Lecture 3: Overview of Exercise Metabolism ● The ATP dependent processes in contracting skeletal muscle ○ Cross­bridge cycling (myosin ATPase) (major consumer of ATP, ~60%) ■ Without ATP, the actin­myosin cannot detach and muscle remains rigid ○ Sarcolemmal excitability (Na+/K+ ATPase) maintains cell membrane potential (repolarisation) and therefore excitability of the cell (Na+ pumped out and K+ pumped in) ○ Calcium pumping into SR (SERCA ­ CA2+ ATPase) ■ cleaves ATP to get energy to fuel the reuptake Calcium ions into the sarcoplasmic reticulum ­1​
Muscle [ATP] = 6 mmol.kg​
wet muscle ­1​
­1​
~75% VO​
max (200W) ­ 0.4 mmol ATP kg​
sec​
(15 s) 2​
­1​
­1​
140% VO​
max (420W) ­ 1.0 mmol ATP kg​ sec​(6 s) 2​
­1​
­1​
300% VO​
max (900W) ­ 3.7 mmol ATP kg​
sec​
(<2 s) 2​
Here, >100% VO​
implies that there is an anaerobic source of ATP in these situations. 2​
Secondly, because we know that we ​
can ​
work at 900W for more than 2s, we know there must be other source of ATP readily available for working muscle. ● The energy systems responsible for ATP generation during exercise ○ Substrate level phosphorylation: High energy phosphagens (ATP/CP), “anaerobic” glycolysis. Essentially converts the energy in the bonds of certain substrates to add phosphate to ADP. ○ Oxidative phosphorylation: Oxidative metabolism of CHO and fat Both processes are phosphorylation because we are adding a phosphate back to ADP ­> ATP. “Anaerobic” ATP production (substrate level phosphorylation): PCr + ADP + H+ <­> ATP + creatine (PCr = phospho­creatine) 2ADP ­> ATP + AMP ­​
+​
Glycogen + 3ADP + 3P​
­> 2Lac​
+ H​
+ 3ATP i​
“Aerobic” ATP production (mitochondria): ● Electron donors (NADH) generated from CHO and Fat metabolic pathways ● ADP + Pi ● O​
(respiratory and cardiovascular systems) 2​
Combines all 3 to generate ATP via the electron transport chain ● The relative ATP generating power and capacity of these energy systems The power of the system refers to the rate at which it can generate ATP. The capacity is related to the total amount of ATP that can be generated if it were to utilise all available energy in the body. High power output between PCr and glycolysis but low capacity. CHO and Fat a less powerful. ● The relative importance of the energy systems during high intensity exercise, rest­exercise transitions and prolonged endurance exercise The initial increase in metabolic demand is met by the rather instantaneous substrate­level phosphorylation of PCr and glycolysis (oxygen deficit phase). This is because it takes time for the oxidative phosphorylation capacity to increase. Fuels for high intensity exercise During heavy resistance exercise, the muscle may actually occlude the blood flow, and majority of the energy relied upon is anaerobic. Oxygen uptake is exercise intensity­dependent, here above 300W and oxygen uptake no longer meets the demand and an oxygen deficit begins. ●
The influence of exercise intensity and duration on the relative importance of CHO and fat for oxidative metabolism Fuels for endurance sports: 1. Liver glycogen (200­400 kcal) 2. Muscle glycogen (1800­3000 kcal) 3. Triglycerides (adipose tissue ­ 50,000­100,000 kcal) mobilises free fatty acids (FFA) 4. Triglycerides (muscle ­ 2000­3000 kcal) Fuels for endurance sports ­ influence of exercise intensity Other fat sources = primarily muscle fats As you move to higher intensity, the relative fat oxidation moves down and there is more of a reliance of carbohydrates However, temporally speaking, the longer the exercise the greater the utilisation of fat and the less the utilisation of CHO and plasma glucose.