Circadian clock signals in the adrenal cortex.

Circadian secretion of steroid hormones by the adrenal cortex is required to maintain whole body homeostasis and to adequately respond to or anticipate environmental changes. The richly vascularized zona glomerulosa (ZG) cells in the pericapsular region regulate osmotic balance of body fluid by secreting mineralocorticoids responding to circulating bioactive substances, and more medially located zona fasciculata (ZF) cells regulate energy supply and consumption by secreting glucocorticoids under neuronal and hormonal regulation. The circadian clock regulates both steroidogenic pathways: the clock within the ZG regulates mineralocorticoid production via controlling rate-limiting synthetic enzymes, and the ZF secretes glucocorticoid hormones into the systemic circulation under the control of central clock in the suprachiasmatic nucleus. A functional biological clock at the systemic and cellular levels is therefore necessary for steroid synthesis and secretion.