The role of the adrenal in generating the diurnal variation in circulating levels of corticosteroid-binding globulin in the rat.

Corticosteroid-binding globulin (CBG) levels were measured in serum samples collected sequentially from rats into which indwelling catheters had been inserted. A distinct diurnal variation in CBG levels was found, with the highest levels of binding protein at the beginning of the dark period. CBG levels then decreased until a nadir was reached shortly after the beginning of the light period. To examine the role of glucocorticoid in the generation of this pattern, rats were adrenalectomized 10 days before repeating the experiment. Adrenalectomy abolished both the diurnal variation in CBG levels and the high degree of variation in the levels of binding protein between animals. However, adrenalectomy just before the onset of the dark period did not eliminate the expected decrease in CBG levels. To further explore the role of steroid, animals that had been adrenalectomized for 10 days were given saline containing 25 micrograms/ml corticosterone for a period of 24 h. Despite the attainment of normal plasma corticosterone levels, no decrease in CBG levels was observed. When steroid administration was discontinued, however, CBG levels dropped concurrently with the decreasing steroid. These studies show that a diurnal variation in CBG exists and suggest that it is the result of the diurnal variation in glucocorticoid levels.