Pathways for oxidative fuel provision to working muscles: Ecological consequences of maximal supply limitations

Abstract

The study of metabolic fuel provision and its regulation has reached an exciting stage where specific molecular events can be correlated with parameters of the organism's ecology. This paper examines substrate supply pathways from storage sites to locomotory muscle mitochondria and discusses ecological implications of the limits for maximal flux through these pathways. The relative importance of the different oxidative fuels is shown to depend on aerobic capacity. Very aerobic, endurance-adapted animals such as long distance migrants favor the use of lipids and intramuscular fuels over carbohydrates and circulatory fuels. The hypothesis of functional co-adaptation between oxygen and metabolic fuel supply systems allows us to predict that the capacity of several biochemical processes should be scaled with maximal oxygen consumption. Key enzymes, transmembrane transporter proteins, glucose precursor supply and soluble fatty acid transport proteins must all be geared to support higher maximal glucose and fatty acid fluxes in aerobic than in sedentary species.

Flight muscle carnitine palmitoyl transferase activity varies with substrate chain length and unsaturation in the hoary bat (Lasiurus cinereus)

“…Lipid metabolism may be limited by muscular uptake and use (Weber 1992;Vock et al 1996;Bonen et al 1999), and CPT is thought to have a large influence on the rate of myocyte lipid oxidation (Sidossis et al 1997;Spurway et al 1997). Thus, the composition of fatty acids in the fuel mixture has the potential to affect the maximal lipid oxidation and overall metabolic rates achieved by bats.…”

Yearly growth and metabolic changes in earthen pond-cultured meagre <em>Argyrosomus regius</em>

“…This result points to an increase in lactate oxidation rates by those tissues involved in thermal accommodation that are able to use lactate as fuel (Vargas-Chacoff et al 2009a, b). A very large fraction of the energy stored as glycogen in white muscle can be provided in the form of lactate, which besides being re-converted to glycogen in situ (Schulte et al 1992) can also be sent to oxidative tissues via the bloodstream (Weber 1992).…”

Seasonal dynamics of flight muscle fatty acid binding protein and catabolic enzymes in a migratory shorebird

“…However, H-FABP is only one component of a complex lipid transport and oxidation system, and a full understanding of the biochemical and physiological mechanisms of endurance flight will require more research. For example, enhanced circulatory delivery of FA by very low density lipoprotein or by albumin with high FA binding capacity has been suggested but not fully explored (21,44). The existence and function in birds of sarcolemmal transport proteins, such as FA translocase (FAT/CD36), plasma membrane FABP, and FA transport protein, and the role of enzymes such as long-chain acyl-CoA synthetase in intracellular FA processing remain to be studied.…”

“…endurance exercise; fuel selection; lipid transport; metabolism THE INSTANTANEOUS COST OF flight is high relative to other forms of locomotion; flying birds expend energy at 10 to 15 times basal metabolic rate (BMR), and the minimum cost of flight may be twice the aerobic limit (V O 2 max ) of similarly sized running mammals (4, 38). In the special case of migratory flight, during which this intensity of exercise is maintained for as long as 50 or even 100 h, energy metabolism is almost completely dominated (85-95%) by the oxidation of exogenous fatty acids (FA) delivered to flight muscles from extramuscular adipose tissue (21,23,44). The use of stored fat as a metabolic fuel makes migratory flight possible, yet there currently exists no general mechanistic understanding of how birds achieve the high rates of exogenous FA transport and oxidation required to support such high-intensity endurance exercise.…”

“…The rate of utilization of exogenous FA appears to be most limited by transport across the sarcolemma (31,43). In running mammals, endurance training leads to improved FA utilization, but it is biased toward increased reliance on intramyocyte triaclyglycerol (44,45). Apparently, a major difference between avian and mammalian fuel metabolism is the enhanced capacity for FA uptake by bird muscles, leading to as much as 20-fold higher rates of exogenous FA oxidation in birds.…”