Differential Expression of Absorptive Cation-Chloride Cotransporters in the Intestinal and Renal Tissues of the European Eel (Anguilla anguilla)

Duplicate pairs of isoforms of each of the NKCC2 and the NCC absorptive cation-chloride-cotransporters have been isolated from the European eel. As with mammalian NKCC2, NKCC2α isoform mRNA expression was restricted to renal tissues, whereas NKCC2β isoform expression was present in intestine and urinary bladder. Similar to mammalian NCC, NCCα mRNA expression was also found in the kidney, whereas, expression of NCCβ mRNA was found at low levels in a number of tissues but particularly in intestine. Following 3 weeks of transfer of yellow or silver (adult life stages) eels from freshwater (FW) to seawater (SW), renal mRNA expression of NKCC2α did not change whereas NCCα expression was reduced although only significantly in silver eels. This suggests that any changes in renal sodium chloride re-absorption in SW-acclimated fish may be due to decreased NCCα cotransporter activity rather than the result of suppression of NKCCα cotransporter activity. Intestinal mRNA expression of NKCC2β generally increased following SW acclimation, although maximal increases occurred later in yellow (7 days) than silver (2 days) eels. Average levels of NKCC2β mRNA abundance in the middle intestine were 89% of those in the anterior, and this was reduced to 44% (of the level in the anterior intestine) in posterior intestine/rectum. Expression of NCCβ was only found in the posterior intestine/rectum. Together these results suggest intestinal sodium chloride absorption may switch from occurring via NKCCβ to NCCβ as imbibed fluid travels down the intestine and the concentration of luminal potassium decreases.