Phosphocreatine Metabolism and Acidosis during Repeated Bouts of Heavy-Intensity Exercise Separated by 15-min Rest

Abstract

Prior heavy-intensity exercise (HVY) results in a reduced reliance on substrate-level phosphorylation during a subsequent exercise bout when the bouts are separated by 6-min rest. Proposed mechanisms for this response have been associated with acidosis and lactate accumulation. PURPOSE: To determine if a similar favourable energetic response in the second exercise is evident when the bouts are separated by 15-min of rest. In this condition intracellular acid-base status will have returned to pre-exercise resting levels prior to the second bout. METHODS: Healthy young adult male subjects (n = 7) performed a 6-min bout of HVY isotonic plantar flexion exercise (HVY1) followed by 15-min of rest, then an identical 6-min bout (HVY2). Muscle phosphocreatine (PCr) and intracellular acid-base status were measured using 3-tesla 31P-magnetic resonance spectroscopy. Each subject repeated the protocol three times. PCr was expressed as percent (%) of average pre-HVY1 resting value. RESULTS: Prior to the onset of exercise, resting intracellular hydrogen ion ([H+]i) was not different (P = 0.11) at the onset of exercise in HVY1 [94 nM (SD 4)] and HVY2 [99 nM (SD 8)]. The increase in [H+]i during exercise was reduced ∼ 45% during HVY2, with end-exercise [H+]i lower (P < 0.01) in HVY2 [139 nM (SD 38)] than HVY1 [167 nM (SD 48)]. Resting [PCr] was greater (P = 0.03) in HVY2 [101.8% (SD 2.1)] than HVY1, while at end-exercise, [PCr] was greater (P = 0.02) in HVY2 [55.1% (SD 15.1)] than HVY1 [48.7 % (SD 10.9)]. CONCLUSIONS: The lower accumulation of [H+]i (and presumably lactate accumulation) in HVY2 (delta [H+]i 40 nM) than HVY1 (delta [H+]i 73 nM), along with the decreased PCr breakdown, are consistent with reduced substrate-level phosphorylation. Therefore, these results demonstrate that the benefits of prior heavy-intensity exercise are evident after 15-min of rest, when intracellular acid-base status has returned to pre-exercise resting levels.