Effect of stimulation frequency on contraction-induced glucose transport in rat skeletal muscle

Previous studies have indicated that frequency of stimulation is a major de terminant of glucose transport in contracting muscle. We have now studied w hether this is so also when total force development or metabolic rate is ke pt constant. Incubated soleus muscles were electrically stimulated to perfo rm repeated tetanic contractions at four different frequencies (0.25, 0.5, 1, and 2 Hz) for 10 min. Resting length was adjusted to achieve identical t otal force development or metabolic rate (glycogen depletion and lactate ac cumulation). Overall, at constant total force development, glucose transpor t (2-deoxyglucose uptake) increased with stimulation frequency (P < 0.05; b asal: 25 +/- 2, 0.25 Hz: 50 +/- 4, 0.5 Hz: 50 +/- 3, 1 Hz: 81 +/- 5, 2 Hz: 79 +/- 3 nmol.g(-1).5 min(-1)). However, glucose transport was identical (P . 0.05) at the two lower (0.25 and 0.5 Hz) as well as at the two higher (1 and 2 Hz) frequencies. Glycogen decreased (P, 0.05; basal: 19 +/- 1, 0.25 H z: 13 +/- 1, 0.5 Hz: 12 +/- 2, 1 Hz: 7 +/- 1, 2 Hz: 7 +/- 1 mmol/kg) and 5' -AMP-activated protein kinase (AMPK) activity increased (P, 0.05; basal: 1. 7 +/- 0.4, 0.25 Hz: 32.4 +/- 7.0, 0.5 Hz: 36.5 +/- 2.1, 1 Hz: 63.4 +/- 8.0, 2 Hz: 67.0 +/- 13.4 pmol.mg(-1).min(-1)) when glucose transport increased. Experiments with constant metabolic rate were carried out in soleus, flexo r digitorum brevis, and epitrochlearis muscles. In all muscles, glucose tra nsport was identical at 0.5 and 2 Hz (P. 0.05); also, AMPK activity did not increase with stimulation frequency. In conclusion, muscle glucose transpo rt increases with stimulation frequency but only in the face of energy depl etion and increase in AMPK activity. This indicates that contraction-induce d glucose transport is elicited by metabolic demands rather than by events occurring early during the excitation-contraction coupling.