Downregulation of adrenocortical cyclo (His-Pro)-binding sites by cyclo (His-Pro) administration to neonatal but not adult rats.

The specific binding of [3H-Pro]cyclo (His-Pro) to a 27,000 g pellet fraction from rat adrenal gland was studied. Binding sites were exclusively localized in the cortical region of the adrenal gland and exhibited a binding affinity (Kd) of 1.79 +/- 0.21 microM and maximal binding capacity (Bmax) of 123.4 +/- 10.2 pmol/mg protein. In vivo administration of cyclo (His-Pro) to neonatal rats led to the downregulation of cyclo (His-Pro)-binding sites with a decrease in Bmax but not Kd. Such changes were not observed after similar treatments of adult rats. These data suggest a receptor-like characteristic of cyclo (His-Pro) binding sites.