Glucocorticoid receptor of frog (Rana esculenta) liver

Cytosolic glucocorticoid receptor in the testis of Bufo arenarum: seasonal changes in its binding parameters

Glucocorticoids (GC) are the hormonal mediators of stress. In mammals, high levels of GC have negative effects on reproductive physiology. For instance, GC can inhibit testicular testosterone synthesis by acting via glucocorticoid receptors (GR), the extent of the inhibition being dependent on GC levels. However, the effect of GC on testicular function and even the presence of GR in amphibians are still unclear. The purpose of this work was to characterise testicular cytosolic GR in Bufo arenarum, determining the seasonal changes in its binding parameters as well as the intratesticular localisation. The binding assays were performed in testis cytosol with [3H]dexamethasone (DEX) and [3H]corticosterone (CORT). Binding kinetics of DEX and CORT fitted to a one-site model. Results were expressed as means +/- standard error. Apparent number of binding sites (Bapp) was similar for both steroids (Bapp DEX = 352.53 +/- 72.08 fmol/mg protein; Bapp CORT = 454.24 +/- 134.97 fmol/mg protein) suggesting that both hormones bind to the same site. Competition studies with different steroids showed that the order of displacement of [3H]DEX and [3H]CORT specific binding is: DEX approximately RU486 approximately deoxycorticosterone (DOC) > CORT > aldosterone > RU28362 > progesterone >>> 11-dehydroCORT. The affinity of GR for DEX (Kd = 11.2 +/- 1.5 nM) remained constant throughout the year while circulating CORT clearly increased during the reproductive season. Therefore, testis sensitivity to GC action would depend mainly on inactivating mechanisms (11beta-hydroxysteroid dehydrogenase type 2) and CORT plasma levels. Since total and free CORT are higher in the reproductive than in the non-reproductive period, the magnitude of GC actions could be higher during the breeding season. The intratesticular localisation of the GR was determined after separation of cells by a Percoll density gradient followed by binding assays in each fraction. DEX binds to two different fractions corresponding to Leydig and Sertoli cells. In conclusion, in the testis of B. arenarum GC could regulate the function of both cellular types particularly during breeding when CORT reaches the highest plasma concentration.

Corticosteroid receptors in liver cytosol of the clawed toad, Xenopus laevis: influence of thyroid and ovarian hormones

The glucocorticoid receptor capacity Ro and the dissociation constant Kd were determined in the liver of Xenopus laevis by Scatchard analysis. In 5-year-old female toads Ro was about three times higher than that in males (153.9, 54.3 fmol/mg protein) and Kd was similar in both sexes (4.0, 4.1 nM). Some of the animals used had abnormal enlarged thyroid glands, atrophic ovaries, or both defects in connection with different levels of Ro, but not of Kd, compared to those of normal animals. Females with ovarian atrophy showed significantly lower Ro values, in the same range as in normal males, and a high liver weight. In male and female toads with enlarged thyroid glands and in animals with both defects a significantly higher Ro occurred compared to that of the corresponding group without this abnormality. To study the influence of thyroid hormones on glucocorticoid receptors, young toads (2-3 years old) received injections of 4-phenyl-2-thiouracil, T3, or T4 on 7 consecutive days. Ro and Kd were determined on the following day. Doses of 50 and 500 ng T3 and of 500 and 5000 ng T4 per gram of body weight and day resulted in an increase of Ro up to 250% of the controls. Injections of T3 were more efficient in males than in females. The effect of thyroxine was about the same in both sexes. These observations suggest that thyroid and ovarian hormones exert an influence on glucocorticoid receptor capacity and may belong to the factors which regulate glucocorticoid receptors.

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.