Influence of platelet volume on the ability of prostacyclin to inhibit platelet aggregation and the release reaction.

The influence of mean platelet volume (MPV) and platelet count on in vitro platelet sensitivity to prostacyclin (PGI2) was studied with human size-dependent platelet subpopulations prepared by counterflow centrifugation. The original unfractionated platelet suspension and each of five size-dependent platelet fractions were suspended in buffer at a platelet count of 2 X 10(8)/ml. The percent decrease in the extent of platelet aggregation and adenosine triphosphate (ATP) release in response to 10 micrograms/ml collagen was determined over a range of PGI2 concentrations in a Lumi-Aggregometer. A significant positive correlation between MPV and the concentration required to give 50% inhibition for both platelet aggregation and ATP release (r = 0.99, p less than 0.001 and r = 0.99, p less than 0.001, respectively) was observed. In separate experiments, the effect of platelet count on the ability of a given dose of PGI2 to inhibit platelet aggregation and ATP release was determined, and a significant inverse correlation was noted (r = 0.99, p less than 0.01 and r = 0.98, p less than 0.01, respectively). Our data indicate that the sensitivity of human platelets to the inhibitory effects of PGI2 is dependent on both the platelet volume and the platelet count. Thus, the presence of a greater platelet mass, resulting from either an increased MPV or an increased platelet count, decreases the inhibitory effectiveness of PGI2 on both platelet aggregation and the release reaction.