title: 2-[125I]Iodomelatonin binding sites in the quail heart: characteristics, distribution and modulation by guanine nucleotides and cations.
abstract: To investigate whether melatonin has a direct action on the cardiovascular system, putative melatonin receptors were studied in quail heart membrane preparations using the specific melatonin agonist 2-[125I]iodomelatonin (125I]Mel, as the radioligand. The [125I]mel binding demonstrated in the mature quail heart was saturable, highly 5.2 pM; Bmax = 1.32 +/- 0.25 fmol/mg protein; n = 8). The linear Scatchard plots and the close to unity Hill coefficient indicated a single class of binding sites. The pharmacological profile was in the affinity order of 2-iodomelatonin = 2-phenylmelatonin > melatonin > 6-chloromelatonin >> 6-hydroxymelatonin > 6-sulphatoxymelatonin >> N-acetylserotonin>>>5-hydroxytryptamine. Guanosine 5'-triphosphate and guanosine 5'-O-(3-thiotriphosphate) (GTP gammaS) dose dependently inhibited the binding. Ten microM GTPgammaS lowered the binding affinity by 50% in saturation studies. The order of potency of inhibition by cations was: Ca2+ > Mg2+ > Li+ > Na+ > K+ > choline chloride. Contrary to most other melatonin binding sites, millimolar concentrations of Ca2+ and Mg2+ did not promote binding in the quail heart membranes. In vitro autoradiography indicated homogenous labeling in the heart. Our results demonstrated [125I]Mel binding sites in the quail heart. That guanine nucleotides and Na+ inhibited the binding indicated that these putative melatonin receptors are coupled to guanine nucleotide-binding proteins (G-proteins).
authors.affiliation: Clarke Institute of Psychiatry, Toronto, Ontario, Canada.
authors.forename: C S
authors.initials: CS
authors.lastname: Pang
authors.forename: P L
authors.initials: PL
authors.lastname: Tang
authors.forename: Y
authors.initials: Y
authors.lastname: Song
authors.forename: G M
authors.initials: GM
authors.lastname: Brown
authors.forename: S F
authors.initials: SF
authors.lastname: Pang
mesh_terms: D000818:Animals; D001345:Autoradiography; D001665:Binding Sites; D001667:Binding, Competitive; D002122:Calcium Chloride / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D002794:Choline / Q000494:pharmacology; D003370:Coturnix; D005260:Female; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology*; D007457:Iodine Radioisotopes; D007700:Kinetics; D015636:Magnesium Chloride / Q000494:pharmacology; D008297:Male; D008550:Melatonin / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000494:pharmacology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D011869:Radioligand Assay; D011956:Receptors, Cell Surface / Q000032:analysis / Q000187:drug effects / Q000378:metabolism*; D044094:Receptors, Melatonin; D012680:Sensitivity and Specificity; D013329:Structure-Activity Relationship
chemical_list: D007457:Iodine Radioisotopes; D011956:Receptors, Cell Surface; D044094:Receptors, Melatonin; D015636:Magnesium Chloride; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D008550:Melatonin; D002122:Calcium Chloride; D002794:Choline