BIO 3303 Lecture 20

BIO 3303

4.5 Muscle and Animal Energetics

Nov. 29, 2012

Muscle Fiber TypesSTO: Slow-twitch oxidative

fibers; Type I• mitochondria rich

FOG: Fast-twitch oxidative glycolytic fibers; Type IIa• mitochondria

FTG: Fast-twitch glycolytic fibers; Type Iix

• few mitochondria

FTGFTG

STOSTO

FOGFOG

Fig. 19.15, Hill et. al., 2008Fig. 19.15, Hill et. al., 2008

Mitochondrial stainMitochondrial stain

Muscle Fiber Types

http://www.medco-athletics.com/lectureseries/neuro.html

• Myofiber expresses single myosin II isoform

• 1 fiber type per motor unit• E.g. Type I, Type II motor unit

• Muscle made up of myofibers of different types• Isoform switching can occur in

response to physiological conditions

• Exercise, temperature

Muscle Fiber Type Functions

Vastus lateralis (thigh muscle) stained for mitochondria

• Type I (STO), and also some Type IIa (FOG)• Mitochondria rich• Oxidative• Endurance exercise

• Type IIx (FTG)• Fewer mitochondria• Glycolytic • Burst exercise

Fig. 7.10, Hill et. al., 2008

Sci. Amer. 283(3):49, 2000

Composition of leg muscle fibres in human groupsGenes and environment (training) play a role in muscle type differences (remodeling more from Type IIx to Type IIa)

STOFOGFTG

Muscle Fibre Summary

STOSTO

oxidativePhosphorylation

many

many

high

red

low

slow

low

slow

slow

Small

Type I

FOGFOG

oxidativephosphorylation

many

many

high

red

intermediate

intermediate

high

fast

fast

Intermediate

Type IIa

FTGFTG

glycolysis

few

few

low

white

high

fast

higher

faster

fast

Large

Type IIx

ATP production

mitochondria

capillaries

[myoglobin]

“colour”

[glycolytic enzymes]

rate of fatigue

myosin ATPase activity

Shortening velocity, Vmax

Ca2+ kinetics

fibre diameter

Myosin isoform

Energy ProductionATP required for muscle contraction and

relaxation (cross-bridge cycling and ATPase pumps)

Sources of ATP:

Fig. 17.14, Hill et. al. 2004

mitochondriamitochondria

1. Phosphagen pools• ATP, ADP• Creatine phosphate (arginine

phosphate in invertebrates, as well as others)

2. Anaerobic glycolysis• 2-3 ATP/glucose produced• Lactic acid produced

3. Oxidative phosphorylation• In mitochondria• 36+++ ATP/ fuel oxidized

Phosphagen poolsCreatine Phosphate (CrP)= ATP stores in sarcoplasm

•CrP + ADP = Cr + ATP•Important for maintaining constant ATP levels (buffer)•Instanteneous supply of ATP to myofibrils•Last only a very short period (few seconds)• When muscle is at rest, phosphocreatine pools are regenerated

Fig. 2.41Creatine phosphate = phosphocreatine

Lactate

Anaerobic glycolysis• Produces 3 ATP per glycogen or

2 ATP per glucose• Glycogen= intramuscular stores

of glucose• Glucose supplied from blood• High intensity exercise fueled

almost entirely by glycogen• In absence of O2, pyruvate

converted to lactate• Accumulation of lactate =

metabolic disturbances

Fig. 2.28

Oxidative phosphorylation

haigis.hms.harvard.edu

• Uses oxygen to make ATP • Can use all 3 fuels to make ATP•Main fuels for exercise: Carbohydrates (glucose and glycogen)

and fatty acids (proteins account for less than 5% of fuels used)• Lots of ATP produced per fuel oxidized• Can provide a constant supply

of ATP at low rates• limited by O2 delivery to

mitochondria

Mitochondrion

Rates of ATP production

Weber J J Exp Biol 2011;214:286-294

• Anaerobic glycolysis = fastest rate of ATP production• Can supply ATP quickly to

