Differential expression of absorptive intestinal and renal tissues of the cation-chloride-cotransporters in 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, NKCC2 alpha isoform mRNA expression was restricted to renal tissues, whereas NKCC2 beta isoform expression was present in intestine and urinary bladder. Similar to mammalian NCC, NCC alpha mRNA expression was also found in the kidney, whereas, expression of NCC beta 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 NKCC2 alpha did not change whereas NCCa 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 NCC alpha cotransporter activity rather than the result of suppression of NKCC alpha cotransporter activity. Intestinal mRNA expression of NKCC2 beta generally increased following SW acclimation, although maximal increases occurred later in yellow (7 days) than silver (2 days) eels. Average levels of NKCC2 beta 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 NCC beta was only found in the posterior intestine/rectum. Together these results suggest intestinal sodium chloride absorption may switch from occurring via NKCC beta to NCC beta as imbibed fluid travels down the intestine and the concentration of luminal potassium decreases. (C) 2007 Elsevier Inc. All rights reserved.