Determining the functional role of waterborne amino acid uptake in hagfish nutrition: a constitutive pathway when fasting or a supplementary pathway when feeding?

Hagfish are unique among aquatic "vertebrates" in their ability to absorb amino acids directly from the water via skin and gill epithelia, but it is unknown whether this phenomenon extends beyond a few studied substrates; what effect fed state has on absorption; and what functional role this may play in hagfish nutrition. Using in vivo and in vitro transport assays, uptake and tissue distribution of the waterborne amino acids L-alanine, L-lysine, and L-phenylalanine were examined as a function of fed state. All three amino acids were shown to be taken up from the water (lysine and phenylalanine for the first time). Following immersion in radiolabelled solutions for 24 h, phenylalanine was the amino acid that accumulated at the highest levels in almost all tissues, with the highest accumulation noted in red blood cells and bile, followed by gill and liver. In general, tissues of fed hagfish displayed a significantly reduced phenylalanine accumulation compared to tissues of hagfish fasted for 3 weeks. An in vitro assay showed that phenylalanine was transported across the skin at the highest rate, with the uptake of lysine occurring at the lowest rate. Feeding status had no significant effect on in vitro transport. These data indicate that dissolved organic nutrients are a significant source of nutrition to hagfish, and may be relatively more important during periods of fasting than during periods of feeding when immersed in decaying carcasses.