RT Journal Article SR Electronic T1 Calculation of the Rate of Catecholamine Synthesis from the Rate of Conversion of Tyrosine-14C to Catecholamines JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 90 OP 99 VO 5 IS 1 A1 N. H. NEFF A1 S. H. NGAI A1 C. T. WANG A1 E. COSTA YR 1969 UL http://molpharm.aspetjournals.org/content/5/1/90.abstract AB When tyrosine-14C is infused intravenously at a constant rate into unanesthetized rats, the specific activity of plasma tyrosine increases curvilinearly with time, producing a concomitant increase in the specific activity of tissue catecholamines. By applying steady-state kinetics to this relationship, a simple model was developed from which the fractional rate constant (k) of a catecholamine compartment could be calculated. Catecholamine synthesis rates are obtained by multiplying k by the steady-state amine level. Rates calculated by this method are in good agreement with those calculated from the decline of catecholamine levels after blockade of their synthesis with α-methyltyrosine. By using the present method, it was found that adrenal demedullation results in about a 4-fold increase in the rate of synthesis of heart norepinephrine while brain catecholamine synthesis remains unchanged. It is suggested that catecholamines released from the adrenal medulla may modulate the synthesis of norepinephrine in cardiac sympathetic neurons. ACKNOWLEDGMENT The skillful assistance of Miss Sonja Löfstrandh is greatly appreciated.