Receptor reserve masks partial agonist activity of drugs in a cloned rat 5-hydroxytryptamine1B receptor expression system.

The present study was undertaken to evaluate the behavior of the rat 5-hydroxytryptamine (5-HT)1B receptor stably expressed in a Y-1 cell line, using both radioligand binding ([125I]iodocyanopindolol) and functional assays (inhibition of forskolin-stimulated cAMP release). The measured EC50 values for agonists were lower than expected from the measured Ki values (e.g., 5-HT, EC50 = 0.49 +/- 0.043, nine experiments; Ki = 5.3 +/- 0.82, four experiments). Furthermore, beta-adrenergic antagonists such as propranolol and pindolol, which have been reported to be partial agonists or antagonists at the 5-HT1B receptors in other systems, were found to be full agonists. To investigate the relationship between receptor occupancy and inhibition of cAMP release (and hence the degree of receptor reserve), we used the irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). EEDQ treatment shifted the dose-response curve for 5-HT to the right by 6-7-fold, accompanied by a reduction (30-50%) in maximal response. Analysis of the data by the method of Furchgott revealed a very steep hyperbolic relationship between receptor occupancy and response for 5-HT, with 92 +/- 1.4% (three experiments) receptor reserve at the 50% maximal response level. In contrast to its effect on 5-HT, EEDQ treatment did not significantly shift the dose-response curves for pindolol; rather, only the maximal responses were reduced and a linear relationship was found between receptor occupancy and response for this compound. According to classical receptor theory these data indicate that pindolol is a partial agonist, relative to 5-HT, but because of the high density of 5-HT1B receptors in this system the difference between the intrinsic activities of the two drugs is masked. Therefore, one has to be cautious when interpreting functional data in transfected systems that often display large receptor reserve.