Corticosteroid receptors in liver cytosol of the clawed toad, Xenopus laevis: daily and seasonal variations

In postmetamorphic Xenopus laevis liver cytosol the glucocorticoid binding capacity R0 and the dissociation constant Kd were determined. The receptor assay included an incubation period of 24 hr at 0-4 degrees with sodium molybdate to stabilize the receptor. Dexamethasone, triamcinolone acetonide, and corticosterone as tritiated ligands were compared regarding the R0 (67.6, 57.2, and 30.7 fmol/mg protein), the Kd (3.54, 0.56, and 9.03 nM), and the rate of dissociation in young specimens of X. laevis. In adult toads the [3H]dexamethasone receptor binding capacity was threefold higher in females than in males (153.86 +/- 12.19, 54.29 +/- 4.5 fmol/mg protein)--with about the same Kd (3.97 +/- 0.57, 4.08 +/- 0.28 nM). Young toads were kept under an artificial light regime (light from 600 to 1800 hr) and dexamethasone binding was measured every 3 hr. Unlike Kd, R0 showed a significant diurnal variation with maximal values at 600 and 1800 hr, which occurred about 9 hr after a maximal level of corticosterone in serum was reached (900, 2100). Seasonal variations of the [3H]dexamethasone and [3H]corticosterone binding capacity were different in both sexes of adult X. laevis. Maximal values in males were found in June/July and October/November. In females, the R0 was increased in the second half of the year with the maximum in August (275.5 +/- 45.02 fmol/mg protein). No correlation between R0 and the concentrations of corticosterone or aldosterone existed.