TRANSPORT AND METABOLISM OF L-GLUTAMATE DURING OXYGENATION, ANOXIA, AND REOXYGENATION OF RAT CARDIAC MYOCYTES

The intracellular glutamate concentration of oxygenated, isolated adul t rat heart cells incubated with 0.15 mM glutamate amounts to 2.89 +/- 0.6 mM. Under these conditions the velocity of glutamate transport wa s 24.3 +/- 1.6 pmol . min(-1). mg protein(-1) and occurs via a high-af finity carrier characterized by an apparent affinity (K-m) value of 0. 18 +/- 0.03 mM. At high glutamate concentrations (>1 mM) this high-aff inity transport system is superimposed by additional uptake processes of a low affinity but a high capacity for glutamate. The 1.6-fold incr eased uptake of glutamate observed during 30 min of anoxic incubation of cardiomyocytes does not prevent an intracellular decrease in this a mino acid to a concentration of 0.49 mM. After 15 min reoxygenation of cardiomyocytes the intracellular glutamate content increases to the c ontrol values of oxygenated cells. Only 2.4% of the glutamate increase after reoxygenation is due to the transport of glutamate from the inc ubation medium. The competitive inhibitor of transaminases, aminooxyac etate, prevents both the observed intracellular decrease in glutamate during anoxia and the increase in intracellular glutamate after reoxyg enation of cardiomyocytes. Half of the amino groups needed for the syn thesis of glutamate originate from intracellular alanine, which increa ses during anoxia and is metabolized during reoxygenation of cardiomyo cytes. The velocity of the glutamate uptake of cardiomyocytes incubate d in a medium containing 10 mM L-glutamate amounted to 728 +/- 140 pmo l . min(-1). mg protein(-1). During anoxic incubation of cardiomyocyte s at this high extracellular glutamate concentration, the intracellula r glutamate breakdown may be compensated by a simultaneous uptake of t his amino acid via the transport processes characterized by a high cap acity.