Binding sites for [3H]-melatonin in human platelets.
A number of in vitro effects of melatonin on human platelets were revealed in previous studies. In order to examine whether high affinity binding sites for [3H]-melatonin were present in membrane preparations of human platelets, a rapid filtration procedure through Whatman GFB paper was employed. Maximal melatonin binding was attained within 3 hr at 0 degree C. Scatchard analysis indicated a single population of binding sites with a dissociation constant (Kd) = 4.1 +/- 0.5 nM and maximal number of binding sites (Bmax) = 24.2 +/- 1.9 fmol/mg protein (mean +/- SEM of five experiments). When various indole analogs were tested for their ability to inhibit [3H]-melatonin binding, the following Ki (nM) were obtained: 6-chloromelatonin (11.4), 2-iodomelatonin (22.0), melatonin (24.7), 5-methoxytryptophol (49.9), N-acetylserotonin (68.9), 6-hydroxymelatonin (78.2), 5-methoxytryptamine (184). Serotonin was a potent inhibitor of [3H]-melatonin binding with a Ki = 20.6 nM. Except for 2-methylserotonin and alpha-methylserotonin, a number of serotonin agonists and antagonists tested did not affect melatonin binding to platelet membranes. Binding experiments carried out at either 0800 or 2000 did not reveal time-dependent differences in Kd or Bmax. The results suggest that high affinity melatonin acceptors are present in human platelets.
Vacas
M I
MI
Departamento de Fisologia, Facultad de Medicina, Buenos Aires, Argentina.
Del Zar
M M
MM
Martinuzzo
M
M
Cardinali
D P
DP
D000328:Adult; D001667:Binding, Competitive; D001792:Blood Platelets / Q000378:metabolism*; D002940:Circadian Rhythm / Q000502:physiology; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008550:Melatonin / Q000031:analogs & derivatives / Q000097:blood*; D008875:Middle Aged; D011869:Radioligand Assay; D044094:Receptors, Melatonin; D017981:Receptors, Neurotransmitter / Q000378:metabolism*; D012702:Serotonin Antagonists / Q000097:blood; D017366:Serotonin Receptor Agonists / Q000097:blood
D044094:Receptors, Melatonin; D017981:Receptors, Neurotransmitter; D012702:Serotonin Antagonists; D017366:Serotonin Receptor Agonists; D008550:Melatonin