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Correlation between myocardial malate/aspartate shuttle activity and EAAT1 protein expression in hyper- and hypothyroidism. Am J Physiol Heart Circ Physiol 2005 May;288(5):H2521-6

Date

12/24/2004

Pubmed ID

15615843

DOI

10.1152/ajpheart.00991.2004

Scopus ID

2-s2.0-18044364838   21 Citations

Abstract

In the heart, elevated thyroid hormone leads to upregulation of metabolic pathways associated with energy production and development of hypertrophy. The malate/aspartate shuttle, which transfers cytosolic-reducing equivalents into the cardiac mitochondria, is increased 33% in hyperthyroid rats. Within the shuttle, the aspartate-glutamate carrier is rate limiting. The excitatory amino acid transporter type 1 (EAAT1) functions as a glutamate carrier in the malate/aspartate shuttle. In this study, we hypothesize that EAAT1 is regulated by thyroid hormone. Adult rats were injected with triiodothyronine (T3) or saline over a period of 8-9 days or provided with propylthiouracil (PTU) in their drinking water for 2 mo. Steady-state mRNA levels of EAAT1 and aralar1 and citrin (both cardiac mitochondrial aspartate-glutamate transporters) were determined by Northern blot analysis and normalized to 18S rRNA. A spectrophotometric assay of maximal malate/aspartate shuttle activity was performed on isolated cardiac mitochondria from PTU-treated and control animals. Protein lysates from mitochondria were separated by SDS-PAGE and probed with a human anti-EAAT1 IgG. Compared with control, EAAT1 mRNA levels (arbitrary units) were increased nearly threefold in T3-treated (3.1 +/- 0.5 vs. 1.1 +/- 0.2; P < 0.05) and decreased in PTU-treated (2.0 +/- 0. 3 vs. 5.2 +/- 1; P < 0.05) rats. Aralar1 mRNA levels were unchanged in T3-treated and somewhat decreased in PTU-treated (7.1 +/- 1.0 vs. 9.3 +/- 0.1, P < 0.05) rats. Citrin mRNA levels were decreased in T3-treated and unchanged in PTU-treated rats. EAAT1 protein levels (arbitrary units) in T3-treated cardiac mitochondria were increased compared with controls (8.9 +/- 0.4 vs. 5.9 +/- 0.6; P < 0.005) and unchanged in PTU-treated mitochondria. No difference in malate/aspartate shuttle capacity was found between PTU-treated and control cardiac mitochondria. Hyperthyroidism in rats is related to an increase in cardiac expression of EAAT1 mRNA and protein. The 49% increase in EAAT1 mitochondrial protein level shows that malate/aspartate shuttle activity increased in hyperthyroid rat cardiac mitochondria. Although hypothyroidism resulted in a decrease in EAAT1 mRNA, neither the EAAT1 protein level nor shuttle activity was affected. EAAT1 regulation by thyroid hormone may facilitate increased metabolic demands of the cardiomyocyte during hyperthyroidism and impact cardiac function in hyperthyroidism.

Author List

Ralphe JC, Bedell K, Segar JL, Scholz TD

Author

Jeffrey L. Segar MD Professor in the Pediatrics department at Medical College of Wisconsin




MESH terms used to index this publication - Major topics in bold

Amino Acid Transport System X-AG
Animals
Aspartic Acid
Cardiomegaly
Excitatory Amino Acid Transporter 1
Gene Expression
Glutamate Plasma Membrane Transport Proteins
Hyperthyroidism
Hypothyroidism
Immunoblotting
Malates
Myocardium
RNA, Messenger
Rats
Rats, Sprague-Dawley
Symporters
Triiodothyronine
jenkins-FCD Prod-478 d1509cf07a111124a2d122fd3df854cc0b993c00