[¹²⁵I]CGP64213 binding on membranes prepared from HEK 293 cells expressing the indicated receptor was measured in the absence and presence of increasing concentrations of the indicated compounds. IC₅₀ values were determined as described underExperimental Procedures. Experiments were conducted in the presence of 1 mM Ca²⁺, or in the presence of 1 mM EGTA with no added Ca²⁺. Values are means ± S.E.M. of at least three independent experiments performed in triplicate. The ratio of the mean IC₅₀ determined in the absence of Ca²⁺ over those determined in the presence of Ca²⁺ is indicated.