Regulation of receptors allozvs flzeir responses to be modified rapidly and appro~~ afely accordizzg to fire zzeeds of fke ezzviro~ ment. Multiple meckaniszns are izzuolved izz the loss of sensitivity that follows exposure to agonists. Receptor seqzzestration, a rapid and transiezzt event, and receptor downregulafiozz, wlzich requires nzore prolonged agonist exposure, conttibute to this effect. However, in this article Bob Lefkowitz and colleagues focus primarily on tlze recezzt developments izz zzndersfanditzg mechanisms of rapid desensitization itzvolving receptor p~ zosp~ zo~ lufion. Vurious molecular biological tecktziques Izave beezz zzsed to denzonstrate the importanf roles of two pa~‘cu~ ar kizznses-@ ARK arzd protein kizzase A-in this regard.

Receptor-mediated responses are subject to regulatory influences which adjust them to the chang-ing environment and needs of the organism. A classical example is desensitization-the tendency of responses to reduce in intensity over time despite the continued presence of a constant stimulus. Desensitization occurs in bacteria (chemotaxis), yeast (pheromone