Corticosterone's metabolite is an agonist for Na+ transport stimulation in A6 cells

This study tests the hypothesis, in A6 epithelia, that 1) corticosterone stimulates active Na+ transport (short-circuit current, Isc) by an additional receptor mechanism to the type I (mineralocorticoid) and type II (glucocorticoid) mechanisms shared with aldosterone (Aldo) and 2) that the agonist may be 6 beta-OH-corticosterone made in the effector cell. The dose-response relationship of corticosterone at 24 h resolves into two components, by curve fitting, with a 50% effective concentration (EC50) for 10% of maximum Isc stimulation of 2 X 10(-9) M and an EC50 for the other 90% of 3 X 10(-7) M. The EC50 of the smaller component correlates with the apparent dissociation constant (K'd) of corticosterone for high affinity (type II) nuclear binding sites shared with Aldo. In unlabeled analogue competition studies Aldo and corticosterone displaced nuclear binding equally below 10(-8) M [3H]corticosterone, indicating only shared sites. However, nonshared saturable sites (displaced by corticosterone but not by Aldo) were found at [3H]-corticosterone concentrations above 10(-8) M. Concentration-binding curves performed with [3H]corticosterone, in presence of 1,000 X Aldo to displace shared sites, revealed a single class of binding sites with a half-maximal saturation of 2 X 10(-7) M, which is quite similar to the EC50 of the lower affinity component of Isc stimulation by corticosterone at 24 h. Reversed phase high-pressure liquid chromatography of nuclear extracts indicates that the saturable component of bound [3H] was 6 beta-OH-[3H]corticosterone derived from [3H]corticosterone. Thus, A6 cells metabolize corticosterone to 6 beta-OH-corticosterone, which in turn occupies lower-affinity receptors not shared with Aldo or corticosterone, to mediate most of the active Na+ transport stimulation by corticosterone.