Adrenergic receptors. Methods of determination and mechanisms of regulation.

The radioligand binding technique has been proven useful in the study of altered responsiveness after exposure to adrenergic agents. A reduction in receptor number has generally been reported after interventions serving to increase the stimulatory input to the receptors prior to assay. Conversely a decrease in receptor stimulation has been demonstrated to induce an elevation in receptor density. These phenomena have been described for alpha- as well as for beta-adrenoceptors in various tissues under a variety of experimental conditions such as prolonged agonist exposure, chronic receptor blockade, denervation and interference with catecholamine turnover. A number of clinically relevant cardiovascular experimental models have been characterized by altered adrenoceptor densities, possibly reflecting a compensatory resetting of sympathetic tone in response to an aberrant haemodynamic pattern. The mechanisms underlying receptor density changes as those discussed have been suggested to involve an internalization process which may involve phospholipase and transglutaminase activation in the cell membrane.