Characteristics of L-aspartate transport and expression of EAAC-1 in sarcolemmal vesicles and isolated cells from rat heart

Objective: L-Aspartate is an important intermediary metabolite in the heart and has also been implicated in myocardial protection, but little is known about its transport across the cardiac sarcolemma. In this study we have t ested the hypothesis that the high affinity sodium-dependent aspartate tran sporter, EAAC-1 is expressed in heart and have also characterised aspartate transport into the myocardium. Methods: Characteristics Of L-[C-14]asparta te uptake into rat heart were investigated using sarcolemmal vesicles and i solated myocytes. The expression of EAAC-1 in the two preparations was also investigated by western blotting. Results: The K-m and V-max of L-aspartat e uptake was 9.78 +/-0.7 muM and 1.17 +/-0.27 pmol/mg/s in vesicles compare d to 6.53 +/-1.24 muM and 13.65 +/-1.0 pmol/mul/s in cells. In vesicles, L- aspartate uptake was dependent on external sodium and internal potassium, a nd was rheogenic. In Cells, L-aspartate uptake was also dependent on extern al sodium. Addition of unlabelled L- and D-aspartate and L-glutamate signif icantly inhibited L-[C-14]aspartate uptake in both preparations but D-gluta mate had no effect. An antibody to the aspartate transporter, EAAC-1 recogn ised a protein of appropriate size in both vesicles and cells. Conclusions: L-aspartate uptake in heart is mediated by a high affinity sodium-dependen t transporter. This is accompanied by the expression in heart of EAAC-1. Th e physiological significance of this transporter with respect to aspartate utilisation in the heart is discussed. (C) 2001 Elsevier Science B.V. All r ights reserved.