Autoradiographic localization of α1-adrenoceptors in the dog prostate and urethra with 3H-prazosin

The regional distribution of α₁-adrenoceptors in the dog urethra has been studied by quantitative autoradiography using the specific α₁-antagonist ³H-prazosin as a ligand. The binding of ³H-prazosin to urethral tissue sections was saturable, reversible, and of high affinity (Kd = 0.7 nM); it occurred at a single population of sites and possessed the pharmacological features of the α₁-adrenoceptor. The binding parameters of ³H-prazosin on urethral tissue sections were found similar to those obtained on urethral membranes. Autoradiographic results revealed that the α₁-adrenoceptors are heterogeneously distributed along the dog urethral axis. The highest densities of ³H-prazosin binding sites were found in the preprostatic urethra. Comparison between autoradiographic and histological slides revealed that the ³H-prazosin binding sites were only localized on the smooth muscle fibres of the dog urethra.