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

Abstract

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 tested the hypothesis that the high affinity sodium-dependent aspartate transporter, EAAC-1 is expressed in heart and have also characterised aspartate transport into the myocardium. Characteristics of L-[14C]aspartate uptake into rat heart were investigated using sarcolemmal vesicles and isolated myocytes. The expression of EAAC-1 in the two preparations was also investigated by western blotting. The K(m) and V(max) of L-aspartate uptake was 9.78+/-0.7 microM and 1.17+/-0.27 pmol/mg/s in vesicles compared to 6.53+/-1.24 microM and 13.65+/-1.0 pmol/microl/s in cells. In vesicles, L-aspartate uptake was dependent on external sodium and internal potassium, and was rheogenic. In cells, L-aspartate uptake was also dependent on external sodium. Addition of unlabelled L- and D-aspartate and L-glutamate significantly inhibited L-[14C]aspartate uptake in both preparations but D-glutamate had no effect. An antibody to the aspartate transporter, EAAC-1 recognised a protein of appropriate size in both vesicles and cells. L-aspartate uptake in heart is mediated by a high affinity sodium-dependent transporter. This is accompanied by the expression in heart of EAAC-1. The physiological significance of this transporter with respect to aspartate utilisation in the heart is discussed.