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 +/- 0.04 nM). The density of [3H]
bunazosin binding sites (Bmax) was 676 +/- 33 fmol/mg.
protein. [3H]
Bunazosin rapidly associated with its binding sites in membranes of human prostates and reached steady state by 20 min. 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 alpha 1
adrenoceptor antagonists competed with [3H]
bunazosin for the binding sites in the rank order: R-(-)-YM-12617 greater than
prazosin greater than
SGB-1534 greater than
bunazosin greater than
terazosin greater than
naftopidil greater than
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 alpha 1,
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 alph 1
adrenoceptor binding sites in human prostates.