L-tyrosine fails to potentiate several peripheral actions of the sympathomimetics

We have recently reported the ability of L-tyrosine (L-TYR) to potentiate the anorectic activity of various mixed-acting sympathomimetics including phenylpropanolamine (PPA), l-ephedrine (EPH) and d-amphetamine (AMP). Included in those studies was the attenuation of L-TYR's effect when coadministered with L-valine, a large neutral amino acid which competes with L-TYR for uptake into the brain, suggesting a central locus for the action of L-TYR. To determine to what extent L-TYR can potentiate peripheral actions, we investigated the effects of L-TYR with either PPA (20 mg/kg), EPH (20 mg/kg) or AMP (1.75 mg/kg) on gastric transit, gastric retention and intrascapular brown adipose tissue thermogenesis. In each of the paradigms studied, PPA, EPH and AMP significantly increased the expected sympathomimetic-mediated response, but no potentiation of L-TYR was observed. These results are consistent with the hypothesis that L-TYR potentiates the anorectic activity of the mixed-acting sympathomimetics largely via an action at a central locus.