The ACTH-R is localized in all three zones of the adrenal cortex (37). Results from binding studies indicate that in the adrenal cortex the ACTH receptors can be subdivided into a type with a KD of 1 nM, but with only 60 binding sites per cell and into a second type with a KD of 300 nM, but with several orders of magnitude more binding sites (about 600,000) per cell. The presence of high and low affinity receptors for ACTH means that the adrenal cortex is highly sensitive to the concentration of ACTH in the systemic circulation.