the ( - ) stereoisomers of epinephrine and norepinephrine are 30 times more potent in competing for the site than the corresponding ( + ) stereoisomers . alpha-adrenergic antagonists , such as phentolamine and phenoxybenzamine , potently compete for the binding sites while the beta-adrenergic antagonist propranolol does not .
C0014563
epinephrine
C0028351
norepinephrine