Abnormal aggregation accompanies abnormal platelet Ca2+ handling in arterial thrombosis.

The resting levels of cytoplasmic Ca2+ (measured by Quin 2 fluorescence) and dense tubular Ca2+ (measured by chlorotetracycline, CTC, fluorescence) are shown to be higher in platelets from patients with arterial thrombosis than from normal donors. Turbidimetric studies of aggregation of diluted platelet-rich plasma (PRP) at 135 microM Ca2+ showed increased rates of aggregation for patients relative to normal controls. For ADP-stimulated aggregation, increased maximal rates (Vmax) and decreased doses for half-maximal rates were observed. With collagen-stimulated aggregation, patient samples showed only decreased ED50 values relative to normal controls. The changes in these values are linearly correlated with the elevation of resting dense tubular Ca2+ level determined by the calcium-CTC test carried out at 2 mM external Ca2+. For ADP-stimulated aggregation this relationship can be mimicked by pre-incubating normal platelets with subcritical concentrations of the Ca2+ ionophore A23187. These results suggest that elevated cytoplasmic and dense tubular Ca2+ in the "resting state" is a major factor in arterial thrombosis, rendering the platelet more sensitive to the stimulation by physiologic agents.