Abstract
A receptor is a molecule that specifically binds a hormone, forming a hormone-receptor complex. This binding initiates cellular events that are ultimately manifested as the physiological effects of the hormone. Until recently information pertaining to gastrointestinal hormone receptors in mucosa has been garnered solely through the classical pharmacological methods of studying agonist-antagonist interactions. The heuristic advantage of studying properties of material hormone receptors, however, has been widely recognized by those interested in other areas of endocrinology.
A simplified scheme diagramming the release of a hormone and its combination with a receptor is illustrated in Fig. 1. In order to understand the regulation of this system one can use sensitive radioimmunoassay methods to measure the concentration of hormone in the blood. This same measurement allows the investigator to determine the stimuli and inhibitors of the release of that hormone. Since the overall effect depends on the concentration of the hormone-receptor complex much information can be acquired concerning the physiology of the system.
Recently, however, it has become apparent that the concentration of receptors can also vary, and, hence, impart a degree of regulation on the system. Regulation of hormone receptors is now recognized to play an important role in the hormone response of target cells (1). The concentration of receptors for a specific hormone may be regulated by other hormones. The levels of oxytocin receptors in the uterus and mammary gland, for example, are probably regulated by both estrogen and progesterone (2). In other cases a particular hormone may regulate the numbers of its own receptors. Insulin downregulates the number of its receptors (3), while prolactin increases its membrane receptors in both the liver and mammary gland (4, 5). Thus, the responses to several hormones are regulated not only by the amount of hormone reaching the receptor but also by the number of receptors present.
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