Calcium and the Fc receptor on human platelets.
We have described the calcium dependence of the IgG Fc receptor (Fc-R) on human platelets by analyzing the direct binding of radiolabelled Fc fragments, monomers and dimers of IgG. Specific binding to platelets was undetectable at 37 degrees C in a calcium-free preparation but readily detected when calcium was restored. Scatchard analysis of the binding data for the calcium-restored platelets permitted calculation of the available Fc-R and the Ka of binding for the different IgG ligands. The mean Ka of binding for 12 normal subjects varied from 10(7) to 10(8) L/M, with an equal receptor number measured by Fc fragments and dimers of IgG, but a lesser amount for monomeric IgG. There was no apparent difference in Fc-R number for platelets from 6 normal male versus 6 normal female subjects. At 4 degrees C binding was detectable for dimers and polymers of IgG in a calcium-free preparation and this was markedly increased with recalcification. Thus, our data are consistent with an Fc receptor population on human platelets whose avidity for binding is significantly enhanced in a calcium-restored medium.
Clark
W F
WF
Department of Medicine, Victoria Hospital, London, Ontario, Canada.
Tevaarwerk
G J
GJ
Reid
B D
BD
Hall
S
S
Caveney
A
A
Parbtani
A
A
Kreeft
J
J
D001792:Blood Platelets / Q000648:ultrastructure*; D002118:Calcium / Q000494:pharmacology*; D005260:Female; D006801:Humans; D007141:Immunoglobulin Fc Fragments / Q000378:metabolism; D007074:Immunoglobulin G / Q000378:metabolism; D007457:Iodine Radioisotopes; D008297:Male; D011485:Protein Binding; D011487:Protein Conformation; D011961:Receptors, Fc / Q000032:analysis / Q000187:drug effects / Q000502:physiology*; D013696:Temperature
D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D007457:Iodine Radioisotopes; D011961:Receptors, Fc; D002118:Calcium