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Substrate utilization during graded aerobic exercise in rainbow trout
1 Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S
4K1
2 Department of Medicine, McMaster University, Hamilton, Ontario, Canada L8S
4K1
* Author for correspondence (e-mail: richarjg{at}mcmail.cis.mcmaster.ca )
Accepted 18 April 2002
A biochemical approach was employed to examine the oxidative utilization of
carbohydrate and lipid in red muscle of rainbow trout (Oncorhynchus
mykiss) during sustained swimming at 30 and 60% of their critical
swimming speed (Ucrit; for 2, 15 and 240 min) and during
non-sustainable swimming at 90% Ucrit (for 2, 15 and 45
min). Measurements included pyruvate dehydrogenase (PDH) activity, creatine
phosphate, ATP, glycogen, glycolytic intermediates, acetyl-CoA, acetyl-,
total-, free-, short-chain fatty acyl- and long-chain fatty acyl- carnitine,
intramuscular triacylglycerol and malonyl-CoA concentrations, and whole body
oxygen consumption
(
O2). During the
first 2 min at 30 and 60% Ucrit, oxidation of endogenous
glycogen by PDH activation increased 4- and 8-fold, respectively, yielding
1.5- to 2.5-fold increases in acetyl-CoA and 2- to 6-fold increases in
acetyl-carnitine concentrations. Within 15 min, PDH activity returned to
control values (153.9±30.1 nmol g-1 wet tissue
min-1); after 240 min there were small 1.7- to 2.6-fold increases
in long-chain fatty acyl-carnitine and approx. 50% decreases in malonyl-CoA
concentrations, indicating an overall enhancement of lipid oxidation.
Sustainable swimming at 30 and 60% Ucrit was further
characterized by 1.5- and 2.2-fold increases in
O2,
respectively. Non-sustainable swimming at 90% Ucrit was
characterized by a sustained tenfold (approx.) elevation of red muscle PDH
activity (approx. 1600 nmol g-1 wet tissue min-1).
Significant 67% decreases in white muscle creatine phosphate and 73% decreases
in glycogen levels, without matching increases in lactate levels, point to
significant recruitment of white muscle during high-speed swimming for power
production, and the potential export of white muscle lactate to red muscle for
oxidation. Overall, sustainable exercise at 30 and 60%
Ucrit is supported by approximately equal contributions of
carbohydrate (approx. 45%) and lipid (approx. 35%) oxidation, whereas
non-sustainable swimming is supported primarily by carbohydrate oxidation with
only moderate contributions from lipid oxidation.
Key words: Swimming, red muscle, white muscle, pyruvate dehydrogenase, lipid, carbohydrate, lactate shuttling, malonyl-CoA, rainbow trout, Oncorhynchus mykiss
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