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First published online October 19, 2007
Journal of Experimental Biology 210, 3749-3756 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.008763

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An examination of the metabolic processes underpinning critical swimming in Atlantic cod (Gadus morhua L.) using in vivo ³¹P-NMR spectroscopy

Glenn J. Lurman^*, Christian H. Bock and Hans-O. Pörtner

Alfred Wegener Institut fuer Polar und Meeresforschung, Am Handelshafen 12, 27570, Bremerhaven, Germany

^* Author for correspondence (e-mail: Glenn.Lurman{at}awi.de)

Accepted 23 August 2007

Traditionally, critical swimming speed has been defined as the speed when a fish can no longer propel itself forward, and is exhausted. To gain a better understanding of the metabolic processes at work during a U_crit swim test, and that lead to fatigue, we developed a method using in vivo ³¹P-NMR spectroscopy in combination with a Brett-type swim tunnel. Our data showed that a metabolic transition point is reached when the fish change from using steady state aerobic metabolism to non-steady state anaerobic metabolism, as indicated by a significant increase in inorganic phosphate levels from 0.3±0.3 to 9.5±3.4 mol g^–1, and a drop in intracellular pH from 7.48±0.03 to 6.81±0.05 in muscle. This coincides with the point when the fish change gait from subcarangiform swimming to kick-and-glide bursts. As the number of kicks increased, so too did the Pi concentration, and the pH_i dropped. Both changes were maximal at U_crit. A significant drop in Gibbs free energy change of ATP hydrolysis from –55.6±1.4 to –49.8±0.7 kJ mol^–1 is argued to have been involved in fatigue. This confirms earlier findings that the traditional definition of U_crit, unlike other critical points that are typically marked by a transition from aerobic to anaerobic metabolism, is the point of complete exhaustion of both aerobic and anaerobic resources.

Key words: Atlantic cod, Gadus morhua, critical swimming speed, in vivo ³¹P-NMR spectroscopy, high-energy phosphates, Gibb's free energy, intracellular pH

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