Effects of l-tyrosine on mixed-acting sympathomimetic-induced pressor actions
We previously reported the ability of l-tyrosine (l-TYR) to potentiate the anorectic activity of various mixed-acting sympathomimetics i3cluding [R * S * ]-(+/-)-norepinephrine [phenylpropanolamine (PPA)], [1R,2S]-(-)-ephedrine (EPH), and [S]-(+)-amphetamine (AMPH) in hyperphagic rats. Included in those studies was the attenuation of l-TYR's effect when coadministered with l-valine, a large neutral amino acid that competes with l-TYR for uptake into the brain, suggesting a central locus for the action of l-TYR. Additional studies demonstrated the inability of l-TYR to alter the peripherally mediated PPA-, EPH-, and AMPH-induced increases in gastrointestinal transit time and retention and intrascapular brown adipose tissue thermogenesis. Because the mixed-acting sympathomimetics are known to increase blood pressure, these studies examined the ability of l-TYR to influence the pressor responses to PPA, EPH, and AMPH (0.03-1 mg/kg) in urethane-anesthetized rats. Each of the mixed-acting sympathomimetics significantly increased mean arterial, systolic, and diastolic blood pressures when administered alone, but no potentiation by l-TYR was observed. These results demonstrate the inability of l-TYR to potentiate the peripheral vasopressor effects of PPA, EPH, and AMPH. These data, in conjunction with our previous findings, suggest that the potentiation by l-TYR of the mixed-acting sympathomimetics is largely restricted to centrally mediated responses.
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