Abstract
Summary
The resting levels of cytoplasmic Ca
2+
(measured by Quin 2 fluorescence) and dense tubular Ca
2+
(measured by chlorotetracycline, CTC, fluorescence) are shown to be higher in platelets from patients with arterial thrombosis than from normal donors. Turbidmetric studies of aggregation of diluted platelet-rich plasma (PRP) at 135 μM Ca
2+
showed increased rates of aggregation for patients relative to normal controls. For ADP-stimulated aggregation, increased maximal rates (V
max
) and decreased doses for half-maximal rates were observed. With collagen-stimulated aggregation, patient samples showed only decreased ED
50
values relative to normal controls. The changes in these values are linearly correlated with the elevation of resting dense tubular Ca
2+
level determined by the calcium-CTC test carried out at 2 mM external Ca
2+
. For ADP-stimulated aggregation this relationship can be mimicked by pre-incubating normal platelets with subcritical concentrations of the Ca
2+
ionophore A23187.
These results suggest that elevated cytoplasmic and dense tubular Ca
2+
in the “resting state” is a major factor in arterial thrombosis, rendering the platelet more sensitive to the stimulation by physiologic agents.
