Feeding a low energy diet and refeeding a control diet affect glycolysis differently in the slow- and fast-twitch muscles of adult male Wistar rats.

Muscle glycogen concentrations in underfed (HYPO) and refed rats (RE) in an earlier study did not correlate with fatigue. We hypothesized that underfeeding slowed glycolysis in the slow-twitch soleus, but not in the fast-twitch extensor digitorum longus (EDL). Thirty adult male Wistar rats were randomly assigned to receive one of two isovolemic and micronutrient-complete liquid diets, a control (CN) energy-complete diet for 10 d or a diet 80% lower in energy (HYPO) for 7 d producing a 20% loss of initial weight. Rats were refed an energy-complete diet for 1 or 4 d (RE1, RE4). Rats were then anesthetized, and the soleus and EDL muscles of the hindlimbs were isolated and electrically stimulated in situ. The pre- and postfatigued muscles were freeze-clamped, lyophilized and stored at -70 degreesC until assayed for specific glycolytic and Krebs cycle metabolites. The HYPO diet caused significantly slower glycolysis in the stimulated soleus but not the EDL compared with the CN diet as supported by the following: 1) a lower fructose-1,6-bisphosphate (F-1,6-P2)/fructose-6-phosphate (F-6-P) ratio; 2) a greater glucose-6-phosphate (G-6-P)/lactate ratio; 3) a lower lactate/glycogen ratio; and 4) lower lactate concentration. Four days of refeeding normalized the F-1,6-P2/F-6-P ratio, but did not improve the lactate/glycogen or the G-6-P/lactate ratios. We conclude that undernutrition compromises glycolysis only in slow-twitch muscles and that 4 d of refeeding restores phosphofructokinase activity.