Corticotropin receptors, cyclic AMP and steroidogenesis.

The detection and characterization of the physiologically relevant receptors for corticotropin (ACTH) in rat adrenocortical cells is described. By the use of a radioligand with full biological potency and high specific radioactivity (1800 +/- 75 Ci/mmol), a single class of receptors with an apparent Kd of 1.41 +/- 0.21 nM was detected 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 but dissociated from that for steroidogenesis. These data are best explained by the receptor-reserve model in which occupancy of a small fraction of the receptors is sufficient for inducing maximal steroidogenesis.