Different mechanisms for the receptor mediated antimineralocorticoid action of two new spirolactone derivatives.

The introduction of a methoxycarbonyl residue in position 7 of spirolactone produced a molecule (ZK 91587) that exhibited dramatically increased affinity for the rat renal mineralocorticoid receptor (MR) that was lacking in RU 26752 with a propyl group in this same 7 position. The binding of 3H-ZK 91587 was specific to MR in renal cytosol and was not obtained with either serum or cytosol from non-target organs such as liver and lung. The RU 26752-receptor complex was more unstable than aldosterone-MR complex at 35 degrees C but underwent complete thermal activation on DNA cellulose. Contrarily, ZK 91587 did not permit thermal activation at all and also rendered the MR highly labile at 35 degrees C. Unactivated aldosterone and RU 26752-MR complexes sedimented largely in the 7 S region during sucrose density gradient centrifugation, and this shifted to 4 S following thermal activation. Paradoxically, under these very conditions, the molybdate stabilized, nonactivated, ZK 91587-MR complex was distributed almost equally into 7 S and 4 S regions which was not altered further by the activation process. If it is assumed that ZK 91587 exerts an antagonist action by inhibiting MR activation, or by MR labilization at body temperature, RU 26752 would seem to act at a step beyond the activation process. These form new tools to dissect receptor structure and function and necessitate a reevaluation of current notions regarding hormone action.