Thresholds for decrease in intracellular pH and increase in blood lactate during progressive exercise: 31P-MRS study

Area of Science:

• Exercise Physiology
• Biochemistry
• Muscle Metabolism

Background:

• Understanding the relationship between intramuscular and intravascular metabolic events during exercise is crucial for interpreting physiological responses.
• Previous research has explored energy metabolism, but direct comparisons of intramuscular pH and plasma lactate kinetics during progressive arm exercise are limited.

Purpose of the Study:

• To compare intramuscular and intravascular events related to energy metabolism during progressive arm exercise.
• To investigate the relationship between changes in intracellular pH and plasma lactate concentration.
• To identify potential physiological indices of muscle fatigue.

Main Methods:

• Utilized 31P-Magnetic Resonance (MR) spectroscopy to measure intracellular pH (pHT) and phosphocreatine/inorganic phosphate (Pi/PCr) ratios in wrist flexor muscles.
• Monitored plasma lactate concentration (La) in venous blood samples collected at one-minute intervals.
• Determined exercise thresholds for pH decline (pHT), Pi/PCr increase (PT), and lactate increase (LT) using piecewise linear regression analysis.

Main Results:

• The threshold for plasma lactate increase (LT) occurred at a significantly lower percentage of maximal work rate (24.8%) compared to the thresholds for intracellular pH decline (pHT, 43.0%) and Pi/PCr increase (PT, 42.5%).
• Intracellular pH remained relatively stable until approximately 43% of maximal work rate.
• Plasma lactate concentration began to rise earlier, indicating the onset of lactic acidosis before significant intracellular metabolic disturbances were detected.

Conclusions:

• Lactic acidosis may precede detectable changes in intracellular pH during progressive arm exercise.
• The threshold for intracellular pH decline (pHT) may reflect the capacity of remaining intracellular biochemical homeostasis.
• pHT could serve as a physiological index for muscle fatigue during exercise.