Regulation of p70S6k, GSK-3beta, and calcineurin in rat striated muscle during aging.

In this study we compared the content and phosphorylation levels of several molecules believed to regulate muscle hypertrophy and fiber type changes in the extensor digitorum longus (EDL), soleus, diaphragm, and heart of adult (6 months), aged (30 months), and very aged (36 months) Fischer 344 x Brown Norway rats. With aging, the mass of the EDL and soleus decreased significantly (approximately 38% and approximately 36%, respectively), the diaphragm's mass remained unchanged while the mass of the heart increased (approximately 35%). Western blotting demonstrated that calcineurin (CnA), the 70-kDa ribosomal S6 kinase (p70(S6k)), glycogen synthase kinase-3beta (GSK-3beta), and the phosphorylated forms of GSK-3beta and p70(S6k) (p-GSK-3beta(Ser9) and p-p70(S6kThr389)) were regulated differently with aging and between muscle types. Total p70(S6k), GSK-3beta, and p-GSK-3beta(Ser9) decreased in the aged-atrophic EDL and soleus while p-p70(S6kThr389) increased. Although total p70(S6k) content diminished in the continuously active diaphragm, phosphorylation of p70(S6k )remained unchanged. Conversely, the expression of GSK-3beta and p-GSK-3beta(Ser9) increased in the diaphragm. With aging, the amount of p-p70(S6kThr389) decreased approximately 56% in the heart while p-GSK-3beta( Ser9) increased approximately 193%. Interestingly, CnA content remained unchanged in the diaphragm, increased approximately 204% in the EDL, and decreased approximately 30% and approximately 65% with aging in the soleus and heart, respectively. These results indicate remarkable differences in the regulation of molecules thought to govern protein synthesis and changes in contractile protein expression.