Hormone-Receptor Interaction
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 to those hormones. These protein receptors should not be viewed as static fixtures associated with cells, but like any cellular structure 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. Because there are a finite number 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 a ditional effect (37). Further, because 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 consequences For example, with hear to may receive a that blocks the 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.