A second major group are the steroids which are formed from cholesterol. Leukotrienes and prostaglandins fall into these categories. Leukotrienes are produced by white blood cells while prostaglandins are produced by many tissues and serve as local or tissue hormones. Autocrines frequently function to shut down their own production, so that the levels do not become to high. Hormone Receptors Hormones combine with receptors on target cells to form a hormone-receptor complex which activates the target cells.
Activation can be either stimulatory or inhibitory.
Down regulation occurs when prolonged exposure to high hormone titers leads to less vigorous response as the receptors are desensitized. Mechanisms of Hormone Action Hormones may be either stimulatory or inhibitory. Hormones are usually secreted in relatively small amounts and yet their actions are widespread and selective. The following mechanisms help explain hormone actions. Second Messenger Systems Water-soluble hormones utilize second messenger systems of hormone action.