Effect of (plus or minus) dihydroxy ephedrine and (plus or minus) dihydroxy pseudoephedrine on adrenergic transmission in mesenteric arteries.

1. The effect of (+/-) dihydroxy ephedrine (DHE), (+/-) dihydroxy pseudoephedrine (DHPE) and isoprenaline on the vasoconstrictor responses of the perfused mesenteric arteries of the rat produced by stimulation of their periarterial sympathetic fibres, and in the responses to injected noradrenaline, has been studied.2. DHE and DHPE inhibited the responses to nerve stimulation and increased the responses to injected noradrenaline.3. Isoprenaline slightly decreased the basal perfusion pressure and reduced the responses to both nerve stimulation and injected noradrenaline. Since these effects of isoprenaline were abolished by propranolol, they are presumably mediated by beta-receptors.4. The inhibitory effect of DHE and DHPE on the responses to nerve stimulation was not abolished by propranolol.5. The results suggest that DHE and DHPE impair adrenergic transmission by partially replacing the adrenergic neurotransmitter, thereby acting as a false neurotransmitter.