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The concentration-dependent effects of clonidine, isomers of epinephrine, norepinephrine (NE), isoproterenol, cobefrin and alpha-methyldopamine, and related desoxy analogs (epinine, dopamine, N-isopropyldopamine) were examined on human platelets. The rank order of aggregatory potency (pD2 values) was R(-)-epinephrine (6.3) greater than R(-)-NE (5.9) greater than (+/-)-erythro-cobefrin (5.3) greater than S(+)-epinephrine (4.7) = S(+)-NE (4.7) = clonidine (4.7) = dopamine (4.6) greater than epinine (4.4) greater than S(+)-alpha-methyldopamine (4.3) = R(-)-alpha-methyldopamine (4.3) greater than (+/-)-threo-cobefrin (3.7). The isoproterenol isomers and N-isopropyl-dopamine were inactive as agonists. In 9 of 16 platelet-rich plasma preparations, R(-)-epinephrine, R(-)-NE, and (+/-)erythro-cobefrin were agonists and the remaining analogs blocked R(-)-NE-induced aggregation with a rank order of inhibitory potencies (pKB values) of clonidine (6.2) greater than S(+)-alpha-methyldopamine (5.0) greater than dopamine (4.6) = R(-)-alpha-methyldopamine (4.4) greater than or equal to S(+)-NE (4.3) greater than N-isopropyldopamine (4.1) greater than S(+)-isoproterenol (3.7) = R(-)-isoproterenol (3.5). Each compound was also able to reverse prostaglandin E1 (PGE1) (0.1 microM)-induced blockade of the maximal aggregation response to ADP. At high concentrations, R(-)-isoproterenol was more potent than either the S(+)-isomer or desoxy analog, N-isopropyldopamine, in the reversal of PGE1 inhibition of ADP aggregation. Phentolamine blocked these alpha 2-adrenoceptor-mediated actions against PGE1 on ADP aggregation. The rank order of potency for the reversal of PGE1-mediated inhibition of ADP aggregation by these catecholamines was similar to that observed for platelet aggregation. Our results indicate that (i) the stereochemical requirements for the interaction of catecholamines with platelet alpha 2-adrenoceptors are in agreement with the Easson-Stedman hypothesis and other alpha-adrenoceptor tissues; (ii) catecholamines lacking a benzylic hydroxyl group in the R-configuration and/or possessing an N-isopropyl group were alpha 2-adrenoceptor antagonists; (iii) clonidine gave quantitatively different responses compared with catecholamines for interaction with alpha 2-adrenoceptors; and (iv) inhibition of platelet adenylate cyclase is correlated to the inhibition of epinephrine-induced aggregation response for this series of compounds.
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G Shams, and
R L Schotzinger, and
D D Miller, and
D R Feller
July 1987,
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