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

Previous studies have indicated that frequency of stimulation is a major determinant of glucose transport in contracting muscle. We have now studied whether this is so also when total force development or metabolic rate is kept constant. Incubated soleus muscles were electrically stimulated to perform 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 total force development or metabolic rate (glycogen depletion and lactate accumulation). Overall, at constant total force development, glucose transport (2-deoxyglucose uptake) increased with stimulation frequency (P < 0.05; basal: 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 Hz: 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, flexor digitorum brevis, and epitrochlearis muscles. In all muscles, glucose transport was identical at 0.5 and 2 Hz (P > 0.05); also, AMPK activity did not increase with stimulation frequency. In conclusion, muscle glucose transport increases with stimulation frequency but only in the face of energy depletion and increase in AMPK activity. This indicates that contraction-induced glucose transport is elicited by metabolic demands rather than by events occurring early during the excitation-contraction coupling.