title: The role of the agonist binding site in Ca(2+) inhibition of the recombinant 5-HT(3A) receptor.
abstract: The mechanism and site of action of Ca(2+) at the recombinant murine 5-hydroxytryptamine (5-HT)(3A) receptor were investigated using whole-cell voltage clamp, radioligand binding and single-cell Ca(2+) imaging. Inhibition of the 5-HT (3 microM)-induced response by 10 mM Ca(2+) reached a plateau at 68.5% inhibition, with half-maximal effect at 2.6 mM. This was due to an increase in EC(50) from 2.35 to 3.87 microM and a 30% reduction in I(max). Ca(2+) also resulted in the inhibition of binding of both 5-HT(3) receptor agonist [3H]m-chlorophenylbiguanide and antagonist [3H]granisetron due to an increase in K(d), with no change in B(max). An increase in EC(50) from 2.6 (1 mM Ca(2+)) to 4.7 microM (10 mM Ca(2+)), with no change in maximal [Ca(2+)](i), was observed from Ca(2+) imaging studies. Largely similar effects were observed with Mg(2+). The combined data suggest that Ca(2+) acting at a site that directly or indirectly influences the agonist binding site plays a significant role in its inhibitory effect at the 5-HT(3) receptor.
authors.affiliation: Instituto de Ciencias Biomédicas, Facultad de Medicina, Universidad de Chile, Chile.
authors.forename: M I
authors.initials: MI
authors.lastname: Niemeyer
authors.forename: S C
authors.initials: SC
authors.lastname: Lummis
mesh_terms: D001665:Binding Sites / Q000502:physiology; D001667:Binding, Competitive / Q000187:drug effects; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002413:Cations, Divalent / Q000494:pharmacology; D002460:Cell Line; D004274:DNA, Recombinant / Q000235:genetics; D004305:Dose-Response Relationship, Drug; D006801:Humans; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D011985:Receptors, Serotonin / Q000187:drug effects* / Q000235:genetics / Q000502:physiology; D044406:Receptors, Serotonin, 5-HT3; D012701:Serotonin / Q000378:metabolism / Q000494:pharmacology; D017366:Serotonin Receptor Agonists / Q000378:metabolism* / Q000494:pharmacology; D014407:Tumor Cells, Cultured
chemical_list: D002413:Cations, Divalent; D004274:DNA, Recombinant; D011985:Receptors, Serotonin; D044406:Receptors, Serotonin, 5-HT3; D017366:Serotonin Receptor Agonists; D012701:Serotonin; D008274:Magnesium; D002118:Calcium