The aldosterone antagonist, spironolactone, has been shown to block the effects of exogenously administered androgen in rat. This suggests that interaction of the drug with androgen at the target tissues may occur. In this paper we have studied the possible interaction of spironolactone with the 5α dihydrotestosterone (DHT)1 receptor of rat ventral prostate. The competitive receptor assay used involves precipitation of the 105,000 x g supernatant of the homogenized tissue with protamine sulfate, removal of the unprecipitated cytosol, and incubation of the precipitate in the presence of the appropriate [3H]DHT steroid solution at O C for 18 hours. Using this method the Kd (dissociation constant) for DHT in the rat prostate was in the range of 1.9 4.0 x 10-9M and the binding capacity was 0.21 pmol/mg protein. Spironolactone was found to interfere with the binding of DHT to the precipitated cytosol and displayed an estimated Kd of 1.3 4.6 x 10-8M. Several digitalis preparations were similarly studied. Digitoxin and digitoxigenin also interfered with the binding of [3H]DHT and had an estimated Kd of 0.8 3.6 x 10-8M. Digoxin interacted less strongly and its estimated Kd was 10-6M. We believe these results suggest an interaction of spironolactone and digitalis with the DHT receptor and may help explain some of their antiandrogenic actions in the rat and in man.
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