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First published online December 1, 2006
Journal of Experimental Biology 209, 4885-4894 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02597

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Postprandial changes in plasma free amino acid levels obtained simultaneously from the hepatic portal vein and the dorsal aorta in rainbow trout (Oncorhynchus mykiss)

Anders Karlsson^1,2, Erika J. Eliason³, Liv Torunn Mydland¹, Anthony P. Farrell³ and Anders Kiessling^1,2,*

¹ Aquaculture Protein Centre (APC), Centre of Excellence, Norwegian University of Life Sciences, PO Box 5003, N-1432 Aas, Norway
² Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, PO Box 5003, N-1432 Aas, Norway
³ Department of Zoology and Faculty of Land and Food Systems, University of British Columbia, BC, Vancouver, V6T 1Z4, Canada

^* Author for correspondence at address 2 (e-mail: anders.kiessling{at}umb.no)

Accepted 16 October 2006

For the first time, changes in plasma concentrations of free amino acid (AA) and their metabolites were followed simultaneously in pre- and post-hepatic blood following a single meal in non-anaesthetized and free-swimming fish. Rainbow trout (Oncorhynchus mykiss), kept in 10°C water and fitted with cannulae in the hepatic portal vein (HPV) and the dorsal aorta (DA), were force-fed 1% of their body mass and blood samples were taken from both cannulae at 0, 3, 6, 12, 24 and 48 h postprandially to follow the free AA profile. Almost all free AAs increased rapidly within the first 3 h and only a few free AAs did not change significantly over time. By 6 h, the total free AA concentration had peaked in blood taken from both the DA (7107±369 nmol ml^-1) and HPV (9999±572 nmol ml^-1). However, individual free AAs showed three main profiles beyond this time: for type I, a peak concentration occurred only at 6 h; for type II, there was a more gradual rise in concentration to a peak at 24 h; and for type III there were two peaks, at 6 h and 24 h. All free AAs returned to or were lower than baseline levels within 48 h, with the exception of threonine and proline. The total free AA concentrations were consistently higher (P<0.05) in the HPV than in the DA at 3 h, 6 h, 12 h and 24 h. Our data provide clear evidence that, during the first pass through the liver, hepatic modification altered individual free AA concentrations as indicated by variable ratios among the simultaneous blood samples. Furthermore, the elevation of ammonium and urea in the HPV indicates intestinal catabolism of ingested free AA before release into the HPV. In conclusion, the dual HPV and DA cannulation shows promise as a useful technique for qualitative and quantitative investigations of absorption and turnover of nutrients, especially if the measurements can be combined with reliable estimates of blood flow and labelled substances.

Key words: Hepatic portal vein cannulation, plasma free amino acids, urea, ammonium, digestibility, protein metabolism, rainbow trout, Oncorhynchus mykiss