Electroneutral cation-Cl- cotransporters NKCC2ß and NCCß expressed in the intestinal tract of Japanese eel Anguilla japonica.

In the present study, we aimed to elucidate the mechanisms of intestinal Na(+) and Cl(-) absorption in Japanese eel, focusing on electroneutral cation-Cl(-) cotransporters, NKCC2ß and NCCß, expressed in the intestinal tract. First, we cloned cDNAs encoding NKCC2ß and NCCß from the intestinal tract of Japanese eel. In both freshwater- and seawater-acclimated eels, quantitative PCR analysis showed that NKCC2ß was predominantly expressed in the anterior and posterior intestines, and that NCCß expression was specifically high in the rectum. According to immunohistochemistry with anti-eel NKCC2ß (reacting with NKCC2ß but not with NCCß) and T4 antibody (reacting with both NKCC2ß and NCCß), NKCC2ß was localized in the apical surface of the epithelial cells in the anterior and posterior intestines, whereas NCCß was likely to be distributed to that in the rectum. Furthermore, a specific NCC inhibitor, hydrochlorothiazide, inhibited of Na(+) and Cl(-) absorption, as well as water absorption, in the rectal sac preparations from seawater eel, indicating the involvement of NCCß in ion absorption in the rectum. Our findings indicate that NKCC2ß expressed in the anterior and posterior intestines and NCCß in the rectum are importantly involved in ion absorption to reduce osmolality of ingested seawater prior to water absorption in seawater-acclimated eel.

Rectal water absorption in seawater-adapted Japanese eel Anguilla japonica.

Marine teleosts drink large amounts of seawater to compensate for continuous osmotic water loss. We investigated a possible significant role of the rectum in water absorption in seawater-adapted eel. In rectal sacs filled with balanced salt solution (BSS) and incubated in isotonic BSS, water absorption was greater in seawater-adapted eel than in freshwater eel. Since rectal fluid osmolality was slightly lower than plasma osmolality in seawater-adapted eel, effects of rectal fluid osmolality on water absorption were examined in rectal sacs filled with artificial rectal fluid with different osmolality. Rectal water absorption was greater at lower rectal fluid osmolality, suggesting that an osmotic gradient between the blood and rectal fluid drives the water movement. Ouabain, a specific inhibitor of Na+/K(+)-ATPase, inhibited water absorption in rectal sacs, indicating that an osmotic gradient favorable to rectal water absorption was created by ion uptake driven by Na+/K(+)-ATPase. Expression levels of aquaporin 1 (AQP1), a water-selective channel, were significantly higher in the rectum than in the anterior and posterior intestines. Immunoreaction for Na+/K(+)-ATPase was detected in the mucosal epithelial cells in the rectum with more intense staining in the basal half than in the apical half, whereas AQP1 was located in the apical membrane of Na+/K(+)-ATPase-immunoreactive epithelial cells. The rectum is spatially separated from the posterior intestine by a valve structure and from the anus by a sphincter. Such structures allow the rectum to swell as intestinal fluid flows into it, and a concomitant increase in hydrostatic pressure may provide an additional force for rectal water absorption. Our findings indicate that the rectum contributes greatly to high efficiency of intestinal water absorption by simultaneous absorption of ions and water.