The effect of acidosis on the labelling of urinary ammonia during infusion of [amide-¹⁵N]glutamine in human subjects

Abstract

In three experiments [amide-15NJglutamine was infused intravenously in male volunteers. After 4-8 h of infusion acidosis was achieved by an oral dose of CaCl, (1 mmol/kg). In one subject acidosis was maintained for 5 d. The acid load produced an approximately 3-fold increase in urinary NH, excretion, with a small (approximately 20%) and transient increase in the isotope abundance of urinary NH,. Estimates of glutamine production rate (flux) were obtained in two experiments. There was no evidence that it was increased in acidosis. The extra NH, production by the kidney represented only a very small part, about 3%, of the total glutamine production rate. Acidosis : Glutamine metabolism: Urinary ammoniaThe principal objective of the work reported here was to examine the effect of acidosis on the labelling of urinary NH, during infusion of an amino acid labelled with 15N. In the endproduct method for measuring whole-body protein turnover with a I5N-labelled amino acid, N flux is determined from the isotope abundance in urinary urea or NH, or both (Waterlow et al. 1978; Fern & Garlick, 1983; Fern et al. 1985a, b). The method has been applied in a number of situations, for example in premature babies (Catzeflis et al. 1985), in malnourished children (Jackson et al. 1983) and adults (Soares et al. 1991), in pregnant women (de Benoist et al. 1985) and in the elderly (Golden & Waterlow, 1977).Being simple and non-invasive, this method is particularly suitable for application to people who are ill. Because many sick patients have some degree of acidosis it is important to know whether and how far acidosis p e r se alters the labelling of urinary NH,. To the extent that this occurs, it would introduce an error into the calculation of turnover rate. In this work we have chosen to use as the tracer glutamine (GLN) labelled in the amide (C-5)-NY rather than [15N]glycine, which has been more generally used for measurements of protein turnover, because the amide-N of GLN is the major precursor of urinary NH,. Further studies on acidosis with [15N]glycine as tracer will be reported later.

Enrichment in Urinary Ammonia and Urea with Hourly Oral Doses of [15N]glycine: Evidence for a Step Function and a Circadian Rhythm in Protein Turnover

“…One explanation for this behaviour could be a change in the pattern of amino acids, of varying enrichment, from which the end-product is derived. For example, with an infusion of [l5N]amide glutamine, which is the main precursor of urinary ammonia, acute acidosis produced an increase in the labelling of ammonia, suggesting an increase in the precursor pool of the proportion of highly labelled glutamine compared with other poorly labelled precursor amino acids [16]. With infusions of [15N]glycine, changes in the opposite direction might be expected, i.e.…”