Cytosolic glucocorticoid receptors in skeletal muscle of the newborn calf.

Abstract

A study was conducted to provide information on the characteristics of skeletal muscle glucocorticoid receptors and to determine the response of serum cortisol and thyroxine secretion to colostrum feeding in the neonatal calf. Twenty-four calves (12 males and 12 females) were obtained immediately postpartum and randomly assigned to one of two treatments after being blocked by breed and sex. The treatments consisted of: 1) pooled colostrum and 2) pooled whole milk, with both treatments being force-fed at birth, 12, 24 and 36 h postpartum. Muscle biopsies (20-30 g) were surgically removed from the right semitendinosus at 36 h postpartum from 14 of the 24 calves (7 male and 7 female). There were no glucocorticoid receptor differences in muscle samples from either the colostrum or milk-fed calves. Binding of dexamethasone to skeletal muscle cytosol indicated a moderate to high affinity (Kd = 2.34 X 10(-8)M) and a low capacity (37.61 fmol/mg cytosolic protein) binding receptor. Competition assays indicated that other non-glucocorticoid hormones had relatively high affinity for the glucocorticoid receptor. The average initial cortisol concentration was highest at birth, 127.9 (SE = 25) and 172.4 (SE = 29) ng/ml for colostrum and milk-fed calves, respectively. Following the initial peak at birth, serum cortisol concentrations declined with time (P less than .05) for both treatments. Serum thyroxine concentrations increased (P less than .05) to reach a peak of 23.3 (SE = 3.1) and 21.0 (SE = 2.9) ug/dl for colostrum and milk-fed calves, respectively, then steadily declined.