ALDOSTERONE RESPONSIVENESS OF A6 CELLS IS RESTORED BY CLONED RAT MINERALOCORTICOID RECEPTOR

A6 cells, derived from Xenopus laevis renal tubule, form a high-resist ance ion-transporting monolayer when grown on permeable supports and c an generate a short-circuit current (SCC) that is stimulated by high l evels of the mineralocorticoid aldosterone. Surprisingly, A6 SCC is mo re responsive to glucocorticoids than to mineralocorticoids, suggestin g the possibility that these cells do not contain transcriptionally ac tive mineralocorticoid receptor (MR) and that glucocorticoid receptor (GR) mediates MR-like responses in these collecting duct-like cells. W e have examined the response of both SCC and a transfected reporter ge ne to mineralocorticoids and glucocorticoids in the presence and absen ce of transfected rat MR (rMR). We found that, in the absence of trans fected MR, a reporter gene that can be activated by MR or GR was more responsive to glucocorticoids such as dexamethasone and RU-28362 than to mineralocorticoids such as aldosterone. Transfected rMR underwent m ineralocorticoid-dependent nuclear localization and restored both tran scriptional sensitivity of a reporter gene and SCC response to mineral ocorticoids. These data demonstrate that A6 cells contain transcriptio nally active GR but not MR and thus suggest a molecular basis for the defect in A6 cell SCC response to aldosterone. Our results also demons trate that GR is capable of mediating hormone stimulation of SCC, a cl assic mineralocorticoid response. Finally, the observation that hetero logous expression of rMR can localize normally to the A6 nucleus in a hormone-dependent fashion and restore both the transcriptional and SCC response to mineralocorticoids suggests that MR function is conserved in species as distantly related as toads and mammals.