Plasma membrane receptors.

Receptors are macromolecules in or on cells that me- diate physiological responses upon binding specific li- gands. Over the past few years, important advances have been made in the understanding of the biology of receptors, both at a basic and at a clinical level. Receptors bind their ligands with remarkable selectiv- ity and with high affinity, permitting a given physiological response to be elicited with great specificity in the target tissue. The specific binding of agonists to receptors leads to the generation of signals that influence cell functions. The signal may be expressed as an altered rate of enzyme activity or ion transport, which then leads to the characteristic physiological responses within the cell.