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The action of metoclopramide, a drug which is used as an antiemetic and prokinetic, on human (h)5-HT 3A receptors, stably expressed in HEK-293 cells, was studied with patch clamp and [ 3 H]-radioligand binding techniques. At clinical concentrations, metoclopramide inhibited peak and integrated currents through h5-HT 3A receptors concentration-dependently (IC 50 = 0.064 and 0.076 μM, respectively) when it was applied in equilibrium [60 s before and during 5-HT (30 μM) exposure]. The onset and offset time constants of metoclopramide action were 1.3 s and 2.1 s, respectively. The potency of metoclopramide when exclusively applied during the agonist pulse decreased more than 200-fold (IC 50 = 19.0 μM, peak current suppression). Metoclopramide (0.10 μM) did not alter the EC 50 of 5-HT-induced peak currents. In contrast to the lack of competitive interaction between metoclopramide and 5-HT in this functional assay, metoclopramide inhibited specific [ 3 H] GR65630 binding to human 5-HT 3A receptors in a surmountable manner. This seeming discrepancy between functional studies and radioligand binding experiments may be accounted for by (1) the slow kinetics of inhibition of peak currents by metoclopramide compared with the fast onset and offset kinetics of 5-HT-induced currents and (2) the low efficacy of metoclopramide in inhibiting radioligand binding (e.g., only 20% binding inhibition compared with 79% peak current suppression by 200 nM metoclopramide). |
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