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Title: Metabolic organization of freshwater, euryhaline, and marine elasmobranchs: Implications for the evolution of energy metabolism in sharks and rays
Authors: Speers-Roesch, B.
Ip, Y.K. 
Ballantyne, J.S.
Keywords: Chiloscyllium punctatum
Himantura signifer
Intermediary metabolism
Ketone body
Non-esterified fatty acid
Potamotrygon motoro
Taeniura lymma
Issue Date: Jul-2006
Citation: Speers-Roesch, B., Ip, Y.K., Ballantyne, J.S. (2006-07). Metabolic organization of freshwater, euryhaline, and marine elasmobranchs: Implications for the evolution of energy metabolism in sharks and rays. Journal of Experimental Biology 209 (13) : 2495-2508. ScholarBank@NUS Repository.
Abstract: To test the hypothesis that the preference for ketone bodies rather than lipids as oxidative fuel in elasmobranchs evolved in response to the appearance of urea-based osmoregulation, we measured total non-esterified fatty acids (NEFA) in plasma as well as maximal activities of enzymes of intermediary metabolism in tissues from marine and freshwater elasmobranchs, including: the river stingray Potamotrygon motoro (300 mmol l-1 plasma urea); and the euryhaline freshwater stingray Himantura signifer, which possesses intermediate levels of urea. H. signifer also were acclimated to half-strength seawater (15‰) for 2 weeks to ascertain the metabolic effects of the higher urea level that results from salinity acclimation. Our results do not support the urea hypothesis. Enzyme activities and plasma NEFA in salinity-challenged H. signifer were largely unchanged from the freshwater controls, and the freshwater elasmobranchs did not show an enhanced capacity for extrahepatic lipid oxidation relative to the marine species. Importantly, and contrary to previous studies, extrahepatic lipid oxidation does occur in elasmobranchs, based on high carnitine palmitoyl transferase (CPT) activities in kidney and rectal gland. Heart CPT in the stingrays was detectable but low, indicating some capacity for lipid oxidation. CPT was undetectable in red muscle, and almost undetectable in heart, from C. punctatum as well as in white muscle from T. lymma. We propose a revised model of tissue-specific lipid oxidation in elasmobranchs, with high levels in liver, kidney and rectal gland, low or undetectable levels in heart, and none in red or white muscle. Plasma NEFA levels were low in all species, as previously noted in elasmobranchs. D-β-hydroxybutyrate dehydrogenase (D-β-HBDH) was high in most tissues confirming the importance of ketone bodies in elasmobranchs. However, very low D-β-HBDH in kidney from T. lymma indicates that interspecific variability in ketone body utilization occurs. A negative relationship was observed across species between liver glutamate dehydrogenase activity and tissue or plasma urea levels, suggesting that glutamate is preferentially deaminated in freshwater elasmobranchs because it does not need to be shunted to urea production as in marine elasmobranchs.
Source Title: Journal of Experimental Biology
ISSN: 00220949
DOI: 10.1242/jeb.02294
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