Porcine adrenal adrenocorticotropic hormone receptors: characterization, changes during neonatal development, and response to a stressor.

ACTH receptors were characterized in the porcine adrenal cortex using a biologically active [125I] Tyr23,Phe2,Nle4ACTH(1-38) analog, and a 149,000 X g(av) total particulate tissue preparation. Binding of this labeled ACTH analog to adrenocortical ACTH receptors was demonstrated to be time and temperature dependent, tissue and hormone specific, saturable, reversible, have a binding optimum at pH 7.5, and be dependent on the presence of Ca++. Scatchard analyses of saturation data using the computer program LIGAND indicated a single class of high affinity binding sites [association constant (Ka) = 2.00 +/- 0.25 10(9) M-1)]. Hill plots of saturation data had slopes (b = 0.83 +/- 0.09) that did not differ from 1, and hence also indicated a single class of binding sites. The rate constant of association (k1 = 3.72 +/- 1.19 X 10(8) min-1M-1) was compatible with the known rapidly occurring action of ACTH in pigs in vivo. Kinetic experiments also indicated the ACTH receptors to be of high affinity (4.18 +/- 0.54 X 10(10) M-1; n = 4). Receptor-associated degradation occurring to analog during incubations involved cleavage of the peptide into fragments rather than deiodination and could be partially prevented by use of 0.01% bacitracin. Subsequently, a study was conducted to determine changes in the ability of piglets to respond, in terms of plasma cortisol, to the physiological and psychological stressor of overnight maternal deprivation (MD) at various neonatal ages (days 3-31 of age); to measure age-related and stressor-associated changes in adrenal ACTH receptors; and to associate any changes in adrenal response with changes in ACTH receptors. Plasma cortisol obtained by venipuncture did not vary with age in either control or MD piglets but was elevated by MD (16.98 +/- 2.5 vs. 68.3 +/- 8.2 ng/ml). There was no effect of MD and associated hormonal perturbations on adrenal ACTH receptors. However, there were significant age-related differences in these receptors, with a 1.6-fold increase in concentrations (55.6 +/- 12.0 fmol/mg protein), and a 63% decrease (P less than 0.05) in affinity (4.19 +/- 0.86 x 10(8) M-1) evident at 17 days of age compared with 10 and 24 days of age. The significance of these age-related changes in ACTH receptors for adrenal function remains to be determined; however, it is evident that in neonatal pigs, there is no stress nonresponsive period.