pH and magnesium alter 45calcium binding to platelets at sites other than glycoproteins I or IIb/IIIa. 
Calcium is a cofactor of human platelet aggregation.  Moreover a direct correlation between the ability of platelets to bind this divalent cation and to aggregate has been demonstrated.  Since magnesium can substitute for calcium in supporting aggregation, especially in the presence of low calcium concentrations, and platelet aggregation is inhibited at low pH, the present study was designed to examine the effects of magnesium and low pH on 45calcium binding to human platelets, and to determine whether such effects might be associated with calcium binding to glycoproteins I (GPI) or IIb/IIIa (GPIIb/IIIa), the putative fibrinogen receptor.  45Calcium binding to aspirin-treated platelets that had been depleted of surface-associated calcium by brief exposure to EDTA was evaluated.  Magnesium (5-10 mM) or a change in hydrogen ion concentration to decrease the pH from 7.5 to 6.0 was found to inhibit the binding of 45calcium to platelets from healthy donors by 34 +/- 6 and 32 +/- 8% (mean +/- SD, n = 13), respectively.  Similar results were obtained with platelets incubated with chymotrypsin to selectively remove GPI or platelets from a patient with the Bernard Soulier Syndrome, congenitally deficient in GPI.  In contrast, calcium binding to platelets from two patients with thrombasthenia, lacking GPIIb/IIIa, was reduced 49 +/- 6% and 42 +/- 8% (n = 4) by magnesium and hydrogen ions, respectively.  This apparently increased inhibition was attributed to the combined effects of an overall decrease (approximately 50%) in calcium binding to thrombasthenic platelets compared with that in control platelets, and a similar absolute reduction in calcium binding in the presence of magnesium and/or hydrogen ions.  No additional inhibition of 45calcium binding was noted in the presence of magnesium and at low pH, indicating that magnesium and hydrogen ions may affect the same platelet membrane binding sites.  The data suggest that although modulation of platelet aggregation by magnesium and pH is accompanied by changes in platelet-associated calcium, calcium binding to the three major platelet membrane glycoproteins, GPI, IIb, and IIIa is unaffected.