Ontogenetic changes in porcine adrenocortical adrenocorticotropic hormone receptors.

1. Binding of [125I]ACTH(1-38) analog to adrenal receptors was measured in fetal pigs (Sus domesticus) at 15-day intervals from midpregnancy (60 days) to near term (105 days; pregnancy length 114 days). 2. Binding was greatest at day 60 (0.42 +/- 0.03 fmol/200 micrograms protein or 0.50 +/- 0.08 fmol/50 micrograms DNA), and least at day 105 (0.13 +/- 0.03 fmol/200 micrograms protein or 0.16 +/- 0.04 fmol/50 micrograms DNA). Total adrenal binding was constant (0.61 +/- 0.02 fmol/paired adrenals). 3. Scatchard analyses at day 60 and day 105 showed comparable apparent affinities of ACTH receptors (Ka day 60 = 1.51 +/- 0.72 x 10(9) M-1 vs Ka day 105 = 1.94 +/- 0.78 x 10(9) M-1). 4. DNA per paired adrenals and membrane-associated protein increased 1.6-fold, providing a constant protein: DNA ratio. Concentrations of adrenal cortisol were constant from 60 to 90 days of gestation age but increased dramatically by day 105. 5. These data suggest that during 60-105 days of gestation age the number of ACTH receptors per cell is reduced.