Plasma Free Fatty Acid Metabolism • Plasma FFA during exercise result primarily from mobilized lipid stores in adipose tissue • Adipose tissue is the most important store of energy in mammals • % body fat typically 10 – 25 %
FFA Mobilization FFA mobilization is dependent upon • Rate of lypolysis in the adipocyte • plasma transport capacity of FFA • rate of reesterification of FFA Conversion back to triglyceride
Lipolysis • Estimated by measuring glycerol in the plasma • Glycerol appears in the plasma only as a result of lipolysis • Cannot be reused by the adipocyte once liberated (glycerol kinase)
A Quick Note About Lipogenesis • Glycerol 3-P is used as the triacycl glycerol backbone (Houston fig 10.6) • Glycerol 3-P derived from dihydroxyacetone phosphate (from glycolysis) • Glycerol cannot be converted to glycerol 3-P in the adipocyte
Can also use appearance of FFA as estimate of lipolysis • This is balance between lypolysis and reesterification (TG formation) • FFA can be used by adipocyte to form TG • Gives the NET lipolytic rate
Acute Exercise • In general lipolysis is increased with exercise • In isolated gluteal adipocytes • Following 30 min of cycling, catecholamine –stimulated glycerol release was ^ 35-50 % compared to pre-exercise
Using microdialysis probe (in vivo measurement) during 30 min cycling • Glycerol release from abdominal adipocytes was increased • Typically, in animals and humans, glycerol release increases 4-5 fold in prolonged moderate intensity exercise (3 – 4 hr)
Hormonal Regulation • Two most important hormonal regulators are catecholamines and insulin • Catecholamines typically stimulate lipolysis • Insulin stimulates lipogenesis and inhibits lipolysis
Hormonal Regulation During Exercise • -adrenergic activity is inhibitory • -adrenergic activity is stimulatory • At rest -adrenergic activity inhibits activation of lipolysis • During exercise -adrenergic activity stimulates lipolysis
How do we know? • Phentolamine (-adrenergic blocker) doubled glycerol concentration in resting humans • Increased lipolysis • Propanolol (-adrenergic blocker) did not alter glycerol concentration
During Exercise • Propanolol reduces the exercise induced elevation of glycerol by 65% • Also impairs endurance performance • Phentolamine has no effect
Insulin • Insulin levels are decreased during exercise • Directly related to work intensity • Mediated by -adrenergic inhibition • Fasting, fat-feeding and insulin deprivation in diabetics result in elevated FFA and glycerol in plasma
Hormone Sensitive Lipase • Hormones regulate lipolysis via their effects on hormone sensitive lipase (HSL) • HSL hydrolyzes FFA from glycerol backbone • HSL is regulated by its phosphorylation state • Phosphoylation of the regulatory site activates lipolysis
A note about FFA mobilization • As exercise duration increases, FFA mobilization increases,… depending • FFA must be carried in the blood by albumin • FFA/albumin ratio can increase 20 fold during prolonged exercise • The increased FFA/albumin ratio favors reesterification
Perfusion to adipose tissue • Increased perfusion to adipose tissue increases FFA mobilization • During prolonged exercise, perfusion to adipose tissue can increase 3-4 fold • This can compensate for the FFA/albumin ratio • Implications for endurance training??
Lactate and lipolysis • Lactate reduces NET lipid mobilization • Increases reesterification, but doesn’t affect lipolysis • Implications for training??
FFA Permeation Across Membranes • Is FFA movement into the cell simple diffusion or carrier mediated? • Traditional thought was simple diffusion, but recent evidence argues for carrier mediation
Support for Carrier Mechanism • During exercise, FFA flux into the cell is too high to be a result of mass action • Cellular uptake of FFA can be saturated • A specific membrane fatty acid binding protein (FABPpm) has been identified
What’s this mean? • During exercise in humans, FFA transport is saturated as unbound FFA concentrations increase in the plasma (2-3 hr extensions) • Maximal velocity of palmitate uptake is increased with muscular contraction and reduced with low CHO availability
What’s that mean? • Increased FFA availability in the plasma does not necessarily translate to increased uptake of FFA in the cell • Fat loading???
What happens once FFA gets inside the cell? • Lipids don’t like water (hydrophobic), so special carrier proteins are necessary in the cytoplasm • FABPc have been isolated from muscle • High levels in SO fibers, intermediate in FOG, and low in FG
Energy or Storage • Once in the cell, the FFA can be oxidized or reesterified to intramuscular TG pool • During exercise, FFA will go predominately toward oxidation for energy generation
The Substrate Utilization Paradox • As exercise intensity increases, the relative contribution from fat oxidation decreases • During light to moderate exercise though, the increase in oxygen consumption offsets the relative decrease in contribution from fat • Up to ~60 – 70 % • No lactate accumulation
Also, as duration of exercise progresses, relative contribution from fat metabolism increases • Decrease in RER after several hours of light intensity exercise • Determined by substrate availability and oxidative capacity
FFA Oxidation Rate • To a certain extent FFA oxidation is dependent or related to FFA concentration in the plasma • At low intensity (30% VO2max) gradual increases in FFA levels in plasma resulted in increased turnover of radiolabelled oleate
In general, fat oxidation and uptake increase at the onset of exercise • Mobilization from the adipose tissue is not sufficient to meet this increased demand • Transient decrease in FFA levels • As exercise continues, FFA concentrations in plasma rise
FFA Oxidation Plateau • FFA concentration in plasma and FFA oxidation are related except… • When lactate begins to accumulate (> 70 % VO2max) • When FFA levels are extremely high (plateaus) • With endurance training, the FFA oxidation plateau is eliminated • increased FABPpm??
Regulation of Oxidation by CPT-I • CPT-carnitine palmitoyltransferase I • Transport acyl carnitine across mitochondrial membrane • Acyl carnitine-FFA attached to carnitine carrier protein • FFA can’t get into the mitochondria without carnitine
Elevations in glucose activate fatty acid synthesis • Fatty acid synthesis intermediates (malonyl co-A) inhibit CPT-I • In effect inhibits fatty acid entry into mitochondria • Fasting induced hypoglycemia removes inhibition of CPT-I • Increases oxidation of FFA
Contradiction • In situ and experimental invivo conditions show that reduced glucose availability reduces rate of exogenous FFA oxidation • The old “Fat burns in the flame of carbohydrate” maxim • But, Krebs intermediates were maintained • Palmitate supraphysiologic?? • Mechanisms for this phenomena not determined
Intramuscular TG Utilization • Intramuscular triglyceride oxidation is dependent upon exercise intensity and duration • In animals, whole body exercise to exhaustion results in decreases in intramuscular TG content • Lower intensity exercise, results are equivocal
In humans using various modes of exercise, TG content of VL decreased 25-50 % • Exercise prolonged at 55-70 % VO2max • During intense exercise 5 min in duration, TG decreased 29 % • Significant contribution of oxidative metabolism at 5 min