Characterization and Localization of Prostatic Alpha 1 Adrenoceptors Using Radioligand Receptor Binding on Slide-Mounted Tissue Section

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Abstract
Alpha1 adrenoceptor binding sites have been characterized in prostatic tissue homogenates using radioligand receptor binding studies. The objective of the present study was to characterize and localize prostatic alpha1 adrenoceptor binding sites using slide-mounted tissue sections and the ligand 3H-prazosin. The present study demonstrated that preincubation is not required; the optimal incubation interval is 40 minutes; and a 1-minute wash (once or twice) maximizes the proportion of specific 3H-prazosin binding. Saturation studies were performed at 8 different concentrations of 3H-prazosin ranging between 0.0625 nM. to 8.0 nM. The binding of 3H-prazosin was consistently saturable and of high affinity. The mean Kd and Bmax determined from 6 saturation studies was 4.16 × 10−10 M. and 1.30 fmol./mg. wet weight, respectively. The pharmacology of these 3H-prazosin binding sites was characterized using competitive displacement experiments. The mean IC50 corrected for prazosin, phentolamine and yohimbine was 7.8 × 10−10 M., 6.0 × 10−10 M. and 2.1 × 10−10 M. The rank order of the IC50 corrected values indicates that alpha1 binding sites were measured under the assay conditions. In the present study, the mean values for Kd, Bmax and IC50 corrected are similar to values previously reported using prostatic tissue homogenates. Prostatic tissue sections were apposed to x-ray film after being incubated with 3 nM. 3H-prazosin (total prazosin binding) and 3 nM. 3H-prazosin + 8 μM. prazosin (nonspecific prazosin binding). The autoradiograms were analyzed using a computerized analyzing system. The specific radioactive densities of 3H-prazosin in the stroma and glandular epithelium were 1099 ± 48 pCi/mg. and 163 ± 42 pCi/mg. The present study validates the technique of assaying alpha1 adrenoceptor binding sites on slide-mounted prostatic tissue sections and provides further evidence that alpha1 adrenoceptor binding sites are localized primarily to the stromal elements of the prostate.