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Time course of myosin heavy chain transitions in neonatal rats: importance of innervation and thyroid state

G R Adams1, S A McCue, M Zeng

  • 1Department of Physiology and Biophysics, University of California Irvine, California 92697-4560, USA.

The American Journal of Physiology
|April 13, 1999
PubMed

Insights

Postnatal rat muscles develop adult myosin heavy chains (MHCs) for weight-bearing. Both thyroid hormone and innervation are essential for achieving this adult MHC phenotype, especially in fast-twitch muscles.

Area of Science:

  • Muscle physiology
  • Developmental biology
  • Endocrinology

Background:

  • Postnatal limb muscles undergo myosin heavy chain (MHC) transitions from embryonic (Emb) and neonatal (Neo) to adult isoforms.
  • This adaptation is crucial for weight-bearing and functional maturation.

Purpose of the Study:

  • To characterize the developmental transition of six MHC isoforms in rat limb muscles.
  • To investigate the roles of innervation and thyroid hormone status in achieving the adult MHC phenotype.

Main Methods:

  • Neonatal rats were made hypothyroid using propylthiouracil (PTU).
  • Unilateral denervation of leg muscles was performed at 15 days postpartum.
  • MHC profiles of plantaris (PLN) and soleus (Sol) muscles were analyzed at various postnatal ages (7, 14, 23, 30 days).

Main Results:

  • Control muscles achieved adult MHC profiles by day 30.
  • Hypothyroidism (PTU) augmented slow MHC in soleus but repressed IIb MHC in plantaris, retaining neonatal MHC.
  • Denervation reduced Type I MHC in soleus and IIb MHC in plantaris, increasing IIa and IIx MHC expression in both muscles.

Conclusions:

  • Both intact thyroid hormone status and innervation are obligatory for the normal attainment of the adult MHC phenotype.
  • Fast-twitch muscles appear particularly dependent on these factors for proper MHC maturation.

Keywords:
NASA Discipline MusculoskeletalNon-NASA Center

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