Glycolytic intermediates in human muscle after isometric contraction

Area of Science:

• Exercise Physiology
• Muscle Metabolism
• Biochemistry

Background:

• Muscle fatigue during sustained isometric contractions involves complex metabolic shifts.
• Understanding glycogen utilization and glycolytic intermediate accumulation is crucial for exercise science.

Purpose of the Study:

• To investigate glycogen utilization and glycolytic metabolite changes in quadriceps muscle during isometric fatigue.
• To examine the recovery kinetics of glucose 6-phosphate and lactate post-contraction.
• To elucidate the role of intracellular glucose accumulation and its regulation.

Main Methods:

• Sustained isometric quadriceps contraction to fatigue at 66% maximum voluntary contraction force.
• Analysis of muscle glycogen, glycolytic intermediates (including glucose 6-phosphate), and lactate.
• Measurement of metabolite changes during a 4-minute recovery period with and without circulatory occlusion.

Main Results:

• Significant glycogen utilization (80.4 mmol/kg d.m.) and accumulation of glycolytic intermediates (82.9 mmol/kg d.m.).
• Hexose phosphates (35.4%) and lactate (59.3%) were major contributors to the increase.
• Glucose 6-phosphate and lactate showed distinct recovery half-times (2.0 and 2.5 min, respectively).
• Muscle free glucose increased, confined intracellularly, linked to hexokinase inhibition by glucose 6-phosphate.
• Circulatory occlusion impaired lactate and glucose 6-phosphate recovery.

Conclusions:

• Isometric exercise induces substantial glycogen breakdown and glycolytic intermediate accumulation.
• Intracellular glucose accumulation is regulated by hexokinase inhibition, influenced by glucose 6-phosphate levels.
• Muscle-blood lactate gradient facilitates faster lactate disappearance after isometric contractions compared to dynamic exercise.
• Local circulation is vital for the efficient recovery of muscle metabolites post-exercise.