Differential effect of aging on protein kinase C activity in rat adipocytes and soleus muscle.

Protein kinase C (PKC) has been postulated to play an important role in glucose transport in insulin-sensitive tissues such as rat adipocytes and skeletal muscle. Since glucose transport decreases in old rats, we examined age-related changes in PKC. Cytosolic PKC-dependent histone-phosphorylating enzyme activity and PKC-beta immunoreactivity of both adipocytes and soleus muscles increased progressively with age (or weight) in rats weighing less than 400 g. In comparing PKC enzyme activity and PKC-beta immunoreactivity in young rats (180 +/- 32 g; mean +/- SE, body weight) versus old rats (658 +/- 108 g), both cytosolic and membrane-associated PKC were greater in adipocytes of old rats (relative to adipocytes of young rats), whereas in the soleus muscle of old rats cytosolic PKC activity was diminished and membrane-associated PKC was increased (relative to solei of young rats). The latter redistribution of soleus PKC may be due to endogenous hyperinsulinemia, which is known to occur in old rats and which may have stimulated the translocation of PKC from cytosol to membrane in the soleus. Whatever the cause, decreases in cytosolic PKC in the soleus muscle may limit acute PKC translocation responses to insulin or other agents in old rats.