Influence of thyroid status on the differentiation of slow and fast muscle phenotypes.

Muscle phenotype is determined by combined effects of intrinsic genetic and extrinsic factors like innervation, hormonal levels and mechanical factors or muscle activity. We have been studying the effect of altered thyroid hormone levels on the expression of myosin heavy chain (MyHC) isoforms in control and regenerating soleus and extensor digitorum longus muscles of euthyroid, hypothyroid or hyperthyroid female inbred Lewis rats. The fiber type composition has been determined according to the mATPase activity and immunocytochemical staining of MyHC isoforms, the content of MyHC isoforms has been determined by SDS-PAGE, the mRNA levels have been measured by RT-PCR and the ultrastructural transformation has been analyzed by electron-microscopy.

Body, heart, thyroid gland and skeletal muscle weight changes in rats with altered thyroid status.

In the present paper we describe changes of anatomical parameters in inbred Lewis strain rats, namely their body weight, body weight gain per week, absolute and relative heart, thyroid gland and skeletal muscle weights, that are assumed to reflect experimentally altered thyroid status. The hyperthyroid state was induced by DL-thyroxine or Na 3,3',5-triiodo-L-thyronine, while methimazole was employed for inducing hypothyroidism. We have found that when compared to euthyroid rats, hypothyroidism resulted in a significantly lower body weight gain, absolute and relative heart weight and, in contrast, in a significant increase of absolute and relative thyroid gland weight.

Early changes in extrafusal and intrafusal muscle fibers following heterochronous isotransplantation.

The ultrastructure of regenerating intrafusal and extrafusal fibers was studied 18 h to 30 days after heterochronous isotransplantation, in which bupivacaine-treated extensor digitorum longus (EDL) or soleus muscles from early postnatal rats were intramuscularly grafted into EDL muscles of adult inbred recipients. As in other models of mammalian muscle regeneration, surviving satellite cells gave rise to presumptive myoblasts, multiplying within the preserved basal lamina tubes at day 4 after grafting. Myoblasts fused to form myotubes with central myonuclei by day 6 after grafting.

Status epilepticus causes necrotic damage in the mediodorsal nucleus of the thalamus in immature rats.

Status epilepticus (StE) in immature rats causes long-term functional impairment. Whether this is associated with structural alterations remains controversial. The present study was designed to test the hypothesis that StE at an early age results in neuronal loss.

Regulation of myosin expression in developing and regenerating extrafusal and intrafusal muscle fibers with special emphasis on the role of thyroid hormones.

Expression of the muscle phenotype is the result of interaction between intrinsic and extrinsic factors, the latter including innervation, mechanical influences and hormonal signals. This minireview summarizes some of the current knowledge regarding the regulation of myosin heavy chain (MHC) isoform transitions during muscle development and regeneration. It describes the role of genetic factors, neural and mechanical influences and it focuses on the contribution of thyroid hormones to the differentiation of muscle fiber phenotypes as shown by the regulation of the expression of MHC isoforms and development of myofibrillar ATPase activity.