Thyroid hormones differentially modulate enolase isozymes during rat skeletal and cardiac muscle development

During muscle development, an isozymic transition of the glycolytic enzyme enolase occurs from the embryonic and ubiquitous alpha alpha-isoform to the muscle-specific beta beta-isoform Here, we demonstrate a stimulatory role of thyroid hormones on these two enolase genes during rat development in hi ndlimb muscles and an inhibitory effect on the muscle-specific enolase gene in cardiac muscle. in hindlimb muscles the ubiquitous alpha-transcript lev el is diminished by hypothyroidism, starting at birth. On the contrary the more abundant muscle-specific beta-transcript is insensitive to hypothyroid ism before establishment of the functional diversification of fibers and is greatly decreased thereafter Our data support the hypothesis of a role of thyroid hormones in coordinating the expressions of contractile proteins an d metabolic enzymes during muscle development. The subcellular localization of isoenolases, established here, is not modified by hypothyroidism. Our r esults underline the specificity of action of thyroid hormones, which modul ate differentially two isozymes in the same muscle and regulate, in opposit e directions, the expression of the same gene in two different muscles.