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The Journal of Experimental Biology 205, 2745-2754 (2002)
© 2002 The Company of Biologists Limited

Glycerol and NEFA kinetics in long-term fasting king penguins: phase II versus phase III

S. F. Bernard, C. Fayolle, J.-P. Robin and R. Groscolas^*

Centre d'Ecologie et Physiologie Energétiques, Centre National de la Recherche Scientifique, 67087 Strasbourg, France

^* Author for correspondence (e-mail: rene.groscolas{at}c-strasbourg.fr)

Accepted 20 May 2002

In spontaneously fasting birds such as penguins, below a body mass threshold corresponding to the phase II—phase III transition, a metabolic and hormonal shift occurs and feeding behaviour is stimulated (`refeeding signal'). The major aim of this study was to determine whether a decrease in non-esterified fatty acid (NEFA) release from adipose tissue could be a component of this signal. Lipolytic fluxes and primary triacylglycerol:fatty acid (TAG:FA) cycling were determined in vivo in breeding, fasting king penguins (Aptenodytes patagonicus) using continuous infusions of 2-[³H]glycerol and 1-[¹⁴C]palmitate under field conditions. In phase II (after approximately 8 days of fasting, large fat stores, body protein spared, N=8), the rate of appearance (R_a) of glycerol and of NEFA were 5.7±0.8 and 10.5±0.4 µmol kg^-1 min^-1, respectively, and the percentage of primary TAG:FA cycling was 41±7%. In phase III (after approximately 25 days of fasting, fat stores reduced by fourfold, increased body protein catabolism, N=9), R_a glycerol kg^-1 body mass remained unchanged, whereas R_a glycerol kg^-1 fat mass and R_a NEFA kg^-1 body mass were increased by 2.8-fold and 1.5-fold, respectively. Increased R_a glycerol kg^-1 fat mass was possibly the result of a 3.5-fold increase in circulating glucagon, the increased R_a NEFA kg^-1 body mass being attributable to decreased primary TAG:FA cycling. Thus, triggering of the refeeding signal that redirects the behavior of fasting, incubating penguins from incubation towards the search for food after entrance into phase III cannot be ascribed to a reduction in lipolytic fluxes and NEFA availability.

Key words: lipolysis, triacylglycerol:fatty acid cycling, fat stores, refeeding signal, bird, king penguin, Aptenodytes patagonicus

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