Effects of L-glutamate on the responses to nerve stimulation in rat isolated atria

Endothelium-dependent production and liberation of kynurenic acid by rat aortic rings exposed to L-kynurenine

Rat aortic slices produced and liberated the endogenous antagonist of glutamate receptors, kynurenic acid, upon exposure to L-kynurenine. Endothelium-denuded slices did not synthesize any measurable amount of kynurenic acid, indicating its endothelial origin. Aortic kynurenic acid production was diminished by modification of the ionic milieu, hypoxia and hypoglycemia, as well as by L-glutamate and L-aspartate, endogenous glutamate receptor agonists, and aminooxyacetic acid, a non-selective inhibitor of aminotransferases and mitochondrial respiration. These data pave the way for future research aimed to clarify the role of kynurenic acid in the physiology and pathology of the endothelium and vasculature.

Kynurenic acid and kynurenine aminotransferase in heart

Kynurenic acid (KYNA) is a tryptophan metabolite and represents the only known endogenous compound acting as an antagonist to excitatory amino acid receptors in the mammalian CNS. Blocking of these receptors in CNS by KYNA affects cardiac function. As it is not known whether human heart is able to synthesize this neuromodulatory amino acid, we investigated the biosynthesizing enzyme of kynurenine aminotransferase (KAT) in the human heart and compared the activity with that of the human brain. The activities of heart and brain KATs were assayed by the conversion of L-kynurenine (L-KYN) to KYNA and quantitated by HPLC with fluorescence detection. Using either pyruvate or 2-oxoglutarate as cosubstrates, heart KAT was found to have a shallow pH optimum between 8 and 9. Highest heart KAT activity was seen in the presence of 2-oxoglutarate, followed by pyruvate. 2-oxoadipate, and 2-oxoisocaproate. Kinetic analyses, performed at pH 8.5, and using various concentrations of L-KYN (from 0.125 to 22.8 mM) in the presence of 2-oxoglutarate (1 and 5 mM) or pyruvate (5 mM) revealed apparent K(m) values in the millimolar range, for L-KYN 1.5, 27, and 20 mM, respectively. Heart KAT activities were compared with those in human brain KAT I and KAT II showing different pH optima 7.4 and 9.6, respectively. In contrast to brain KAT I, heart KAT activity was not inhibited by an excess of 2 mM L-tryptophan, L-glutamine, or L-phenylalanine at pH 9.6, as well as at pH 8 or 7.4. Our study demonstrates that human heart is capable of synthesizing KYNA from low concentrations of L-KYN selectively. A shallow pH optimum of KAT activity, i.e. between 8.0 and 9.0, pronounced 2-oxoacid specificity, and a lack of sensitivity to inhibition by L-glutamine, L-phenylalanine, and L-tryptophan indicate that the heart KAT system displays enzymatic characteristics different from those of human brain KAT I or KAT II. Fluctuation of L-KYN and 2-oxoacid levels may markedly influence the KYNA synthesis and subsequent KYNA effect on cardiac activity. KYNA synthesis in the human heart suggests a neurophysiologic role. Our studies from the basis for purification and further characterization of KAT protein in human heart as well as for physiologic studies.

Effects of the in vitro treatment with gangliosides on the release of endogenous amino acids from rat hypoxic atria

1. In the rat isolated atria the in vitro exposure to 60 min of hypoxia in the absence of glucose followed by 30 min of reoxygenation increased the release of the amino acids glutamate (Glu) and taurine (Tau). The efflux of the remaining amino acids assayed (aspartate, glycine and alanine) did not change throughout the period studied. 2. The increase in Tau release started 45 min after the onset of the hypoxic period whereas that of Glu started during the reoxygenation phase. These increases were not observed when glucose was present during the hypoxic period. 3. The in vitro pretreatment for 2 h with 50 microM bovine brain gangliosides (BBG) prevented the increases in the release of Tau and Glu induced by the hypoxia reoxygenation. 4. These results constitute a further example where BBG appears to exert a protective role in cardiac tissues submitted to injuries.