Corticotropin Is the Pituitary Hormone That Regulates Glucocorticoid Synthesis by the Adrenal Cortex
The control of the secretion of the glucocorticoids by the zona fasciculata and zona reticularis is by the tropic hormone (corticotropin) (Figure 34-9). A negative-feedback system exists, whereby glucocorticoids inhibit the release of hypothalamic corticotropin-releasing hormone, which in turn results in decreased corticotropin secretion by the pituitary gland.
Some evidence indicates that glucocorticoids also have a negative-
FIGURE 34-9 Regulation of cortisol secretion by the Iiypothalamopituitary axis. Plus signs indicate stimulation; minus signs indicate inhibition. CRH, Corticotropinreleasing hormone. (From Hedge GA, Colby HD, Goodman RL: Clinical endocrine physiology, Philadelphia, 1987, Saunders.)
FIGURE 34-10 Circadian changes in cortisol secretion in normal horses (open circles), in comparison with no circadian change in horses with equine Cushing's disease (solid circles). (From Dybdal N: Studies on equine Cushing's disease, Davis, University of California-Davis, 1990 [PhD thesis).)
feedback effect at the level of the pituitary gland. The potency of a glucocorticoid in negative-feedback inhibition of corticotropin is directly related to its glucocorticoid potency; for example, cortisol has more potent negative-feedback effects than corticosterone and has more potent glucocorticoid effects.
The negative-feedback control system that exists for the secretion of glucocorticoids does not result in the maintenance of uniform hormone concentrations in blood throughout the day. Sleep and activity patterns are superimposed on the negative-feedback system, so a predictable circadian rhythm occurs in which concentrations of glucocorticoids are lowest late at night and highest in the early-morning hours (Figure 34-10).
Another factor that can modify the negative-feedback control of glucocorticoids is stress. Stress can result from physical or psychological stimuli that are harmful to the individual. The effects of stress, as with the factors that influence circadian rhythms of glucocorticoid secretion, are mediated through the CNS. The glucocorticoid response to stress is immediate: concentrations of cortisol increase rapidly to reach, within minutes, values that are several-fold higher than normal. The glucocorticoid response is proportional to the severity of the stress; that is, lower levels of stress result in less cortisol production than do higher levels of stress.