Background: Hyperthyroid induced cardiac hypertrophy is related to increased cardiac workload. These changes are associated with an upregulation of metabolic pathways associated with energy production. The malate/aspartate shuttle, necessary to transfer the reducing equivalents produced by glycolysis into the mitochondria, is increased 33% in hyperthyroid rats. Of the shuttles two inner membrane protein carriers, the aspartate-glutamate carrier is rate-limiting. The Excitatory Amino Acid Transporter, Type 1 (EAAT1) has recently been shown to function as a glutamate carrier in the malate/aspartate shuttle. We hypothesize that EAAT1 is upregulated by thyroid hormone.Methods: Adult Sprague-Dawley rats were infused with, tri-iodothyroxine (T3), propylthiouracil (PTU), or saline over a period of 8 days. Serum free T3 levels were measured. Rats were euthanized, hearts weighed, and tissue frozen. Northern Blot analysis was performed on total RNA using a unique 350 bp 32-P labeled EAAT1 ribonucleotide probe and normalized to 18S rRNA. A spectrophotometric assay of the malate/aspartate with glutamate and lactate as substrates was performed on isolated mitochondria. Results are displayed as oxidation rate/min/mg mitochondrial protein. Protein lysates from mitochondria were used for immunoblot analysis with human anti-EAAT1 IgG.Results: EAAT1 steady-state mRNA levels were increased in the T3-treated rats compared to controls (0.031+/−0.005 vs. 0.011+/−0.002; P<0.05), and decreased in PTU-treated rats vs. controls (0.0011+/−0.0002 vs. 0.0015+/−0.0001; P<0.05). EAAT1 mitochondrial protein levels were increased in T3-treated rats vs. controls (8.9+/−0.4 vs. 5.9+/−0.6; P<0.005). EAAT1 protein levels were below detection in PTU-treated rats. Malate/aspartate shuttle activity was unchanged by PTU infusion.Conclusions: Hyperthyroidism in rats is related to an increase in expression of both EAAT1 mRNA and protein in cardiomyocytes. This 49% increase in the rate-limiting aspartate-glutamate carrier of the malate/aspartate shuttle correlates with the observed increase in shuttle activity. The upregulation of EAAT1 by thyroid hormone may facilitate the cardiomyocyte response to hyperthyroidism and the associated increased metabolic demand placed upon the cell.