Imidazoline ReceptorsIdazoxanReceptors, DrugAgmatineImidazolesBenzofuransReceptors, Adrenergic, alpha-2Adrenergic alpha-2 Receptor AgonistsClonidineAdrenergic alpha-AntagonistsDioxanesAdrenergic alpha-AgonistsYohimbineOxazolesAdrenergic alpha-2 Receptor AntagonistsMedetomidineImidazolinesAntihypertensive AgentsIsoindolesRadioligand AssayRats, Sprague-DawleyDose-Response Relationship, DrugHeart AtriaMedulla OblongataLigandsBinding, CompetitiveAtrial Function