Effects of prior exercise and a low-carbohydrate diet on muscle sarcoplasmic reticulum function during cycling in women

The effects of exercise and diet on sarcoplasmic reticulum Ca2+-cycling properties in female vastus lateralis muscle were investigated in two groups of women following four different conditions. The conditions were 4 days of a low-carbohydrate (Lo CHO) and glycogen-depleting exercise plus a Lo CHO diet ( Ex + Lo CHO) ( experiment 2) and 4 days of normal CHO ( Norm CHO) and glycogen-depleting exercise plus Norm CHO ( Ex + Norm CHO) ( experiment 1). Peak aerobic power (Vo(2) (peak)) was 38.1 +/- 1.4 (SE); n = 9 and 35.6 +/- 1.4 ml (.) kg(-1) (.) min (-1); n = 9, respectively. Sarcoplasmic reticulum properties measured in vitro in homogenates (mu mol (.) g protein(-1) (.) min(-1)) indicated exercise-induced reductions ( P < 0.05) in maximal Ca2+-ATPase activity (0 > 30, 60 min > fatigue), Ca2+ uptake (0 > 30 > 60 min, fatigue), and Ca2+ release, both phase 1 (0 > 30 > 60 min, fatigue) and phase 2 (0 > 30, 60 min, fatigue; 30 min > fatigue) in Norm CHO. Exercise was without effect in altering the Hill slope (n(H)), defined as the slope of relationship between Ca2+-ATPase activity and Ca2+ concentration. No differences were observed between Norm CHO and Ex + Norm CHO. Compared with Norm CHO, Lo CHO resulted in a lower (P < 0.05) Ca2+ uptake, phase 1 Ca2+ release (0 > 30 min), and nH. Ex + Lo CHO resulted in a greater ( P < 0.05) Ca2+ uptake and nH compared with Lo CHO. The results demonstrate that Lo CHO alone can disrupt SR Ca2+ cycling and that, with the exception of Ca2+ release, a glycogen-depleting session of exercise before Lo CHO can reverse the effects.