ACTH RECEPTOR LEVELS IN LAMB ADRENALS AT LATE GESTATION AND EARLY NEONATAL STAGES

Subcellular preparations (membrane pellet 20,000 x g) from lamb adrenals bind [125I]-ACTH 1-24. The binding observed is specific, saturable, rapidly equilibrated and temperature dependent. Hence, the binding exhibits many of the characteristics which are usually associated with hormone receptors. The apparent dissociation constant found is about 10-7 M. The concentration of these ACTH receptors has been studied in lamb fetal adrenal glands at the end of gestation and in lamb adrenal glands after parturition. The number of ACTH receptors when expressed per 2 adrenal glands is increased 5-fold between Day 123 of gestation (25.2 ± 1.8 nmoles) and birth (120.9 ± 17 nmoles). This increase is achieved in 2 phases and reflects not only an increase in the number of cells, but also an augmented number of receptors per cell. No change in the affinity of the binding has been observed throughout the period studied. There is a close correlation (r = 0.57; n = 28; P < 0.01) between the levels of fetal plasma corticosteroids and the number of ACTH receptors during late pregnancy. These results suggest that receptor modulation is one of the factors involved in the gradually developing sensitivity of the ovine fetal adrenals to ACTH just before parturition.