Figure 743

Tyrosine hydroxylase inhibitor. Metyrosine (a-methyl-para-tyrosine) is an inhibitor of tyrosine hydroxylase, the enzyme that catalyzes the conversion of tyrosine to dihydroxyphenylalanine [6,9]. Because this first step is rate limiting, blockade of tyrosine hydroxylase activity results in decreased endogenous levels of circulating cate-cholamines. In patients with excessive production of catecholamines, metyrosine reduces biosynthesis 36% to 79%; the net physiologic effect is a decrease in peripheral vascular resistance and increases in heart rate and cardiac output resulting from the vasodilation. The degree of vasodilation is dependent on the degree of blockade of adrenergic vascular tone. NE—norepinephrine.

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