The number and affinity of binding sites for tritiated prostacyclin (PGI2) in platelets were investigated in twenty-eight patients with spontaneous angina (fourteen in the active and fourteen in the inactive phase) and in nine healthy controls of similar age. Active-phase patients had significantly lower numbers of both classes of platelet PGI2 receptors (high affinity and low affinity) than controls or inactive-phase patients. In contrast, the affinity for PGI2 was not significantly different in the three groups. In six active-phase patients restudied in the inactive phase the previously low number of PGI2 receptors had returned to normal. These results suggest that the instability of angina is associated with functional changes not confined to the coronary vasculature.
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