Characterization of corticotropin receptors on adrenocortical cells.

Abstract

The binding of ACTH to receptors on isolated rat adrenocortical cells was investigated with the aid of [[125I]ITyr23, Phe2, Nle4]ACTH-(1-38) (125I-ACTH analog) which retained full biological potency and had a specific radioactivity of 1800 .+-. 75 Ci/mmol. Binding was highly specific to adrenocortical cells, and the radioactive peptide was displaced by low concentrations of ACTH but not by other basic peptides. Binding was rapid, reversible and linearly related to the number of cells. 125I-ACTH analog was not significantly degraded by incubation with the cells at 23.degree. C for 1 h. Scatchard analysis of the binding was compatible with a single class of binding sites with Kd = 1.41 .+-. 0.21 nM, and the number of sites was estimated to be 3840 .+-. 1045 per cell. The binding curve was superimposable on the concentration-response curve for cAMP. Small, but significant amounts of 125I-ACTH analog were bound at concentrations sufficient for maximal stimulation of steroidogenesis. For a series of ACTH analogs, the concentrations of the peptides required for half-maximal stimulation of cAMP production were in excellent agreement with the concentration required for half-maximal inhibition of binding. The adrenocortical cells apparently contain only 1 class of ACTH receptors and stimulation of a small fraction of these receptors (< 3%) may be sufficient for maximal steroidogenesis.