Abstract
This study investigates effects of chronic low frequency stimulation (CLFS) on myosin heavy (MHC) and light chain (MLC) expression in fast-twitch muscles in hypothyroid, euthyroid, and hyperthyroid rats. The changes at both the mRNA and protein level indicated antagonistic effects of thyroid hormone and CLFS: under euthyroid conditions, CLFS mainly elicited a MHCIIb----MCHIId----MHCIIa transition. Whereas CLFS did not induce the slow MHCI in the euthyroid state, this isoform was present in the hypothyroid state and was further enhanced with CLFS indicating the suppressive effect of thyroid hormone to be stronger than the inductive influence of CLFS. Hyperthyroidism alone suppressed the expression MHCIIa and enhanced a MHCIId to MHCIIb transition. This shift to the faster MHC isoforms was only partially counteracted by CLFS. Thus, it appeared that thyroid hormone had a graded suppressive effect on the expression of MHC isoforms in the order MHCIId less than MHCIIa less than MHCI. Elevated neuromuscular activity partially counteracted these hormone effects. Changes in MLC mRNAs were consistent with those in the MHC pattern, i.e. increases or decreases in MHCIIb led to corresponding changes in the expression of MLC3f. A similar relationship existed for the slow MHCI and the slow MLC isoforms.
Highlights
Antagonistic Effects of Chronic Low Frequency Stimulation and Thyroid Hormone on Myosin Expression in Rat Fast-twitch Muscle*
We have previously shown that chronic low frequency stimulation (CLFS) of rat fast-twitch muscle primarily affects the expression of the fast myosin heavy chain (MHC) isoforms [4, 20, 35]
At both the mRNA and protein level, CLFS leads to increases in MHCIIa at the expense of MHCIIb
Summary
Antagonistic Effects of Chronic Low Frequency Stimulation and Thyroid Hormone on Myosin Expression in Rat Fast-twitch Muscle*. From the Faculty of Biology, University of Konstanz, D-7750 Konstanz, Federal Republic of Germany and the &Laboratory of Pathology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania The changes at both the mRNA and protein level indicated antagonistic effects of thyroid hormone and CLFS: under euthyroid conditions, CLFS mainly elicited a MHCIIb + MCHIId + MHCIIa transition. Whereas CLFS did not induce the slow MHCI in the euthyroid state, this isoform was present in the hypothyroid state and was further enhanced with CLFS indicating the suppressive effect of thyroid hormone to be stronger than the inductive influence of CLFS This shift to the faster MHC isoforms was only partially counteracted by CLFS.
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