Exercise enhances skeletal muscle insulin action, and subsequently affects a number of insulin sensitive processes such as glucose uptake and glycogen synthesis. The effects of exercise on insulin action and glucose homeostasis have important implications for the maintenance of good health and in the treatment and prevention of type 2 diabetes. However, the underlying mechanism/s mediating the increase in skeletal muscle insulin action following exercise are equivocal. One hypothesis is that exercise or muscle contraction enhances intracellular insulin signalling events downstream of the insulin receptor. Recently we have demonstrated in mouse skeletal muscle, that prior exercise enhances insulin-stimulated insulin receptor substrate-2 (IRS-2) phosphorylation and associated phosphatidyl-inositol (PI) 3-kinase activity (Howlett et al., 2002). However, no study has examined whether this exercise-mediated effect on IRS-2 signalling also occurs in human skeletal muscle. In light of this, the aim was to examine insulin signalling in human skeletal muscle in response to a hyperinsulinaemic euglycaemic clamp following an acute bout of exercise. Seven untrained males (24 ± 2 yr, 73 ± 3 kg, O2 peak = 3.63 ± 0.22 l.min-1) were studied at rest and after 60 min of strenuous exercise (75 ± 4% O2 peak). Immediately following rest or exercise, a 120 min hyperinsulinaemic (40 mU.m-2) euglycaemic (5 mM) clamp was performed. Muscle biopsies were obtained at rest, post exercise, and 30 and 120 min of hyperinsulinaemia. Plasma insulin levels were similar during hyperinsulinaemia (Rest, 704 ± 34; Exercise, 691 ± 40 pmol.l-1). Insulin-mediated glucose disposal rates were similar during the final 30 min of the clamp (Rest, 9.1 ± 1.1; Exercise, 8.3 ± 1.0 mg.kg-1.min-1). Insulin had no significant effects on IRS-1 and IRS-2 associated PI 3-kinase activity. Exercise, per se, tended to decrease IRS-1 (0.70 ± 0.13 fold) and IRS-2 (0.71 ± 0.10 fold) associated PI 3-kinase activity. Following exercise, insulin-stimulated IRS-2 associated PI 3-kinase activity tended to increase at 30 min (1.29 ± 0.11 fold) and was further enhanced at 120 min (2.83 ± 0.81 fold, p<0.05). In contrast, following exercise insulin-stimulated IRS-1 associated PI 3-kinase activity increased to a peak at 30 min (1.88 ± 0.40 fold, p<0.05), although remained elevated above basal at 120 min (1.67 ± 0.18 fold, p<0.05). Despite the effect of exercise on these proximal insulin signalling proteins, there was no significant effect of exercise on insulin-stimulated activation of downstream insulin signalling proteins, including phosphorylation of Akt (Ser473) and GSK3-β (Ser9). However, exercise did result in an increase in insulin stimulated phosphorylation of GSK3-α (Ser21). In conclusion, prior exercise increases insulin-stimulated IRS-2 signalling in human skeletal muscle. It appears that insulin-stimulated IRS-1 and IRS-2 signalling in human skeletal muscle may be differentially regulated by exercise.
Howlett, K.F., K. Sakamoto, M.F. Hirshman, W.G. Aschenbach, M. Dow, M.F. White and L.J. Goodyear. (2002). Insulin signaling after exercise in insulin receptor substrate-2 deficient mice. Diabetes 51: 479-483.