title: Dilution of plasma with Tris buffer increases measured catecholamines in plasma.
abstract: We investigated the effects of dilution of plasma samples on the measured concentrations of catecholamines. Diluting samples of human plasma 10-, 50-, and 100-fold with Tris buffer (100 mol/L, pH 8.6) improved analytical recovery of internal standards, suggesting that it decreases the commonly observed inhibition of methylation in radioenzymatic assays of catecholamines in plasma. However, the dilution is not associated with a proportional decrease in counted radioactivity. This extra amount of radioactivity, which is unlikely to be nonspecific in origin, accounts for a significant increase in the calculated catecholamine concentration. Tentatively, we suggest that Tris buffer releases both catecholamines and conjugated catecholamines bound to some unidentified low-molecular-mass component of plasma.
mesh_terms: D004298:Dopamine / Q000097:blood*; D004837:Epinephrine / Q000097:blood*; D006801:Humans; D009638:Norepinephrine / Q000097:blood*; D011865:Radioisotope Dilution Technique; D011933:Reagent Kits, Diagnostic; D014325:Tromethamine
chemical_list: D011933:Reagent Kits, Diagnostic; D014325:Tromethamine; D004298:Dopamine; D009638:Norepinephrine; D004837:Epinephrine