High affinity aldosterone binding sites (type I receptors) in the mammalian inner ear

The presence of aldosterone (Type 1) binding sites in the mammalian inner ear has been previously suggested by an increase in inner ear Na, K-ATPase ouabain binding sites in response to the administration of aldosterone in vivo (Pitovski et al., 1993). Type I binding sites have now been identified and characterized in the lateral wall of the basal turn of the cochlea and in the ampullae of the semicircular canals of the guinea pig. In the presence of RU 28362, which blocks low-affinity binding of the labeled hormone to Type II sites, [3H]-aldosterone binds to a single class of high-affinity (Type I) sites with Kd values of 34.7 nM in lateral wall of the basal turn of the cochlea and 31.3 nM in the ampullae of the semicircular canals. Bmax is 17.1 fmol/mg dry tissue for the cochlear sample and 17.4 fmol/mg dry tissue for the ampullae, comparable to reported values in renal tissue (17-31 fmol/mg protein). Thus, the results of receptor-binding experimental protocols with [3H]-aldosterone clearly suggest that these inner ear tissues are a target site of mineralocorticoid action.