The primary wave of epinephrine-induced platelet aggregation represents alpha2-adrenoceptor status

We examined relationships between epinephrine-induced slope of primary wave of aggregation and the alpha 2 -adrenoceptor status on platelets. A concentration (10 -9 to 10 -6 )-dependent increase in slope of primary wave with EC50 of epinephrine at 4.5 ± 0.4 × 10 -7 was observed. In studies on epinephrine binding to alpha 2 adrenoceptors in competition with 3 H-yohimbine to platelets, (IC 50 ) of epinephrine was 4.8 ± 3.4 x 10 -7 M. There was a significant (P<0.02) correlation between EC 50 of epinephrine to evoke biological response and IC 50 of epinephrine to bind to alpha 2 -adrenoceptors (r-0.75). There was no relationship between number of receptor sites or dissociation constant of 3 H-yohimbine binding and primary wave of platelet aggregation. These data show that the slope of primary wave in response to epinephrine reflects alpha 2 -adrenoceptor binding of the agonist.