Inhibition of the anti-natriuretic action of aldosterone by thyroid hormone in the rat

The physiological effects of 3,5',3'-triiodo-L-thyronine alone or combined with cortisol on cultured pavement cell epithelia from freshwater rainbow trout gills

The effects of 3,5',3'-triiodo-L-thyronine (T3; 10 or 100 ng ml(-1)), alone or combined with cortisol (500 ng ml(-1)), on the physiological properties of cultured pavement cell epithelia from freshwater rainbow trout gills were assessed. T3 had dose-dependent effects on electrophysiological, biochemical, and ion transporting properties of cultured epithelia in both the absence and the presence of cortisol. These included reduced transepithelial resistance (TER), increased net Na(+) and Cl(-) movement (basolateral to apical) under asymmetrical culture conditions (freshwater apical/L15 media basolateral), and elevated Na(+)-K(+)-ATPase activity. However, paracellular permeability was elevated only in high-dose T3-treated preparations. In T3 + cortisol-treated epithelia, similar T3-induced alterations in TER, net Na(+) and Cl(-) movement, and paracellular permeability were observed, whereas the activity of Na(+)-K(+)-ATPase was further elevated. Under symmetrical culture conditions (L15 medium apical/L15 medium basolateral), T3 had no effect on transepithelial Na(+) and Cl(-) transport, which was passive. However, T3 + cortisol treatment resulted in active Na(+) extrusion (basolateral to apical). Under asymmetrical conditions, hormone treatment did not change the pattern of ion movement (active Na(+) extrusion, active Cl(-) uptake). These experiments demonstrate that cultured pavement cell epithelia from freshwater rainbow trout are T3-responsive and provide evidence for the direct action of T3 and the interaction of T3 and cortisol on the physiology of this preparation.

Thyroid hormone. Aldosterone antagonism in cultured epithelial cells

Thyroid hormone (T3) has been demonstrated to inhibit the action of aldosterone on sodium transport in toad urinary bladder and rat kidney. We have examined the effect of T3 on aldosterone action and specific nuclear binding in cultured epithelial cells derived from toad urinary bladder. In cell line TB6-C, addition of 5 X 10(-8) M T3 to culture media for up to 3 days results in no change in short-circuit current or transepithelial resistance. This concentration of T3 completely inhibits the maximal increase in short-circuit current in response to 1 X 10(-7) M aldosterone. The inhibition can be demonstrated with 18 h preincubation or with simultaneous addition of T3 and aldosterone. The half-maximal concentration for the inhibition of the aldosterone effect is approx. 5 X 10(-9) M T3. T3 has no effect on cyclic AMP-stimulated short-circuit current in these cells. The effect of T3 on nuclear binding of [3H]aldosterone was examined using a filtration assay with data analysis by at least-squares curve-fitting program. Best fit was obtained with a model for two binding sites. The dissociation constants for the binding were K'd1 = (0.82 +/- 0.36) X 10(-10) M and K'd2 = (3.2 +/- 0.60) X 10(-8) M. The half-maximal concentration for aldosterone-stimulated sodium transport in these cells is approx. 1 X 10(-8) M. Analysis of nuclear aldosterone binding in cells preincubated for 18 h with 5 X 10(-8) M T3 showed a K'd1 = (0.15 +/- 0.10) X 10(-10) M and K'd2 = (3.5 +/- 0.10) X 10(-8) M. We conclude that T3 inhibits the action of aldosterone on sodium transport at a site after receptor binding in the nucleus.