM-12617 > prazosin > SGB-1534 > bunazosin > terazosin > naftopidil > urapidil. In parallel studies with [3H]bunazosin, the Kd and Bmax values for [3H]prazosin binding in human prostates were slightly lower. There was a similarity in the potency and rank order of seven alphai adrenoceptor antagonists for the inhibition of [3H] bunazosin and [3H]prazosin binding in human prostates. The new [3H]bunazosin binding assay in human prostates is remarkable for its low degree of nonspecific binding as compared to [3H]prazosin, especially at high ligand concentrations. Thus, [3H]bunazosin may become a useful radioligand for the further analysis of the alphai adrenoceptor binding sites in human prostates.