Comparative effects of a low-carbohydrate diet and exercise plus a low-carbohydrate diet on muscle sarcoplasmic reticulum responses in males

We employed a glycogen-depleting session of exercise followed by a low-carbohydrate (CHO) diet to investigate modifications that occur in muscle sarcoplasmic reticulum (SR) Ca2+-cycling properties compared with low-CHO diet alone. SR properties were assessed in nine untrained males [peak aerobic power (V-O2 peak) = 43.6 +/- 2.6 (SE) ml.kg(-1).min(-1)] during prolonged cycle exercise to fatigue performed at similar to 58% V-O2 peak after 4 days of low-CHO diet (Lo CHO) and after glycogen-depleting exercise plus 4 days of low-CHO (Ex + Lo CHO). Compared with Lo CHO, Ex + Lo CHO resulted in 12% lower (P < 0.05) resting maximal Ca2+-ATPase activity (V-max = 174 +/- 12 vs. 153 +/- 10 mu mol.g protein(-1).min(-1)) and smaller reduction in Vmax induced during exercise. A similar effect was observed for Ca2+ uptake. The Hill coefficient, defined as slope of the relationship between cytosolic free Ca2+ concentration and Ca2+-ATPase activity, was higher (P < 0.05) at rest (2.07 +/- 0.15 vs. 1.90 +/- 0.10) with Ex + Lo CHO, an effect that persisted throughout the exercise. The coupling ratio, defined as the ratio of Ca2+ uptake to V-max, was 23 -30% elevated (P < 0.05) at rest and during the first 60 min of exercise with Ex + Lo CHO. The similar to 27 and 34% reductions (P < 0.05) in phase 1 and phase 2 Ca2+ release, respectively, observed during exercise with Lo CHO were not altered by Ex + Lo CHO. These results indicate that when prolonged exercise precedes a short-term Lo CHO diet, Ca2+ sequestration properties and efficiency are improved compared with those during Lo CHO alone.