alpha 2B-Adrenoceptor levels govern agonist and inverse agonist responses in PC12 cells

Receptor density is an important determinant of cellular effector responses to receptor activation. We analysed cytosolic Ca(2+) responses to alpha(2)-adrenergic agents in PC12 cells expressing human alpha(2B)-adrenergic receptors (AR) at two densities (3.8 and 1.3 pmol/mg protein). The efficacy (E(max)) of agonists was greater in cells with higher receptor expression; while the potency (EC(50)) of norepinephrine and oxymetazoline was independent of alpha(2B)-AR levels. Several classical alpha(2)-AR antagonists behaved as either partial or inverse agonists in a receptor density-dependent fashion. No apparent structural similarities were found among the inverse agonists, precluding simple predictions of inverse agonist activity. Transfected PC12 cells expressing alpha(2B)-AR at relatively high density would be a useful approach to screen inverse agonists for this class of receptors. Our results further indicate that receptor density significantly influences the properties of ligands, not only of partial agonists as predicted by classical receptor theory, but also of antagonists and full agonists.