Lactate and Hydrogen Accumulation During Exercise

Why do muscles begin to "burn" during exercise?

this happens due to the rapid accumulation of hydrogen ions (H+) due to the production of hydrogen ions, ADP, and phosphate from ATP hydrolysis

What occurs when more ATP is used by the body?

more hydrogen ions are produced, so the muscle tissue pH becomes acidic (initiates muscle "burning")

How are excess hydrogen ions disposed of?

ATP synthase in the ETC utilizes hydrogen ions and lost electrons to produce water as the end product

How does lactate production relate to hydrogen accumulation?

lactate is a marker for hydrogen accumulation and an indirect marker for exercise intensity

How does lactate aid in the disposal of excess hydrogen ions?

the conversion of lactate to pyruvate in the Cori cycle uses up excess H+ and NADH, lowering the acidity of the muscle tissue

how does high-intensity exercise affect glycolysis rate?

glycolysis rate and ATP turnover increase with high-intensity exercise, leading to increased pyruvate production

What happens when mitochondrial oxidation is limited?

pyruvate is reduced to lactate, H+ builds up primarily due to ATP hydrolysis

How do lactate thresholds change with more training?

individuals who are well-trained have higher lactate thresholds and can tolerate more hydrogen ion accumulation, therefore having higher lactate clearance and buffering capacity

How does lactate production sustain glycolysis?

lactate production helps regenerate NAD+, acting as an indirect hydrogen ion shuttle

How do NADH and H+ cross the mitochondrial membrane?

reducing equivalents are transferred via shuttle systems called the Malate-aspartate and Glycerol-2-phosphate shuttles

What is the function of shuttle systems?

they allow the oxidative disposal of H+ in mitochondria, although shuttle capacity is limited

Where is lactate transported?

liver (Cori cycle), oxidative muscle fibers, heart

What is acidosis?

decline in intracellular pH caused by imbalance between H+ production and removal

What is impaired by a low pH?

cross-bridge cycling, calcium's ability to bind to troponin, enzyme activity, muscle contraction

How does training adaptation affect H+ accumulation?

increased mitochondrial density, improved hydrogen shuttle capacity, increased buffer capacity, upregulation of MCT transporters, so acidosis is delayed and performance is improved

What is the Respiratory Exchange Ratio (RER)?

measured at the mouth using indirect calorimetry, reflects the balance between CO2 production and O2 consumption, used to estimate substrate utilization during steady-state exercise (VCO2/VO2)

What does an RER of 0.7 mean?

predominantly fat oxidation

What does an RER of 0.85 mean?

mixed fat and carbohydrate oxidation

What does an RER of 1.00 mean?

predominantly carbohydrate oxidation

What does an RER of over 1.00 mean?

indicates non-metabolic CO2 production

What is the biochemical basis for RER values?

fat oxidation consumes more oxygen per carbon dioxide than carbohydrate oxidation

Why does RER increase with exercise intensity %?

the body is burning more carbohydrates, % energy from fat decreases and % energy from carbs increases

Why are fast-twitch fibers used during high-intensity exercise?

they have higher concentrations of enzymes that break down glucose into ATP in glycolysis (phosphofructokinase and pyruvate kinase)

What happens when epinephrine is released during intense exercise?

epinephrine released from the adrenal glands triggers a cascade of events that breaks down muscle glycogen to provide glucose for energy

Why do we burn more fat as exercise duration increases?

high intensity exercise that burns more carbohydrates cannot be sustained for long periods of time

What occurs is McArdle's Disease?

no glycogen metabolized due to lack of glycogen phosphorylase, motor proteins lack ATP needed for movement, muscle failure and injury results

What is the optimal training intensity for fat burning?

below 60% of VO2 max (below lactate threshold)

When is liver glycogen used primarily?

during glucagon production of glucose when we are in a fasting state (maintaining homeostasis)

When is muscle glycogen used primarily?

during exercise (ex: running a marathon)