Comparison of Cl−‐absorption in the intestine of the seawater‐ and freshwater‐adapted eel, Anguilla anguilla: Evidence for the presence of an Na‐K‐Cl cotransport system on the luminal membrane of the enterocyte

Abstract The mechanism(s) of absorption of chloride ions (Cl‐absorption), as measured by the short‐circuit current (Isc, μA·cm−2), was investigated in the intestinal middle part of the seawater‐(SW) and the freshwater‐(FW) adapted eel. Tissues, mounted in an Ussing chamber and continuously perfused by gravity flow on both sides with 20 mM HCO3− ‐ 1% CO2 Ringer's solution at pH 8.0, developed a serosa‐negative transepithelial voltage (Vt, mV) and an Isc that were both lower in FW‐adapted than in SW‐adapted eels. Bilateral removal of Na+ or Cl− from the bathing solutions reduced both electrical variables to zero. Removal of K+ from or addition of Ba2+ (1 mM) to the luminal bath markedly decreased Isc and Vt. When a K+‐free plus Ba2+‐containing solution perfused the lumen, Isc and Vt were almost completely abolished. Luminal bumetanide (10−5 M), an inhibitor of Na+‐K+‐2Cl− cotransport, reduced Isc and Vt by about 90% in a few minutes. Amiloride (10−3 M) and DIDS (4,4′‐diisothiocyanostilbene‐2,2‐disulphonic acid; 10−4 M), inhibitors of Na+/H+ and Cl−/HCO3− exchangers, and hydrochlorothiazide (10−4 M), an inhibitor of the Na‐Cl symport, had no effect on Isc and Vt when added luminally. Serosal ouabain (10−3 M) abolished Isc and Vt in about 30 min. All the above experimental manoeuvres led to similar results in both SW‐adapted and FW‐adapted eels. The data suggest that in the intestine of both SW and FW eels, Cl−‐absorption is mediated by a luminally located neutral Na+‐K+‐2Cl− cotransport system. The basolaterally located Na+, K+‐ATPase, by generating an inwardly directed electrochemical gradient for Na+, provides the driving force for the active accumulation of Cl− by the electroneutral cotransporter. The K+ accumulated is refluxed by a Ba2+‐inhibitable luminal K+ conductance. © 1992 Wiley‐Liss, Inc.