Hormonal regulation involves a hierarchy of cell types acting on each other either to stimulate or to modulate the release and action of a hormone. Secretion of hormones from endocrine cells is stimulated by chemical signals from regulatory cells that occupy a higher position in this hierarchy (Figure 23.8).

The hierarchical nature of hormone action can be summarized as follows:

1. Hormonal action is controlled ultimately by the central nervous system, which transmits signals to the hypothalamus. It responds by producing factors that either stimulate (called releasing factors) or inhibit the release of hormones from the pituitary.

2. Pituitary hormones do one of the following:

a. They stimulate other endocrine glands, each of which releases a hormone that acts on a target tissue and elicits a specific metabolic response.

b. Alternatively, they act directly on a target tissue. The action of a hormone sets in motion events that ultimately limit that action.

Some pituitary hormones stimulate target tissue directly. For example, prolactin stimulates mammary glands to produce milk.

Most pituitary hormones act on endocrine glands that occupy an intermediate, or secondary, position in the hierarchy, stimulating them to produce hormones that exert the ultimate actions on target tissues. Pituitary hormones that act on other endocrine glands are called tropic hormones or tropins. An example is adrenal corticotropic hormone (ACTH), also called -corticotropin (see here also). This peptide is secreted from the anterior pituitary, and it stimulates the adrenal cortex to produce glucocorticoids and mineralocorticoids, which in turn act on a number of tissues.

The action of a hormone is self-limiting because of the existence of feedback loops, in which secretion of a hormone sets in motion a series of events that leads to inhibition of that secretion (Figure 23.9).