CONTRACTION-INDUCED INCREASE IN MUSCLE INSULIN SENSITIVITY - REQUIREMENT FOR A SERUM FACTOR

The insulin sensitivity of glucose transport is enhanced in skeletal m uscle after a bout of exercise. In a previous study, stimulation of wa shed muscles to contract in vitro, in contrast to exercise, did not re sult in an increase in insulin sensitivity. The purpose of the present study was to explain this apparent discrepancy. We found that, althou gh rat epitrochlearis muscles stimulated to contract in vitro after 15 min of incubation in Krebs-Henseleit buffer did not develop increased insulin sensitivity, muscles stimulated to contract immediately after being dissected showed a small but significant enhancement of the sti mulation of 3-O-methyl-D-glucose transport by 30 mu U/ml insulin. Furt hermore, muscles stimulated to contract in situ and then allowed to re cover in vitro showed as large an increase in insulin sensitivity as t hat which occurs after a bout of swimming. To follow up these findings suggesting involvement of a humoral factor, we incubated epitrochlear is muscles in serum before and during contractile activity in vitro. E pitrochlearis muscle insulin sensitivity was enhanced to as great an e xtent after in vitro contractile activity in serum as after swimming. Experiments involving charcoal treatment, ultrafiltration, or trypsin digestion provided evidence that the serum factor that interacts with contractions to enhance insulin sensitivity is a protein.