Corticosteroid-Binding Globulin in the Rat: Studies on the Sex Difference

Abstract

The activity of corticosteroid binding (CBG) was determined in sera of male and female rats by multiple equilibrium dialysis at 37 C with corticosterone-4-C14. A sex difference in CBG activity was 1st apparent at 30 days of age and reached a maximum level at 48 days when the female animals had approximately twice the binding activity of the males. Prepuberal and postpuberal gonadectomy did not alter the binding activity of female rats but increased that of male animalsby 60-70%. Administration of estradiol did not change female CBG activity but resulted in a 50-70% increase in male rats. Testosterone injection depressed CBG levels of the intact female by 30% but did not alter the male value. Progesterone administration increased the binding activity of intact male and female animals. The injection of testosterone into previously gonadectomized male rats returned the CBG activity to that of sham-operated controls. The data suggested that testosterone suppresses CBG activity. An approximate calculation of the amount of free biologically active corticosterone was performed for intact male and female rats and the values were related to the thymus weight. The results indicated a slightly higher amount of free biologically active peripheral corticosterone in the female animal although the difference was not statistically signficant.