Aldosterone and corticosterone binding and effects on Na+ transport in cultured kidney cells

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Abstract
A continuous line of cells (A6) derived from toad kidney forms epithelia in culture that manifest aldosterone-stimulatable transepithelial Na transport. In this study an efficient filtration assay for nuclear binding of [3H]aldosterone was validated. Specific high-affinity aldosterone and corticosterone binding sites in the particulate (nuclear-enriched) fraction were characterized in intact epithelia. Despite metabolism of both steroids, 2 high-affinity binding sites for each were demonstrable: aldosterone, K''d1 = 0.85 (.+-. 0.19) .times. 10-10 and K''d2 = 1.6 (.+-. 0.42) .times. 10-8 M; corticosterone, K''d1 = 0.52 (.+-. 0.31) .times. 10-10 and K''d2 - 0.32 (.+-. 0.19) .times. 10-8 M. Analog competition-binding studies indicated a qualitative difference in the 2 sites and co-occupancy of both sites by the 2 steroids. The Na transport response to aldosterone and corticosterone approximated a linear function of occupancy of the lower affinity sites. Although the lower affinity sites resemble mammalian glucocorticoid receptors in terms of relative binding affinities for analogs, they are the receptors which mediate the aldosterone and corticosterone stimulation of Na+ transport in these epithelia.