EFFECT OF CIRCULATING EPINEPHRINE ON PLATELET-FUNCTION AND HEMATOCRIT

We investigated the effect of raising arterial plasma epinephrine with in the lower pathophysiological concentration range on various indicat ors of blood platelet function and hematocrit. Epinephrine was raised over 60 minutes by a stepwise increasing intravenous infusion in 40 he althy men aged 20 to 40 years. Platelet count increased progressively with increasing arterial epinephrine to a maximal change of 69+/-6 X 1 0(9)/L in EDTA-anticoagulated blood and a maximal change of 42+/-6 X 1 0(9)/L in acid-citrate-dextrose (ACD)-anticoagulated blood, and the we ight of circulating platelets increased by 29% (P<.001). Platelet size increased significantly in EDTA and decreased in ACD, and the differe nce between EDTA and ACD was significant (P<.0001) for both count and size, suggesting that epinephrine not only recruits platelets into the circulation but also induces some microaggregation in vivo or adhesio n ex vivo. Aggregation of platelets in vitro induced by epinephrine de creased (P<.003 for Delta optical density and P=.038 for maximal optic al density) after epinephrine infusion compared with saline but did no t change when stimulated with ADP or collagen. These findings suggest a selective downregulation of the epinephrine-activating mechanisms co ncomitant with a rise in the platelet content of epinephrine by 81% (P <.001) and no change in the platelet sodium-proton membrane exchange. The release of granular content (beta-thromboglobulin and platelet fac tor 4) to the circulation in response to epinephrine was not significa nt. Thus, under acute conditions it seems that the platelets may prote ct themselves against inappropriate overstimulation by epinephrine. Th e importance of platelet epinephrine uptake is still unknown, but sodi um-proton exchange does not seem to be involved in regulating the effe cts of circulating epinephrine on platelet function. Epinephrine has a pronounced effect on raising hematocrit (maximal change of 1.74+/-0.1 3 X 10(-2), P<.0001).