title: 2-[125I]iodomelatonin binding sites in the quail liver: characterization and the effect of guanosine 5'-O-(3-thiotriphosphate).
abstract: Melatonin receptors were studied in quail livers using the melatonin agonist 2-[125I]iodomelatonin ([125I]MEL) as the radioligand. The specific binding of [125I]MEL to membrane preparations of liver was rapid, stable, saturable, reversible and of high affinity. Scatchard analysis of the specific binding data indicated an equilibrium dissociation constant (Kd) of 19.4 +/- 1.01 pmol/l (n = 7) and a maximum number of binding sites (Bmax) of 1.16 +/- 0.19 fmol/mg protein (n = 7) in the quail liver collected at mid-light. The Hill coefficient approached 1.0, suggesting a single class of [125I]MEL binding sites in the quail liver. The diurnal variation study showed that the value of Kd was 64.4% higher (p < 0.05) at mid-dark compared to mid-light, with no significant change in Bmax. The kinetic analysis showed that the Kd value was 25.0 +/- 3.94 pmol/l at mid-light, which was comparable with values determined from saturation studies. Aside from 2-iodomelatonin, melatonin and 6-chloromelatonin, all indole analogs and neurotransmitters tested in inhibition studies had slight or no displacement of [125I]MEL binding. These studies demonstrated that [125I]MEL binding sites were highly specific for melatonin. The presence of 10 and 50 mumol/l guanosine 5'-O-(3-thiotriphosphate) significantly increased (p < 0.05) the Kd values and depressed the Bmax values, proposing that [125I]MEL binding sites in quail livers were coupled to a G-protein. Our results indicate that melatonin may exert a direct action on liver functions.
authors.affiliation: Department of Physiology, University of Hong Kong.
authors.forename: Q
authors.initials: Q
authors.lastname: Wan
authors.forename: S F
authors.initials: SF
authors.lastname: Pang
mesh_terms: D000818:Animals; D001665:Binding Sites; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002940:Circadian Rhythm / Q000502:physiology; D003370:Coturnix; D004305:Dose-Response Relationship, Drug; D005260:Female; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology*; D007211:Indoles / Q000494:pharmacology; D007457:Iodine Radioisotopes; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008550:Melatonin / Q000031:analogs & derivatives* / Q000378:metabolism; D018377:Neurotransmitter Agents / Q000494:pharmacology; D011784:Quail; D011956:Receptors, Cell Surface / Q000378:metabolism*
chemical_list: D007211:Indoles; D007457:Iodine Radioisotopes; D018377:Neurotransmitter Agents; D011956:Receptors, Cell Surface; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C044556:2-iodomelatonin; D008550:Melatonin