MYDRIASIS ELICITED BY IMIDAZOL(IN)E ALPHA(2)-ADRENOMIMETICS IN COMPARISON WITH OTHER ADRENOCEPTOR-MEDIATED EFFECTS AND HYDROPHOBICITY

alpha(2)-Adrenoceptor agonists cause both mydriasis and platelet aggre gation. This work is aimed at identifying the factors accompanying and affecting mydriatic activity. For eight imidazol(in)e drugs mydriatic , hypotensive and bradycardic activities were determined in rats. The lipophilicity of the agents was determined chromatographically and cal culated theoretically. A correlation was found between the hypotensive and the bradycardic potency and between the mydriatic activity and bo th the hypotensive and bradycardic activity. Mydriatic activity depend ed on the lipophilicity of the agents studied. The human platelet anti aggregatory activity of the drugs did not correlate with either the my driatic or cardiovascular activity and it was independent of lipophili city. The dependence of the centrally induced effects on lipophilicity and the lack of such a dependence in the case of the in vitro alpha(2 )-adrenoceptor-mediated platelet aggregation may be interpreted as res ulting from heterogeneity of the rat cerebral and the human platelet a lpha(2)-adrenoceptors. The alpha(2)-adrenergic activity of drugs in th e model of mydriasis in rats cannot be predicted from their activity i n causing human platelet aggregation in vitro.