fuel high intensity exercise• Oxidized Fatty acids and

carbohydrates (CHO) supply ATP at slower rates: fuel low intensity exercise

Oxidative phosphorylation

Anaerobic glycolysis

Lipids85%

Protein14%

Carbohydrates (CHO)1%

Fuel stores available for locomotion

• CHO stored as glycogen in liver and muscle

• Lipids stored as TAG (triacylglyceride) in adipose tissue

• Proteins = functional tissue

Intramuscular stores: glycogen and TAG •readily available for fast ATP production•high intensity exercise is predominantly fueled by muscle glycogen (as opposed to liver glycogen)

Whole-body stores

http://howmed.net/physiology/skeletal-muscle/

TAG

Burst exerciseHigh intensity exercise can only be

maintained for short periods of time

Fast-twitch (Type II) muscle fibers predominantly used

Resting to all-out effort: 1. Creatine phosphate for a few seconds

• very high rate of ATP production2. Anaerobic glycolysis for a few minutes• Intramuscular glycogen primarily used (faster ATP production)• high rate of ATP production but lower than creatine

phosphate• Exercise intensity decreases→ Exhaustion from energetic shortfalls, ion disturbances, pH

imbalances, etc.

Cheetah: 120km/h for up to 500m

Recovery from burst exercise• Replenish energy stores: glycogen,

ATP, CrP• reestablish ion gradients, Ca2+

stores, pH• lactate removal• glycogen synthesis in muscle• to liver for gluconeogenesis

(Cori cycle)• to heart or RM to be oxidized

Energy for these processes come from oxidative phosphorylation.• Oxygen debt (EPOC) • Varies with intensity of exercise

(EPOC)

Endurance exerciseLow-intensity exercise can be maintained for

long periods of timeSlow-twitch (type I) muscle fibers

predominantly used (mitochondria rich)ATP production from oxidative

phosphorylation• Limited by rate of oxygen delivery to mito•Many capillaries• Small muscle fiber diameter• High [myoglobin]• Strong heart (↑ stroke volume)

• fuel used depends on exercise intensity• rate of ATP production from oxidized CHO > oxidized Fat• Stores of Fat > CHO

• Limited by rate of delivery and availability of fuels

Fuels oxidized during exercise in mammals

Exercise intensity (% VO2max)

Whole-body fuel use• High intensity of exercise fueled

mostly by CHO• Low intensity of exercise fueled

mostly by lipidsEndogenous vs exogenous sources• CHO: endogenous > exogenous• Lipids: endogenous > exogenous

Weber J J Exp Biol 2011;214:286-294

only at high intensity

Carbohydrate stores are limited

Fig. 6.6, Hill et. al. 2004

At low intensity (~70% VO2max)

• CHO = ~1% of whole-body fuel stores• muscle glycogen

depleted first• then liver glycogen

stores (brought to muscle by blood)• Once glucose stores in

the body depleted, intensity of exercise is at its lowest

• fueled only by lipids (lowest rate of ATP synthesis)• “hitting the wall”

→Timing of these very variable, depending on relative intensity of exercise (% VO2max)

or intramuscular TAG

Muscle energeticsCreatine phosphate (CrP)

• Instantenous supply of ATPAnaerobic glycolysis • Fast rate of ATP production

• Less efficient than oxidative phosphorylation, runs out of fuel quickly (short duration)

Oxidative phosphorylation• More efficient• Slow rate ATP production

GlycolysisGlycolysis2-3

HighNo

Carbohydrate

Oxidative phosphorylationOxidative phosphorylation36

LowerYes

Carbohydrate, fatty acids, amino acids

Efficiency (#ATP/glucose)Rate of ATP productionOxygen dependencyFuel

0 10 20 40 60 80

50

100

Prop

ortio

n of E

nerg

y Deli

vere

dExercise Period (s)

