Activities of Octopamine and Synephrine Stereoisomers on Octopaminergic Receptor Subtypes in Locust Skeletal Muscle

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Abstract
The activities of the (−) and (+)- forms of p-, m- and o- octopamine and p- and m- synephrine have been compared on the different subtypes of octopamine receptor present in the extensor-tibiae neuromuscular preparation from the locust hindleg. The rank order of potency of the (−)-forms on the OCTOPAMINE2A receptors was p-synephrine > p-octopamine > m-octopamine > o-octopamine > m-synephrine whilst the rank order of the (+)-forms was p-synephrine > p-octopamine > m-octopamine. (+)-m-Synephrine and (+)-o-octopamine had no effect on this class of receptor when tested up to a concentration of 10−3M. The rank order of potency of the (−)-forms on the OCTOPAMINE2B receptors was p-synephrine > p-octopamine > m-synephrine > m-octopamine > o-octopamine whilst the rank order of the (+)-forms was p-octopamine > p-synephrine > m-octopamine > o-octopamine. (+)-m-Synephrine again had no effect up to a concentration of 10−3M. The rank order of potency of the (−)-forms ontheOCTOPAMINE1 receptors was p-synephrine > p-octopamine > m-synephrine > m-octopamine > o-octopamine, whilst the rank order of the (+)-forms was p-synephrine > p-octopamine > o-octopamine > m-synephrine > m-octopamine. In all cases the (−) enantiomers of all the structural isomers tested were more potent than the corresponding (+)-forms with isomeric activity ratios varying between 2 and 480. The activities of the octopamine and synephrine stereoisomers on the different octopamine receptor subtypes in the locust are compared with their published activities on the subtypes of mammalian α- and β-adrenoceptors.