Effects of the noradrenaline metabolites on the adrenergic receptors.

Abstract

The effects of the five noradrenaline (NA) metabolites and of the O-methylated metabolite of adrenaline, metanephrine, were studied in a tissue with adrenergic beta-receptors (guinea-pig atria) and in a tissue with predominance of alpha-receptors (cat's nictitating membrane). In atria, normetanephrine and metanephrine elicited positive chronotrophic effects which were mediated through the beta-receptors. Both O-methylated metabolites had only 1/1000th of the potency of NA. In the normally innervated nictitating membrane normetanephrine and metanephrine elicited maximal responses of the same magnitude as NA. While normetanephrine had one half of the potency of NA, metanephrine was even more potent than NA. The effects of normetanephrine or metanephrine were mediated through the activation of the alpha-receptors and were not potentiated by either cocaine or denervation. Neither the deaminated nor the deaminated-O-methylated metabolites of NA had activity as agonists on alpha-or beta-receptors in concen-of up to 1×10−4M. These metabolites did not elicit alpha-or beta-receptor block in concentrations of up to 1×10−4M.