1 The activities of the (-)-and (+ )-forms of m-and p-octopamine and m-and p-synephrine on a,-adrenoceptors from rat aorta and anococcygeus and a2-adrenoceptors from rabbit saphenous vein were compared with those of noradrenaline (NA).2 The rank order of potency of the (-)-forms on cx,-adrenoceptors from rat aorta and a2-adrenoceptors was NA > m-octopamine = m-synephrine > p-octopamine = p-synephrine. The two m-compounds were 6 fold less active than NA on x,-adrenoceptors from rat aorta and 150 fold less active on x,-adrenoceptors. The two p-compounds were 1,000 fold less active than NA on both oqadrenoceptors from rat aorta and x2-adrenoceptors. The rank order ofpotency ofthe (-)-forms on oqadrenoceptors from rat anococcygeus was NA = m-synephrine > m-octopamine >p-octopamine = p-synephrine. m-Octopamine was 4 fold less active than NA and (-)-m-synephrine. The two pcompounds were 30 fold less active than NA. 3 The rank order of potency of the (+)-forms was NA>m-octopamine>m-synephrine>p-octopamine >p-synephrine on both a,-and M2-adrenoceptors. The potency ofeach ( + )-form was 1-2 orders of magnitude less than that of the (-) counterpart, the differences being greater for the stereoisomers of synephrine than for those of octopamine on both a,-and M2-adrenoceptors. 4 The yohimbine diastereoisomer antagonists, rauwolscine and corynanthine, were tested against (-)-NA and (-)-m-octopamine-induced contractions in both preparations. Based upon the known selectivities of these isomers for a-adrenoceptor subtypes, it is concluded that the rat aorta contains only a,-adrenoceptors while the rabbit saphenous vein possesses predominantly M2-adrenoceptors.5 Ligand binding data for the octopamine and synephrine stereoisomers at oq-and a2-binding sites from rat cerebral cortex was also obtained. (-)-Forms were more active than (+ )-forms. The rank order of affinity of the (-)-forms for both a,-and M2-binding sites was NA > m-octopamine = msynephrine >p-synephrine >p-octopamine. The relative affinities ofthe members ofthe series against a,-binding sites were very similar to their relative functional activities on rat aorta. However, the affinities of both m-and p-compounds relative to that of ( -)-NA were much greater at the x2-binding sites than were the relative activities in rabbit saphenous vein, possibly suggesting low intrinsic efficacy. Functional antagonist responses to NA by the (-)-octopamine and synephrines could not, however, be demonstrated on rat aorta or rabbit saphenous vein. 6 The activities of m-octopamine and m-synephrine were not significantly different from each other on either a,-adrenoceptors from rat aorta or x2-adrenoceptors; however, m-synephrine is more active than m-octopamine on a,-adrenoceptors from rat anococcygeus. Both m-octopamine and msynephrine can be considered to be naturally occurring x,-selective amines. However, if m-and poctopamine are co-released with NA in amounts proportional to their concentration, it is concluded that their activities on m,-and x2-adrenoceptors are too low to be ...
