Inhibition by prostacyclin of the tumor cell-induced platelet release reaction and platelet aggregation.

Prostacyclin was examined for its inhibitory effects on the tumor cell-induced platelet release reaction. Prostacyclin inhibited in a dose-dependent manner tumor cell-induced release of platelet dense granules and alpha-granules concomitant with an inhibition of platelet aggregation. Release was determined by assay of biochemical markers (serotonin for dense granules and beta-thromboglobulin for alpha-granules). A tenfold higher concentration of prostacyclin was required to inhibit completely serotonin release as compared to the concentration required for beta-thromboglobulin release. Correlative ultrastructural studies demonstrated that prostacyclin at doses of over 10 ng/ml inhibited the ultrastructural changes associated with tumor cell-induced platelet shape change and platelet granule release. Platelet aggregates exhibited the retention of granule reservoirs that could potentially be involved in long-term release of biologically active substances.