Adrenergic Receptor Subtype Activation by (+)-, (-)- and (+/-) - Norephedrine in the Pithed Rat

The ability of (+/-)-norephedrine (phenylpropanolamine) and its component isomers, (+)-and (-)-norephedrine, to activate adrenergic receptor subtypes in the cardiovascular system of the urethane/chloralose-anaesthetized pithed rat has been investigated. At all adrenoceptor subtypes, (-)-norephedrine was the most potent agonist followed by (+/-)- then (+)-norephedrine. The greatest activity was observed at the alpha 1-receptor, with little activity observed at either beta 1 or beta 2-adrenoceptors. Reserpinization shifted the (-)-norephedrine dose-response curve slightly to the right, indicating that only a minor portion of its activity is due to the release of stored endogenous catecholamines. These results suggest that most of the cardiovascular activity of the compounds is through the direct activation of alpha 1-adrenoceptors.