16α,18-dihydroxydeoxycorticosterone and the binding of aldosterone to mineralocorticoid receptors in kidney of adrenalectomized rats

It has been recently suggested that 16α,18-dihydroxydeoxycorticosterone (16α,18-diOHDOC) may act as a "positive allosteric effector" of the binding of aldosterone to mineralocorticoid receptors. To test this hypothesis, a series of in vitro and in vivo studies examining the effect of 16α,18-diOHDOC on tritiated aldosterone (³HA) binding to mineralocorticoid receptors was performed. Using kidney slices from adrenalectomized rats, in vitro incubations were made for 20′ at 37C, over a range of concentrations of ³HA plus tenfold dexamethasone to confine tracer binding to mineralocorticoid receptors. At no concentration of ³HA did 16α,18-diOHDOC enhance binding; at all tracer concentrations a slight competing effect was observed. When ³HA was injected into rats in vivo with and without 16α,18-diOHDOC, a similar insignificant displacement of ³HA binding was seen in renal cytoplasmic fractions from adrenalectomized test rats. Additional in vitro studies were performed in an attempt to elucidate the mechanism of postulated action of 16α,18-diOHDOC. Neither renal cytoplasmic binding of oestradiol, postulated as a secondary pathway for steroid influenced Na⁺ retention, nor the binding of dexamethasone to renal glucocorticoid receptors, was altered by 16α,18-diOHDOC. Binding of tritiated 16α,18-iOHDOC in renal cyto-plasmic fractions was shown to be non specific, in that it could not be displaced by excess unlabelled 16α,18-diOHDOC. Finally, in a series of in vivo experiments using adrenalectomized rats, we could not show any effect of 16α,18-diOHDOC on urinary electrolyte excretion, either alone or in combination with low doses of aldosterone. Accordingly, we can find no evidence for 16α-diOHDOC having a direct effect on the kidney: in particular, there would appear as yet no molecular evidence for 16α,18-diOHDOC being a positive allosteric effector of aldosterone.