Animal age affects soleus myosin heavy chain (MHC) mRNA expression and myostatin protein following hindlimb unweighting (HU)

Abstract

Aim to determine the effect of age (maturity) on myosin heavy chain (MHC) expression in the soleus muscle following HU. We hypothesized a similar shift in soleus MHC expression and myostatin protein following HU in growing and mature rats. Growing (2 mos, n=12) and mature (8 mos, n=12) male rats were ambulatory or HU. Soleus total protein (BCA), myosin (SDS-PAGE), MHC isoforms (RT-PCR), and myostatin (immunoblot) were determined using standard techniques. Protein content and total myosin decreased similarly with HU in growing and mature animals. IIa expression was unchanged with HU in growing animals but significantly upregulated in mature rats. With HU a greater than 500-fold increase in IIb MHC expression was observed in growing rats whereas IIb expression in mature animals was down-regulated 40%. Unweighting resulted in a 0.5-fold myostatin protein increase in young rats, and a 2.5-fold increase in mature animals, compared to control values. It may be concluded that the age of the animal will not affect total protein or total myosin following HU. However, maturity of the animal will influence soleus MHC expression and myostatin protein, with HU. This information may be useful in comparing across studies relating fiber type and inactivity/activity. NA65-12300, AG15796, NAA04CC62G