ATP splitting

CrPCrP splittingsplitting

GlycolysisGlycolysisOxidativeOxidativephosphorylationphosphorylation

0 10 20 40 60 80

50

100

Prop

ortio

n of E

nerg

y Deli

vere

dExercise Period (s)

ATP splitting

CrPCrP splittingsplitting

GlycolysisGlycolysisOxidativeOxidativephosphorylationphosphorylation

Time

Salmon migration – Fuel UsePacific salmon feed, grow and store energy for several years in ocean before undertaking migration• Migrate to natal spawning sites in rivers

• E.g. Fraser River sockeye salmon travel more than 1000 km against current• No feeding during migration, only rely on fuel stores

Fuel used changes throughout migration • Early stages of migration: Lipids predominantly used

→Use triglycerides from muscle and adipose tissue

Glycogen spared throughout migration• Reserved for high

intensity spawning activity

Salmon dies shortly after spawning• Energy stores fully

depleted, tissues digested

Fig. 12.14

Salmon migration – Fuel UseLater stages of migration:• Break down muscle to use protein as fuel

• Break down white muscle (RM spared for slow swimming)• Break down intestinal tract, and other less useful tissues

• Metabolism: the sum of all chemical reactions in a biological entity• Metabolic rate: the rate of conversion of energy to heat and

external work (ATP)• Respirometry: technique for obtaining rates of metabolism and fuel

used by measuring of O2 consumption and CO2 production

Animal Energetics

Animal EnergeticsRates O2 consumption and CO2 production used to determine fuel used

• Type of fuel oxidized reflected in respiratory exchange ratio (RER)

• At rest: predominant fuel = lipids • low RER• During exercise = lipids and CHO depending on intensity• RER increase with intensity of exercise (more CHO oxidized at high intensities)

Table 5.2, Hill et. al. 2004

RER = CO2O2

VO2max VO2max: maximum oxygen consumption

•determined during incremental exercise•Maximum capacity of an individual to transport and use oxygen•RER should equal 1 at VO2max

Endurance athletes have high VO2max

• enhanced O2 delivery to mitochondria

• enhanced oxidative phosphorylation• enhanced fuel delivery

lipidsCHO

µm

ol O

2 k

g-1 m

in-1

Exercise intensity: speed vs %VO2maxRunning speed: 10 km/hr

45% VO2max

80% VO2max

An athlete (high VO2max) and a sedentary person (low VO2max) running side by side• athlete will use a less CHO

because he is running at a lower % VO2max

Migratory birds are exceptional endurance athletesSome Bar-tailed Godwits have longest known non-stop flight: 11,000 km from Alaska to New Zeland•55% of body weight prior to migration is fatArctic Terns have longest distance migration: 70,900km roundtrip from Arctic to Antarctic and back

Migration of semipalmated sandpiper

Weber J J Exp Biol 2009;212:593-597

Refueling stopover in Bay of Fundy during its fall migration from breeding areas in the Arctic.• Double body mass in 2 weeks• Eat Corophium mudshrimp

(“natural doping”)• High amounts of n-3 PUFA• Energy source• Enhance oxidative

capacity of fatty acids

Allows to undertake long flight across Atlantic to South America• 4500km, 3 days at ~60 km/h

Animal energetics: Migratory birds

% li

pid

oxid

ized

exercise intensity (% VO2max)

Migratory birds

mammals

Flying = high intensity exercise•2x the VO2max of same-size mammals→ Efficient oxygen deliveryMigratory birds can oxidize

lipids at high exercise intensities•Most of the energy from body

adipose tissues • Enhanced lipid mobilization,

transport and oxidation• 10x faster than mammals

Animal energetics: Migratory birdsTaking endurance to another extreme: Bar-headed geese fly

over Mt Everest during migration flight→Only 1/3 of the oxygen compared to sea level

Next Lecture

• Monday: Review (last lecture!)

• Lecture 1 to 20

• Post your review topics and question from the whole course before Sunday 11am in designated discussion folder on blackboard site