Analysis of the mineralocorticoid receptor in rat heart with the aid of two new spirolactone derivatives.

Two derivatives of spirolactone, synthesized in an effort to eliminate the obnoxious side effects of the native molecule, were employed to dissect various aspects of the MR structure and function in rat heart. The introduction of a propyl residue in position 7 of spirolactone produced a molecule (RU 26752) that exhibited an increased affinity for the agonist specific MR, and furthermore revealed an antagonist-specific MR population in the target organ heart but absent from nontarget lung and liver. The specificity for both sites increased when a methoxycarbonyl group was introduced in the 7 position (ZK 91587). RU 26752 labilized the MR at 35 degrees C but did not interfere with thermal activation assessed on DNA-cellulose and sucrose density gradients. ZK 91587 was even more effective in labilizing the MR and did not permit thermal activation at all. Whereas only one ionic species was observed with RU 26752 on DE-52 columns, two were evident with ZK 91587. Both antimineralocorticoids were bound to populations of two molecular sizes on Ultrogel columns. Thus, the nature of chemical substitution in the 7 position of spirolactone dramatically alters the receptor-mediated antisteroid action of the resulting molecule. Such differences may permit distinction between agonist versus the antagonist-specific receptor conformations, and could possibly be exploited for the eventual purification of the mineralocorticoid receptor from various organs.