title: Source and physiological significance of plasma 3,4-dihydroxyphenylalanine in the rat.
abstract: To elucidate the source and physiological significance of plasma 3,4-dihydroxyphenylalanine, the immediate product of the rate-limiting step in catecholamine biosynthesis, plasma 3,4-dihydroxyphenylalanine was quantified in conscious rats after administration of reserpine, desipramine, clorgyline, or forskolin, treatments that affect tyrosine hydroxylase activity. Plasma 3,4-dihydroxyphenylalanine was also examined during infusions of norepinephrine with or without clorgyline, reserpine, or desipramine pretreatment. After reserpine, the plasma 3,4-dihydroxyphenylalanine level decreased by 22% and then increased by 40%, a result consistent with modulation of tyrosine hydroxylase activity first by an increased axoplasmic norepinephrine content and then by depletion of norepinephrine stores. After desipramine, the plasma 3,4-dihydroxyphenylalanine level decreased by 20%, reflecting the depressant effect of neuronal uptake blockade on norepinephrine turnover. Forskolin increased the plasma 3,4-dihydroxyphenylalanine level by 30%, consistent with activation of tyrosine hydroxylase by cyclic AMP-dependent phosphorylation. Acute administration of clorgyline was without effect on the plasma 3,4-dihydroxyphenylalanine level. Norepinephrine infusions decreased the plasma 3,4-dihydroxyphenylalanine concentration, as expected from end-product inhibition of tyrosine hydroxylase. Pretreatment with desipramine prevented the norepinephrine-induced decrease in plasma dihydroxyphenylalanine content, indicating that inhibition of tyrosine hydroxylase required neuronal uptake of norepinephrine. Both reserpine and clorgyline augmented the norepinephrine-induced decrease in plasma 3,4-dihydroxyphenylalanine level, suggesting that retention of norepinephrine in the axoplasm--due to inhibition of norepinephrine sequestration into storage vesicles or catabolism--caused further inhibition of tyrosine hydroxylase. Changes in plasma 3,4-dihydroxyphenylalanine concentration during norepinephrine infusions were negatively correlated with those in plasma 3,4-dihydroxyphenylglycol level, a finding consistent with modulation of tyrosine hydroxylase activity by axoplasmic norepinephrine. In reserpinized animals, clorgyline and norepinephrine infusion together decreased the plasma 3,4-dihydroxyphenylalanine content by 50%, a result demonstrating that hydroxylation of tyrosine was depressed by at least half. The results indicate that quantification of plasma 3,4-dihydroxyphenylalanine can provide a simple and direct approach for examination of the rate-limiting step in catecholamine biosynthesis.
mesh_terms: D000818:Animals; D002395:Catecholamines / Q000096:biosynthesis*; D003010:Clorgyline / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology; D003891:Desipramine / Q000494:pharmacology; D004295:Dihydroxyphenylalanine / Q000097:blood*; D004789:Enzyme Activation; D007700:Kinetics; D008297:Male; D009638:Norepinephrine / Q000494:pharmacology; D010766:Phosphorylation; D051381:Rats; D011921:Rats, Inbred WKY; D012110:Reserpine / Q000494:pharmacology; D014446:Tyrosine 3-Monooxygenase / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D002395:Catecholamines; D005576:Colforsin; D004295:Dihydroxyphenylalanine; D012110:Reserpine; D000242:Cyclic AMP; D014446:Tyrosine 3-Monooxygenase; D003010:Clorgyline; D003891:Desipramine; D009638:Norepinephrine