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 (∼ 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.