INFLUENCE OF RECEPTOR DENSITY ON THE PATTERNS OF BETA(2)-ADRENOCEPTORDESENSITIZATION

Sustained stimulation of the beta(2)-adrenoceptor leads to a desensiti zation of the receptor-mediated adenylyl cyclase stimulation. While de sensitization promoted by nanomolar concentrations of isoproterenol in volves the phosphorylation of the beta(2)-adrenoceptor by protein kina se A alone, both protein kinase A- and beta-adrenoceptor kinase-mediat ed phosphorylation leading to the binding of beta(2)-arrestin contribu te to the desensitization evoked by micromolar concentrations of agoni st. In the present study, we assessed the influence of receptor densit y on the patterns of desensitization induced by these two different le vels of stimulation. Murine L cells were transfected with a cDNA encod ing the human beta(2)-adrenoceptor and clonal cell lines expressing va rious levels of beta(2)-adrenoceptor were used for the study. In cell lines expressing the highest number of receptor, approx. 150 000 sites /cell (approx. 3000 fmol/mg of membrane proteins), pretreatment with m icromolar concentrations of isoproterenol causes a desensitization pat tern characterized by a reduction in both the potency and the efficacy of isoproterenol to further stimulate the adenylyl cyclase activity. In contrast, desensitization induced by 10 nM isoproterenol resulted o nly in a decrease in the potency of isoproterenol. This distinct patte rn of desensitization is not seen in cells expressing 12 000 receptors /cell (approx. 200 fmol/mg of membrane proteins) and, in that case, pr etreatment with 10 nM isoproterenol leads to a reduction in both the s ensitivity and the maximal response. Similar effects on the beta-adren oceptor-stimulated adenylyl cyclase were observed in these cells follo wing treatment with dibutyryl cAMP. Receptor density therefore dramati cally influences the pattern of desensitization evoked by low level of stimulation. The results also demonstrate that although different mol ecular events are involved in the desensitization evoked by different levels of stimulation, its phenotypic expression can be qualitatively identical in cells expressing a relatively small number of receptors. Hence, protein kinase A-mediated desensitization cannot be qualitative ly distinguished from the beta-adrenoceptor kinase-mediated process in these cells.