β-Adrenergic activities of octopamine and synephrine stereoisomers on guinea-pig atria and trachea

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Abstract
The activities of the (-)- and (+)-forms of m- and p-octopamine and m- and p-synephrine on β1- and β2- adrenoceptors in guinea-pig atria and trachea have been compared with that of noradrenaline. The rank order of potency of the (-)-forms on β1-adrenoceptors was noradrenaline > m-synephrine. m-octopamine = p-octopamine > p-synephrine. m-Synephrine was 100-fold, m-and p-octopamine about 6000-fold, and p-synephrine about 40 000-fold less active than noradrenaline. The (+)-forms were 1-2 orders of magnitude less active than their (-)-counterparts. The four (-)-compounds were more than four orders of magnitude less active than noradrenaline on β2,-adrenoceptors, and the (+)-forms had no detectable activity in concentrations as high as 10−4 M. If m-and p-octopamine are co-released with noradrenaline in amounts proportional to their concentration, their activities at these structures are too low to be physiologically significant.