Along with other nutrients, animals need trace metals to function. Iron, the oxygen-binding component in the blood pigment hemoglobin, is a critical trace metal and is usually absorbed through the gut. Mammals can alter iron metabolism in order to meet oxygen demand during periods of low environmental oxygen – hypoxia. However, it was unknown whether hagfish – a much more primitive group of animals that exhibit unusual tolerance to hypoxia – change their iron dynamics to achieve such resilience and whether they could take up iron from their environment. Knowing that hagfish can absorb some nutrients through their skin, Chris Glover, of Athabasca University, Canada, and colleagues from several other Canadian institutions set out to determine whether hagfish can take up iron across the skin; if so, how it might compare with gut uptake; and whether changes in iron transport during hypoxia contribute to the impressive hypoxia tolerance of these hardy fishes.
To begin to answer their questions, the researchers measured iron transport across the skin and gut tissue of Pacific hagfish. Put simply, they exposed the external surface of each tissue to a solution containing radioactive iron at a range of concentrations and measured how much radioactivity entered a saline solution on the other side of the tissue after 2 h. They also measured how much radioactivity remained within the tissue itself.
Glover and his colleagues found that the hagfish does take up iron through the skin – and at a higher rate than in the gut. Also, over all but the highest iron concentrations, the uptake in both tissue types initially increased with concentration and then reached a plateau. In addition, most iron absorbed by the skin remained in the skin, while iron absorbed by the gut tended to move across the tissue. The initial increase and subsequent uptake plateau indicate that specific transporter molecules are responsible for moving the iron in both tissues (as these molecules at some point reach a maximum transport capacity). However, the discrepancy in transport rates and in the iron's final destination suggests that each tissue handles iron differently.
To find out whether hagfish alter iron transport during hypoxia – when the oxygen-binding metal is particularly essential – the researchers again measured iron transport, but this time on skin and gut from hagfish exposed to hypoxia for 24 h. They also analyzed the oxygen content of the fish blood, the red blood cell count and volume, the proportion of the blood made up of red blood cells and the blood hemoglobin content to determine whether hypoxia increases iron demand in hagfish as it does in mammals.
Unexpectedly, hypoxia exposure did not significantly change iron transport in either the fish's skin or gut. Also, unlike in mammals, hypoxia did not appear to increase iron demand in the blood. These results suggest that changes in iron transport do not contribute to the hagfish's outstanding hypoxia tolerance and that the relative lack of change in blood parameters may preclude the need for any changes in iron transport in the first place.
This study is the first to characterize trace metal transport in hagfish skin or gut and to identify iron transport in hagfish skin. It also paves the way for future research by raising more questions: what is the nature of the transporter; what happens to the iron in hagfish skin; and is skin iron transport ancient or adaptive? Clearly, these primitive hypoxia heroes are not ready to divulge all their secrets just yet; they've got them clenched in an iron fist – er, fin.