Correlates of urinary excretion of catechols in humans

An endogenous system involving renal uptake of dihydroxyphenylalanine (DOPA) and conversion to the natriuretic catecholamine,:dopamine (DA), may participate in sodium homeostasis. Normal values for and correlates of urinary excretion of free (unconjugated) catechols in humans are incompletely understood. Daily excretion rates of DOPA, DA, the DA metabolite dihydroxyphenylacetic acid (DOPAC), the sympathetic neurotransmitter norepinephrine (NE), the NE metabolite dihydroxyphenylglycol (DHPG), and the adrenomedullary hormone epinephrine (EPI) were measured in 70 American normotensive volunteers of different ages, sex, and race, and 35 Israeli Caucasian males in their sixth decade, with no dietary or activity restrictions. In both groups, the excretion rate of DOPAC (overall mean 12.53 ± 1.08 (SEM) μmoles/day) exceeded by far those of all other catechols combined. Catechol excretion rates were positively inter-correlated in both subject groups - especially DA with DOPA (r=0.71 across all subjects, p<0.001). Excretion rates of catechols were also positively correlated with the excretion rate of sodium in both groups (for DOPA r=0.70 across all subjects, p<0.001; for DA r=0.54, p<0.001; for DOPAC r=0.46, p<0.001; for NE r=0.47, p<0.001; and for EPI r=0.29, p<0.01), but not with urine volume. DA and DOPA excretion rates were higher in Black Americans than in Caucasian Americans (2.50 ± (SEM) 0.19 vs 1.86 ± 0.10 μmoles/d, p<0.01; 0.21 ± 0.02 vs 0.16 ± 0.01 μmoles/d. p<0.01); DA excretion decreased with increasing subject age (r=-0.24); and excretion rates of DOPA were negatively correlated with MAP (r=-0.33, p<0.01). The high rate of urinary excretion of DOPAC probably mainly reflects glomerular filtration or tubular secretion of plasma DOPAC. The multiple positive correlations among excretion rates of catechols indicate a common source, such as coupled synthesis and turnover of catecholamines in the sympathoadrenal system. Positive correlations of excretion rates of DOPA, DA, and DOPAC with ad libitum dietary salt intake are consistent with salt-induced activation of a renal DOPA-DA natnuretic system.