Effect of receptor density on the receptor-effector coupling: use of cloned and stably expressed alpha 1B-adrenoceptors in CHO cells.

Using Chinese hamster ovary cells stably expressing alpha 1B-adrenoceptor as a model, we investigated whether the changes in receptor density may influence the receptor-effector coupling relationship. Among the transfected cells, two clones which showed similar pharmacological properties but markedly differed in receptor density (Bmax were 1600 and 110,000 sites/cell, respectively), were examined. The phenoxybenzamine inactivation method showed that the alpha 1B-adrenoceptor occupancy and transients of cytosolic Ca2+ concentration ([Ca2+]i) response relationship was markedly nonlinear but similar in the two cell lines. The dose-response relationship for norepinephrine-induced [Ca2+]i response showed an increase in maximum effect with no change in agonist potency, and the increase in maximum effect was disproportionate to the difference in receptor density. The results indicate that the classical model of drug-receptor action cannot appropriately describe the coupling of alpha 1B-adrenoceptor to [Ca2+]i response in the single receptor expressing system.