Muscle glutamine production in burn patients: the physiological meaning of tracer estimates.

In a previous issue of Clinical Science, Biolo et al. [1] report data showing that hypercatabolic burn patients have a decreased release of glutamine from the leg muscles. This observation is of clinical relevance since muscle is considered to be the main glutamine producing tissue in man and because the glutamine released from muscle supports the function of the gut and the immune system [2,3]. In healthy controls glutamine is the main amino acid exported from muscle [3–5]. However, in the burn patients an excessive rate of glycolysis appears to increase muscle pyruvate concentrations and, therefore, alanine production [1–3,6] at the expense of glutamine production. A decreased release of glutamine from the leg muscles of burn patients [1] suggests that these patients may have an increased need for glutamine supplementation. The message from this study [1] seems to be in striking contrast to a recent paper from the same group using stable isotope tracers of glutamine [7]. The title of that study was: ‘Accelerated glutamine synthesis in critically ill patients cannot maintain normal intramuscular free glutamine concentration’. Four of the five patients investigated in [7] also were hypercatabolic burn patients with comparable clinical characteristics to the patients in [1]. The main difference between the two studies [1,7] is the method used to investigate glutamine metabolism. In the most recent study [1], only the mass balance of glutamine has been measured (arterio-venous concentration difference¬leg blood flow). This variable indicated that the leg muscles of burn patients released less glutamine than those of control subjects. In the study by Mittendorfer et al. [7] a [5-"&N]glutamine tracer has also been infused in order to measure several intramuscular components of glutamine metabolism. The authors made use of the so-called ‘three pool model ’ for the calculation of intramuscular glutamine kinetics [8]. This model, apart from the estimate of the mass balance, permits calculation of the rate of inward transport of glutamine into the muscle, the rate of outward transport of glutamine from the muscle into the circulation, and the rate of intracellular synthesis and rate of intracellular utilization of glutamine. Both the rate of intracellular synthesis and intracellular utilization of glutamine were 3-fold higher in burn patients than in controls, according to this tracer model. However, the mass balance, which can be calculated from other variables reported in [7], indicated that the net amount of glutamine released from the leg was decreased to the same extent as that observed in the larger population in [1]. Apart from the fact that the titles of the two papers [1] versus [7] are confusing, the question as to what the physiological meaning of the tracer estimate of glutamine synthesis and glutamine utilization is, can be raised. It can simply mean that there is a high rate of futile cycling between glutamine and glutamate (with the amide-N being removed by the action of glutaminase and being incorporated again by the action of glutamine synthetase). It could also mean that glutamine synthesis and oxidation in the tricarboxylic acid cycle (via the subsequent action of glutaminase, alanine aminotransferase and α-ketoglutarate dehydrogenase) occur simultaneously. Independent of the enzymic reactions that occur, the end result still is that the net mass balance (the amount of glutamine released from the leg muscles) is decreased in the patients and that, therefore, the amount of glutamine available to support the gut and immune system is decreased. So, for the clinician who has to judge whether a patient may be in need of glutamine supplementation, the tracer dilution method does not seem to provide relevant or meaningful physiological information. An additional complication is that the validity of the three pool model [8] for the calculation of intramuscular glutamine kinetics has recently been questioned [9,10]. Van Acker et al. [9] have shown that it takes more than 20 h for the intramuscular glutamine pool to reach a steady state (constant enrichment). This is the consequence of the enormous size of the intramuscular glutamine pool in human muscle [9,10]. Theoretical calculations [10] have shown that an earlier claim of Biolo et al. [8] that a steady state was reached in four healthy subjects already at 5 h after the start of the glutamine tracer infusion is highly unlikely, and were not confirmed by experimental data in 20 patients [9]. Failure to freeze dry and clean the muscle biopsies from contamination visible only under a dissection microscope (the recommended procedure in Scandinavian muscle physiology laboratories) may give errors in the estimates of glutamine concentration and enrichment and this may