The primary wave of epinephrine-induced platelet aggregation represents alpha 2-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 x 10(-7) was observed. In studies on epinephrine binding to alpha 2-adrenoceptors in competition with 3H-yohimbine to platelets, (IC50) of epinephrine was 4.8 +/- 3.4 x 10(-7) M. There was a significant (P less than or equal to 0.02) correlation between EC50 of epinephrine to evoke biological response and IC50 of epinephrine to bind to alpha 2-adrenoceptors (r-0.75). There was no relationship between number of receptor sites or dissociation constant of 3H-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.