Expression of glutamate transporters in rat cardiomyocytes and their localization in the T-tubular system.

Glutamate and aspartate play important roles in the intermediary metabolism of the myocardium and have been shown to improve cardiac recovery after hypoxia or ischemia. Limited data are available about the expression of glutamate transporters that are involved in the uptake of glutamate and aspartate in cardiomyocytes. In this study, non-radioactive in situ hybridization (ISH) using complementary RNA probes was applied to detect the glutamate transporters GLT1 variant (GLT1v) and EAAC1 mRNA in rat cardiomyocytes. The transporter proteins were demonstrated by Western blotting and immunocytochemistry using affinity-purified antibodies against transporter peptides. ISH and immunocytochemistry showed that both glutamate transporters are coexpressed in cardiomyocytes. The ISH labeling indicates the distribution of transporter mRNA throughout the cytoplasm of cardiomyocytes. GLT1v and EAAC1 proteins, which showed in Western blots a molecular mass of approximately 60 kD, are strongly enriched and colocalized in the transverse (T)-tubular system of cardiomyocytes. These results may indicate that glutamate/aspartate uptake into cardiomyocytes could be mediated by the high-affinity transporters GLT1v and EAAC1. A high efficiency of glutamate/aspartate transport into cardiomyocytes could be achieved by their localization in the T-tubular system, which consists of tubular invaginations of the sarcolemma extending deep into the cell.