Terbutaline-induced downregulation of beta 2-adrenoceptors without Gi-protein alterations in human lymphocytes

Recent evidence suggests that agonist-induced desensitization of Gs protein-coupled beta-adrenoceptors is accompanied by sensitization of Gi protein-coupled receptors and/or an increase in Gi protein. To find out whether such "cross-regulation" between Gs protein- and Gi protein-coupled receptors can be also demonstrated in vivo in humans, we studied the effects of a 2 week treatment of eight male volunteers with the beta 2-adrenoceptor agonist terbutaline (3 x 5 mg/day) on beta 2-adrenoceptor density and Gi-protein content in lymphocytes and on alpha 2-adrenoceptor density (Gi-coupled receptors) in platelets. Terbutaline decreased the lymphocyte beta 2-adrenoceptor density by about 30%, but had no significant influence on lymphocyte Gi-protein levels (assessed by pertussis toxin-catalyzed [32P]ADP ribosylation) or on platelet alpha 2-adrenoceptor density. We conclude that circulating blood cells are not suitable to demonstrate in humans in vivo a "cross-regulation" between Gs- and Gi-coupled beta- and alpha-adrenoceptors.