ABSTRACT
The measure of biological response elicited by a drug, by binding to receptors, is proportional to the fraction of the receptors that are occupied by that drug. The presence of spare receptors may be demonstrated by using an irreversible antagonist or noncompetitive antagonist. The spare receptor concept explains how the sensitivity of a cell or tissue is dependent not only on the affinity of the receptor for binding, but also on the total concentration of the receptors that could be found on a target tissue or cell. The phenomenon of overshoot follows withdrawal of certain drugs. Overshoot may occur with either agonists or antagonists. Down regulation is a process by which the number of total receptors present on the cell membrane is decreased subsequent to continued stimulation of the receptors by an agonist. The general mechanisms involved in activation of intracellular receptors and the consequent responses are outlined.
