L-glutamate transport in internally dialyzed barnacle muscle fibers

Glutamate-induced calcium increase in myotubes depends on up-regulation of a sodium-dependent transporter

We report a study on the regulation by 2-chloro adenosine (2CA) of a glutamate (Glu) transporter in myogenic C2C12 cells. Long-term 2CA exposition significantly increased the V(max) of the Glu transporter. Moreover, 2CA-treated cells responded to Glu challenge by a rapid and transient increase in their intracellular calcium level. The above reported effects were totally abolished by treating C2C12 cells with the Na(+)-dependent Glu transporter inhibitors DL-threo-b-hydroxyaspartic acid and L-trans-pyrrolidine-2,4-dicarboxylic acid. We propose that the possible link between the Glu uptake increase and the Glu induction of calcium rise could be the depolarizing currents carried by Na(+) coupled with transporter activity.

Effect of carnitine on muscular glutamate uptake and intramuscular glutathione in malignant diseases

Abnormally low intramuscular glutamate and glutathione (GSH) levels and/or a decreased muscular uptake of glutamate by the skeletal muscle tissue have previously been found in malignant diseases and simian immunodeficiency virus (SIV) infection and may contribute to the development of cachexia. We tested the hypothesis that an impaired mitochondrial energy metabolism may compromise the Na+-dependent glutamate transport. A randomized double-blind clinical trial was designed to study the effects of L-carnitine, i.e. an agent known to enhance mitochondrial integrity and function, on the glutamate transport and plasma glutamate level of cancer patients. The effect of carnitine on the intramuscular glutamate and GSH levels was examined in complementary experiments with tumour-bearing mice. In the mice, L-carnitine treatment ameliorated indeed the tumour-induced decrease in muscular glutamate and GSH levels and the increase in plasma glutamate levels. The carnitine-treated group in the randomized clinical study showed also a significant decrease in the plasma glutamate levels but only a moderate and statistically not significant increase in the relative glutamate uptake in the lower extremities. Further studies may be warranted to determine the effect of L-carnitine on the intramuscular GSH levels in cancer patients.

Altered glutamate metabolism is associated with reduced muscle glutathione levels in patients with emphysema

Chronic obstructive pulmonary disease (COPD) is often characterized by an impaired skeletal muscle energy metabolism, which is at least partly related to chronic hypoxia and a reduced diffusing capacity. We have found that muscle glutamate (Glu), which is negatively influenced by these conditions, was reduced in patients with severe COPD. The aim of this study was to investigate whether the reduced intracellular Glu level in patients with emphysema is associated with an increased muscle glycolytic metabolism. Since Glu is an important substrate in the synthesis of glutamine (Gln) and glutathione (GSH), the influence of Glu status on muscle GSH and Gln was also examined. In 13 patients with emphysema and 25 control patients, arterial blood and biopsies from the vastus lateralis muscle were obtained. Expressed as a percentage of the control values, the patients with emphysema had reduced values for muscle Glu (64 +/- 12%; p < 0.001), GSH (76 +/- 23%; p < 0.01), and Gln (93 +/- 5%; p < 0.01), and higher values for lactate (p < 0.01) and pyruvate (p < 0.05). No differences were found in plasma values. Muscle Glu was highly associated with GSH (R(2) = 0.61; p < 0.001), but not with Gln. This study illustrates that reduced Glu levels in skeletal muscle of patients with emphysema are possibly related to an enhanced glycolytic activity and associated with decreased GSH levels.

Elevated venous glutamate levels in (pre)catabolic conditions result at least partly from a decreased glutamate transport activity

Abnormally high postabsorptive venous plasma glutamate levels have been reported for several diseases that are associated with a loss of body cell mass including cancer, human/simian immunodeficiency virus infection, and amyotrophic lateral sclerosis. Studies on exchange rates in well-nourished cancer patients now show that high venous plasma glutamate levels may serve as a bona fide indicator for a decreased uptake of glutamate by the peripheral muscle tissue in the postabsorptive period and may be indicative for a precachectic state. High glutamate levels are also moderately correlated with a decreased uptake of glucose and ketone bodies. Relatively high venous glutamate levels have also been found in non-insulin-dependent diabetes mellitus and to some extent also in the cubital vein of normal elderly subjects, i.e., in conditions commonly associated with a decreased glucose tolerance and progressive loss of body cell mass.