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) Ca(2+)-cycling properties compared with low-CHO diet alone. SR properties were assessed in nine untrained males [peak aerobic power (Vo(2 peak)) = 43.6 +/- 2.6 (SE) ml.kg(-1).min(-1)] during prolonged cycle exercise to fatigue performed at approximately 58% Vo(2 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 Ca(2+)-ATPase activity (V(max) = 174 +/- 12 vs. 153 +/- 10 micromol.g protein(-1).min(-1)) and smaller reduction in V(max) induced during exercise. A similar effect was observed for Ca(2+) uptake. The Hill coefficient, defined as slope of the relationship between cytosolic free Ca(2+) concentration and Ca(2+)-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 Ca(2+) 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 approximately 27 and 34% reductions (P < 0.05) in phase 1 and phase 2 Ca(2+) 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, Ca(2+) sequestration properties and efficiency are improved compared with those during Lo CHO alone.