Hormones are carried by the circulation to all tissues, but they affect only certain tissues. Tissues responsive to specific hormones have specific protein receptors capable of binding those hormones. These protein receptors should not be viewed as static fixtures asso ciated with cells, but like any cellular structures, they are subject to change. The number of receptors varies from 500 to 100,000 per cell, depending on the receptor.
Receptor number may decrease when exposed to a chronically elevated level of a hormone (down-regulation), resulting in a diminished response for the same hormone concentration. The opposite case, chronic exposure to a low concentration of a hormone, may lead to an increase in receptor number (up-regulation), with the tissue becoming very responsive to the available hormone. Since there is a finite num-ber of receptors on or in a cell, a situation can arise in which the concentration of a hormone is so high that all receptors are bound to the hormone; this is called saturation. Any additional increase in the plasma hormone concentration will have no additional effect (41).
Further, since the receptors are specific to a hormone, any chemical similar in “shape” will compete for the limited receptor sites. A major way in which endocrine function is studied is to use chemicals (drugs) to block receptors and observe the conse quences.
For example, patients with heart disease may receive a drug that blocks the receptors to which epinephrine (adrenaline) binds; this prevents the heart rate from getting too high during exercise. After the hormone binds to a receptor, cellular activity is altered by a variety of mechanisms.
Mechanisms of Hormone Action Mechanisms by which hormones modify cellular activity include:
■ alteration of membrane transport mechanisms,
■ altering activity of DNA in the nucleus to initiate or suppress the synthesis of a specific protein, and
■ activation of special proteins in the cells by “second messengers.”