topic 8 again lol

LACTIC ACID METABOLISM

• at physiological pH (~7.4), lactic acid exists almost entirely in its deprotonated (ionized) form: lactate⁻.

• Even during aerobic metabolism, tissues still produce some lactate.

• Why?

  • Lactate dehydrogenase (LDH) is always active.

  • Pyruvate may not always be immediately taken up by mitochondria (due to limited proximity or transport rate), so it gets converted to lactate.

• Not just muscle — other tissues make lactate too:

• Lactate turnover refers to the rate at which lactate is produced and utilized by the body. It's a measure of how much lactate is entering and leaving the bloodstream and being used for other metabolic processes

• Lactate isn’t waste — it’s a valuable:

• This is the Lactate Shuttle Hypothesis (Brooks, 1985):

  • M-type LDH (muscle) favors pyruvate → lactate

  • H-type LDH (heart) favors lactate → pyruvate

• Lactate moves from high glycolytic (Type II) to oxidative (Type I) tissues to be used as fuel.

“Infusion of epinephrine at rest sharply increases lactate released by muscle (↑ glycolysis, no effect on O₂/P)”

EXERCISE CONDITIONS

• As soon as muscle contractions begin, anaerobic glycolysis activates rapidly, leading to lactate production.

  • The muscle starts releasing more lactate than it's consuming, creating a net output.

  • Simultaneously, lactate consumption also rises (e.g., by heart, liver, oxidative fibers).

✅ Net release underestimates total lactate produced

• Some lactate is used within the same muscle or nearby tissues before entering the blood.

• So, the actual lactate production is higher than what you see in blood measurements.

✅ Initial lactate release depends on muscle mass

• The more active muscle mass, the more lactate produced.

• as exercise at same sub max intensity continues , lactate release from muscles level off, consumption increases → drop in blood lactate

✅ Rate of lactate consumption depends on availability

• The body clears lactate faster when there’s more available in blood.

• Uptake by heart, liver, kidneys, Type I fibers increases as lactate levels rise.

✅ “VO₂ ↑ linearly with exercise intensity, but lactate ↑ in a curvilinear fashion”

• Oxygen consumption (VO₂) increases steadily with exercise intensity.

• Lactate stays low at first, then rises sharply once a certain intensity is reached.

• This point is often called the “lactate threshold” or OBLA (onset of blood lactate accumulation).

  • In untrained individuals, blood lactate begins to rise rapidly around 60% VO₂max.

  • In trained endurance athletes, this shift occurs much later — around 85% VO₂max.

lactate rises suddenly because as intensity increases:

• Sympathetic activation (fight or flight response) releases catecholamines (epinephrine, norepinephrine)

• These stimulate glycolysis (↑ glucose breakdown) in muscle

➕ Additional Points:

• Catecholamines stimulate glycolysis but not oxidative enzymes:

  • → More pyruvate is made than mitochondria can handle

  • → More lactate is formed

• Reduced splanchnic blood flow (to liver/kidneys):

  • ↓ lactate uptake by liver and kidneys → more remains in blood

• Substrate crossover:

  • As intensity rises, the body shifts from fat to carbohydrates (CHO) as the primary fuel → ↑ glycolysis → ↑ lactate

• Motor unit recruitment changes:

  • At higher intensities, more Type II (glycolytic) fibers are recruited → produce more lactate

Fiber Type Differences in Lactate Handling

• During glycolysis, glucose → pyruvate → produces NADH

• To keep glycolysis going, cells need to regenerate NAD⁺ (this is what the glycerol phosphate shuttle does)

• If a muscle fiber has strong shuttle activity, it keeps the NAD⁺ / NADH ratio high in the cytosol

• This reduces the need to convert pyruvate into lactate

Lactate utlization (refers tconversion of lactate to pyruvate)

a. accumulate

b. be released from msucle

c. be converted

d. be used to produce glycogen

e. be oxidized as fuel