Gender- and thyroid hormone-related transitions of essential myosin light chain isoforms expression in rat soleus muscle during ageing.

In this brief review, the modulatory influence of essential myosin light chain (MLC) isoforms on muscle cell contractility is discussed. Specific interest is focused on the expression of the MLC1Sa and MLC1Sb isoforms in the slow-twitch soleus muscle in male and female rats, during ageing and after thyroid hormone treatment. According to two-dimensional gel electrophoresis analysis, the MLC1Sa/MLC1SB ratio increased during ageing in both males and females in parallel with the age-related decrease in shortening velocity reported in muscle fibres expressing the slow (type 1) myosin heavy chain (MHC) isoform. However, the MLC1Sa and MLC1Sb isoform expression responded to thyroid hormone treatment in a complex manner which did not parallel the age-related changes in shortening velocity reported in hyperthyroid animals. Thus, if MLC1Sa and MLC1Sb isoforms modulate shortening velocity in type 1 fibres, then other modulators of shortening velocity are not regulated by thyroid hormone in co-ordination with these essential MLCs.