Improved models for pharmacological null experiments: calculation of drug efficacy at recombinant D1A dopamine receptors stably expressed in clonal cell lines.

Modern drug discovery demands accurate knowledge of the drug properties of affinity and efficacy at specific receptor proteins. Furthermore, drugs with well defined properties make better tools with which to explore and understand receptor regulation. The use of clonal cell lines stably expressing a given recombinant receptor may provide a highly useful model in which drug effects may be studied on one receptor subtype at a time. The present report was designed to evaluate the utility of a general method in which a clonal cell line stably expressing a recombinant D1A dopamine receptor was used as a model system for studying drug actions by null models. The null model for receptor occlusion (to calculate agonist Ka) and the null model for relative efficacy (to calculate test agonist affinity and epsilon r) were evaluated in these studies. To initiate these studies, rat C6 glioma cells that do not normally express DA receptors have been modified by stable transfection with the primate D1A DA receptor [Machida et al., 1992 (Molec. Pharmacol. 41: 652-659)] to a density of approximately equal to fmol/mg protein. The recombinant receptors show robust stimulation of cAMP in the stably transfected C6 cells. Calculation of agonist Ka from dose-response data requires that a portion of the cell's receptors be occluded in the absence of changes in post-receptor events leading to the response. Receptor reserve is typically reduced by alkylation, thereby lowering maximal response. Unfortunately, most of the currently available alkylating agents are not selective either for a particular receptor or for receptors vs other proteins within a signaling pathway. Short-term agonist treatment offers a possible complement to the use of non-selective or poorly characterized alkylating drugs for reducing maximum response in appropriate cell systems. The null method of receptor occlusion was used to determine the Ka for dopamine when maximum response was decreased by alkylation vs short-term agonist treatment. Direct non-linear curve fitting was used to analyze the data. In addition to DA, two other compounds were used to reduce receptor reserve to validate the method: fenoldopam (relatively high efficacy) and SKF38393 (low efficacy). Analyses indicated that the affinity of DA was similar whether calculated by alkylation (1.1 +/- 0.58 microM), 75 min DA treatment (0.57 +/- 0.16 microM) or 45 min treatment with DA (0.86 +/- 0.11 microM). Short-term agonist treatment experiments using multiple concentrations of DA, fenoldopam, or SKF38393 to decrease receptor reserve provided additional support for the validity of the Ka determinations using this procedure. Other experiments were conducted according to the null model for relative efficacy in which the affinity for DA is calculated by comparing the DA response before and after receptor occlusion, and the affinity and relative intrinsic efficacy of the test agonist are determined as a function of its actions relative to DA. We used the following four test drugs: + Br-APB, a novel agent with potential dopamine agonist properties, and three high-affinity DA agonists, fenoldopam, R-(-)-apomorphine (APO), and SKF38393. Intrinsic efficacy values relative to that of DA (1.0) were as follows: fenoldopam, 0.46 +/- 0.11; APO, 0.19 +/- 0.13; SKF38393, 0.07 +/- 0.01; and +Br-APB, 0.26 +/- 0.40. The agonist affinities (Ka) were: fenoldopam, 0.018 +/- 0.008 microM; APO, 0.80 +/- 0.18 microM; SKF38393, 0.16 +/- 0.04 microM; BR-APB, 0.43 +/- 0.29 microM; and DA, 0.58 +/- 0.17 microM. EC50/Ka ratios were consistent with relative intrinsic efficacies and Ka values were similar to KL values reported for membrane binding studies. Finally, Monte Carlo simulations were conducted to determine the precision of the parameter estimates...