Responses of protein synthesis in different skeletal muscles to fasting and insulin in rats.

The rate of protein synthesis (Ks) was measured in nine skeletal muscles from young rats (100 g body wt) after fasting and subsequent insulin infusion. An overnight fast (12 h) resulted in a fall in Ks in all muscles, but this ranged from a small nonsignificant decrease in soleus (Sol) to one-half of the fed rate in tensor fascia latae (TFL). After fasting for 36 h, Ks was significantly reduced in all muscles, although there was still a range apparent between different muscles. The response of the individual muscles to fasting was related to fiber type composition, and, in particular, the responses of the commonly studied Sol were characteristic of the highly oxidative fiber composition rather than being peculiar to Sol itself. The magnitude of the increase in Ks during an infusion of insulin (0.5 h) in the 12-h fasted rats was proportional to the magnitude of the initial response to fasting and these changes were largely reflected in changes in the rate of synthesis expressed per unit RNA (KRNA). After 36 h of fasting, Ks in the muscles responded little to the insulin stimulus, and a similar small effect on KRNA, compared with that in the 12-h fasted animals, suggests an apparent insensitivity to insulin after this longer fasting period.