3-Methoxy-4-hydroxyphenylethylene glycol (MHPG) labelled with six deuterium atoms ([2H6]MHPG) was infused into two female greyhound dogs. Plasma and urine were analyzed for endogenous MHPG and [2H6]MHPG and their conjugates by the technique of selected ion monitoring gas chromatography-mass spectrometry. This analysis showed that the major forms of MHPG and [2H6]MHPG in the plasma and urine of the greyhounds were the sulphate conjugates. However, the plasma concentration of [2H6]MHPG sulphate increased continuously during the infusion period and the renal clearance of this compound was found to be much less than that of the endogenous sulphate. The explanation that his was due either to saturation of a renal transport mechanism or to a deuterium isotope effect was eliminated. Stereochemical analysis showed that, while the [2H6]MHPG used for infusion was a racemic mixture of two stereoisomers, much more of the nature(-)stereoisomer of [2H6]MHPG sulphate was excreted in the dog urine. It was also shown that both the (+) and (-)stereoisomers of [2H6]MHPG sulphate were sulphoconjugated at the 4-position of the aromatic ring.
With the more widespread use of sensitive specific methods of measurements of catecholamines and metabolites, more emphasis must be given to assessment of dynamic indices of catecholamine turnover. Wide inter-individual differences in noradrenaline clearance from plasma and in resting endogenous production rate, together with individual differences in responsiveness, frustrate simplistic attempts to relate plasma level of transmitter with turnover. To assess catecholamine function adequately plasma and urinary levels of catecholamine and the main metabolites should be determined together with an index of responsiveness. Central nervous catecholamine turnover cannot be easily assessed from measurement of urinary metabolite excretion Cerebrospinal fluid concentration of MHPG may provide an index of brain noradrenaline turnover.
1. The 24 h urinary excretion of free 4-hydroxy-3-methoxyphenylethylene glycol (HMPG), HMPG conjugated as glucuronide and HMPG conjugated as sulphate was determined in nine healthy volunteer subjects and six patients with phaeochromocytoma. In both groups of subjects most (97%) HMPG was in the conjugated form. 2. Although patients with phaeochromocytoma excreted five- to ten-fold the amounts of each of the forms of HMPG excreted by the control subjects the ratio of these different forms of HMPG to each other did not differ significantly between the groups. 3. In cerbrospinal fluid and brain about 80% of HMPG was free and of the conjugated HMPG in these two tissues most was in the glucuronide form. 4. In is concluded from these data that both HMPG sulphate and HMPG glucuronide have a substantial peripheral origin and that measurements of their urinary excretion cannot be used as an index of brain catecholamine turnover.
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