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

The mechanism(s) of absorption of chloride ions (Cl-absorption), as measured by the short-circuit current (I_sc, 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 (V_t, mV) and an I_sc 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 I_sc and V_t. When a K⁺-free plus Ba²⁺-containing solution perfused the lumen, I_sc and V_t were almost completely abolished. Luminal bumetanide (10⁻⁵ M), an inhibitor of Na⁺-K⁺-2Cl⁻ cotransport, reduced I_sc and V_t 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 I_sc and V_t when added luminally. Serosal ouabain (10⁻³ M) abolished I_sc and V_t 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.