Week 6 - Lactate Metabolism

Question 1

How does lactic acid exist in the human body? Why

Answer 1

As lactate and H+ ion as it fully dissociates at physiological pH - 7.4.

Question 2

What evidence is there/ isn’t there that H+ ions are the cause of fatigue?

Answer 2

As muscle force (jump height) decreases H+ ion concentration increases - supportive.
The H+ ion concentration does not decrease during recovery despite an increase in jump height or muscle force. You can recover force with a really low pH - no cause and effect.

Question 3

Otto Meyerhof idea and evidence against

Answer 3

Glucose –> glycose-6-phosphate –> pyruvic acid —> lactic acid when no/low oxygen so with high levels of oxygen you would expect little lactic acid but this is not the case - at rest still lots. At rest lactate outnumbers pyruvate 50:1.

Question 4

What did Fletcher and Hopkins (1907) conclude?

Answer 4

The presence/lack of oxygen causes lack of/build up of lactic acid.
In a frog study it disappeared when in presence of oxygen.

Question 5

Hill and Lupton (1923) Conclusion

Answer 5

After exercising you have to repay the oxygen that wasn’t available at the start of exercise. Rise in lactate coincides with O2 deficit in exercising muscle.

Question 6

What is the lactate threshold?

Answer 6

The intensity where blood lactate increases exponentially. Also known as the lactate inflection point.
Can’t sustain 5/6mmol for very long.

Question 7

How can lactate actually be used?

Answer 7

As a correlator for fatigue. Indirect measurement.
Lactate threshold can be used to set training intensities but also as an alternative to VO2 max testing in patients who cannot reach exhaustion.

Question 8

What is the cori cycle?
What does it do?

Answer 8

A cycle between the liver and skeletal muscle.
Lactate is transported to the liver in the venous system where it is converted to glucose through gluconeogenesis.H

Question 9

How much lactate is removed through the cori cycle during exercise?

Answer 9

About 25%.

Question 10

What has a major effect on the cori cycle?

Answer 10

Alcohol - impairs liver function, puts the liver in an overly stressed state so cannot complete the cori cycle - added risk when max testing so checked for during screening.

Question 11

What ground-breaking suggestion did George Brooks make?

Answer 11

Lactate is actually a substrate for aerobic metabolism - the same as pyruvate.

Question 12

Give the two types of lactate shuttle

Answer 12

Intracellular - within the cell
Intercellular - between the cells

Question 13

Describe intracellular lactate shuttle

Answer 13

Within the cell you have lactate producing parts and mitochondria to oxidise the lactate - lactate moved to mitochondria

Question 14

Describe the intercellular lactate shuttle

Answer 14

Using the entire body to try and get rid of the lactate.
Type IIa fibres produce the most lactate and have few mitochondria so the lactate needs to be transported out of the cell.
Monocarboxylate transporter 4 (MCT4) does this by picking up the lactate and moves it through the venous and arterial system to type I fibres - lots of mitochondria.
Taken into the cell using MCT1. Lactate converted back to pyruvate.

Question 15

How does lactate accumulate during exercise?

Answer 15

Lactate production (Rate of appearance - Ra) increases.
Rate of disappearance (Rd) does not increase to the same extent as the Ra.

Question 16

Give some reasons for the oxygen debt

Answer 16

Elevated hormone levels
Elevated core body temperature
Restoration of oxygen stores in muscle and blood
Elevated heart rate and breathing rate post-exercise

Question 17

What causes the reduction in muscle pH?

Answer 17

Not the H+ ions produced from lactic acid dissociation.

It is the H+ ions produced during ATP hydrolysis - breaking the ATP to release energy.

Question 18

How is lactate useful?

Answer 18

Lactate can buffer the H+ ions and actually prevent acidosis.
Prevents pyruvate accumulation and supplies NAD which is needed in phase 2 of glycolysis.

Question 19

How to calculate the lactate threshold

Answer 19

Plot work (speed/power) on the x axis and blood lactate concentration on the y axis.
A curve is fitted and the lactate threshold determined using y= mx+ c
The point where lactate increases exponentially is the point of lactate threshold.

Question 20

Describe the training adaptions for blood lactate levels.

Answer 20

There is a rightward and downward shift in the blood lactate concentration curve I.e. work harder before they start to accumulate blood lactate.
Due to an increased number of mitochondria – can cope with more H+ ions as more mitochondria doing respiration and using up the H+ ions produced.
They can work at higher speed, higher power, greater oxygen consumption but not be producing as much lactate.

Question 21

What is maximum lactate steady state? MLSS

Answer 21

How much lactate you can cope with and sustain without getting fatigued.
The criteria is that blood lactate varied by <1mmmol/litre during the last 20 minutes of constant intensity exercise.
In untrained individuals, this sustained power will be lower.