Muscle Cells and Contraction mitochondria sarcoplasmic reticulum contractile

Muscle Cells and Contraction mitochondria sarcoplasmic reticulum contractile filaments

Muscle Types • Type I - “Red”, “Slow” – Contracts relatively slowly – Many mitochondria – Good blood supply • Type IIb - “White”, Fast” – Contracts relatively rapidly – Few mitochondria – Poor blood supply – Packed full of contractile filaments

Gentle exercise • Type I muscles recruited – good blood supply, heaps of mitochondria • Most readily available fuel is glucose – Glucose transporters move to cell surface BLOOD MUSCLE glucose Transporters

Consequences of Glucose Usage • Muscle glucose uptake leads to a decrease in blood glucose concentration • Need to keep blood glucose constant! – Small change in blood glucose causes. . . insulin glucagon

Effects of Low Insulin and High Glucagon – Stimulation of glycogen breakdown in liver – Stimulation of fat breakdown in white adipose tissue LIVER WAT GLYCOGEN FAT glucose fatty acids glucose

Glucose Recycling • Glucose stores (glycogen) are limited – Cannot convert fatty acids into glucose • GLUCOSE CONSERVATION AND RECYCLING – Fatty acids substitute for glucose as a fuel – Fatty acids prevent glucose from being wastefully oxidised

Fatty Acid and Glucose Oxidation glucose GLYCOLYSIS pyruvate fatty acids FATTY ACID OXIDATION acetyl Co. A CO 2

Fatty Acid Oxidation inhibits Glucose Oxidation glucose to liver GLYCOLYSIS inhibits pyruvate fatty acids FATTY ACID OXIDATION acetyl Co. A CO 2 lactate

Summary of Gentle Exercise – Initially, glucose is used – Then fatty acids take over and glucose is recycled LIVER GLYCOGEN lactate MUSCLE glucose WAT FAT glucose lactate CO 2 fatty acids CO 2

Moderate Exercise • As the pace increases, the rate of fatty acid utilisation increases, but. . – The enzymes that catalyse fatty acid oxidation soon reach their maximum capacity – During running, fatty acid oxidation alone is not sufficient to maintain ATP production – Inhibition on glucose oxidation is removed • Glucose oxidation occurs! • Less glucose recycling • Liver glycogen stores depleted faster

Summary of Moderate Exercise LIVER GLYCOGEN lactate MUSCLE glucose WAT FAT fatty acids glucose fatty acids lactate CO 2

Strenuous Exercise – As the intensity of the exercise increases further, muscle glycogen is broken down. LIVER GLYCOGEN lactate glucose WAT FAT fatty acids MUSCLE GLYCOGEN glucose fatty acids CO 2

Glycogen Depletion during a Marathon Race Glycogen (mg/g) 100 75 slower 50 25 faster 0 0. 5 1. 0 1. 5 2. 0 Time (hours) 2. 5

Why Glycogen is Important • When glycogen has run out, only fatty acid oxidation can be used for ATP generation • Power output is lower when using only fatty acids • “Hitting the Wall” • Cannot sprint if there’s no glycogen

Strategies • Start the event with more glycogen than your competitors • Spare the glycogen by making more use of fatty acids • Use fatty acids sooner so less glycogen is used in the early stages

Glycogen Supercompensation – After extensive depletion, glycogen resynthesis overshoots. 100 75 50 pre-exercise level 25 0 0 1 2 3 Time (days) 4 5

Glycogen Loading - Classical 100 – Interrupts training! – Potentially dangerous – Uncomfortable – Character building! 75 50 Hi. FAT Hi. CHO 25 0 0 1 2 3 Time (days) 4 5

Glycogen Loading - Tapered • Fits well into normal training • No dangerous full glycogen depletion. 100 75 50 always High Carbohydrate 25 0 0 1 2 3 Time (days) 4 5

Glycogen Sparing • Increase the use of fatty acids – Carnitine helps fatty acids enter mitochondria – Training increases the activity of fatty acid oxidation enzymes • Start fatty acid release from White Adipose Tissue early – Strong cup of coffee! – NOT glucose drinks BEFORE an event insulin glucagon = inhibition of fatty acid release

Fitness • Better cardiovascular system • Increased vascularisation of muscles – better oxygen supply – better fuel supply (especially fatty acids) • More mitochondria – Higher capacity to burn fatty acids • Conversion of Type IIb to Type I?

Sprinting • Uses Type IIb muscles – Poor blood supply – Packed full of contractile filaments – Few mitochondria – VERY rapid consumption of ATP • Fuel selection problem – Fatty acids? » oxygen supply, mitochondria – Blood Glucose? » transporter recruitment, blood supply

Anaerobic Glycogen Utilisation GLYCOGEN GLYCOLYSIS ADP ATP pyruvate lactate to blood stream acetyl Co. A • Inefficient and incomplete • High turnover • Accumulation of lactate • Takes time to stimulate glycogen breakdown

Buying time with Creatine Phosphate creatine phosphate + ADP ® ATP + creatine • Less than 5 seconds supply of creatine phosphate – Enough to get glycogen mobilisation going • Adrenaline stimulates massive glycogen breakdown.

Fatigue • Role of lactate? ? • Acidity interferes with many processes – Enzymes involved in glycolysis – Contractile process – Calcium movements • All very controversial! – But one things for sure: very low p. H and running out of glycogen are definitely bad news – OK, so that was TWO things…

Fatigue 2 • Use p. H buffers? – Bicarbonate widely used – Certainly works but check with doctor first! • Does glycogen ever run out in a sprint? – Not in short events (100 m) – Important in longer events and multi-heat competitions

Does Glycogen run out during Sprinting? 100 • Have to ensure adequate glycogen resynthesis between races • Rapidly absorbed carbohydrate 75 50 25 depleted 0 0 1 2 3 Time (hours) 4 5

Sprint Training • Increased power output – Increased cell size – More contractile filaments – STEROIDS! • Co-ordination • Conversion of Type I to Type IIb? • Adequate glycogen stores

Lessons • Muscle Contraction – Muscle Types • Getting energy from fat and carbohydrate – No work, no energy consumption! – Increase work, increased oxidation of fuel • Fuel mix during. . – Walking, Jogging, Running and Sprinting • Training effects

How the Fuel Mix Changes LIGHT INTENSE Fatty acids CO 2 Glucose CO 2 Glycogen lactate MODERATE Fatty acids CO 2 Glucose CO 2 SPRINTING Creatine P creatine Glycogen lactate

Take Home Message! • Glycogen is an important fuel to all athletes – Using fatty acids quickly and copiously reduces the use of valuable glycogen • Carbohydrate intake has to be high to allow adequate glycogen synthesis