Mechanisms of phenylalanine-induced pressor effects in conscious rats

Phenylalanine and tyrosine reportedly decrease blood pressure in conscious restrained rats. However, tyrosine has recently been found to increase blood pressure in anesthetized animals, questioning the generality of findings obtained in restrained animals. The present study therefore evaluated the effects of phenylalanine on mean arterial pressure (MAP) and heart rate (HR) in unrestrained, conscious rats. Phenylalanine (0.32-1.33 mmol/kg i.p.) increased MAP and decreased HR, effects that were antagonized by carbidopa and prazosin but not by desipramine. In addition, both DOPA and tyrosine (1.33 mmol/kg) increased MAP. In contrast, phenylalanine-induced increases in plasma concentrations of its indirectly acting sympathomimetic amine metabolite, phenethylamine, were small and temporally unrelated to the phenylalanine-induced MAP elevation, and desipramine inhibited MAP increases produced by exogenous phenethylamine. These observations indicate that phenylalanine increases MAP in conscious, unrestrained animals by augmenting peripheral catecholamine synthesis and release rather than by affecting phenethylamine bioavailability.