BICARBONATE ABSORPTION IN EEL INTESTINE - EVIDENCE FOR THE PRESENCE OF MEMBRANE-BOUND CARBONIC-ANHYDRASE ON THE BRUSH-BORDER MEMBRANES OF THE ENTEROCYTE

Bicarbonate absorptive fluxes through the isolated intestine of the Eu ropean eel (Anguilla anguilla) were evaluated by the pH-stat method un der short-circuited conditions. It was found that bicarbonate absorpti ve flux was dependent on the luminal Na+ and was inhibited by luminal 4-acetamido-4' stilbene-2-2' disulfonic acid (SITS; 2.5 x 10(-4) M) an d luminal acetazolamide (10(-4) M), while luminal amiloride (1 mM) was without effect. Furthermore, by using brush border membrane vesicles (BBMV) isolated from eel intestine, the existence of two carbonic anhy drase (CA) isoforms, one tightly associated to the brush border membra ne (BBM) and the other soluble in the cytosol, was demonstrated. The m embrane-bound CA differs from the cytoplasmic isoform in that 1) it is relatively resistant to treatment with 0.045% lauryl sulfate sodium s alt (SDS); 2) it is less inhibitable by ethoxzolamide and sulfanilamid e; and 3) its Km(app) is significantly lower than that of the cytoplas mic isoform. These results suggest that a BBM-bound CA isozyme would p lay an important role in bicarbonate absorption from the lumen, facili tating the HC0(3)(-) transfer through the luminal membrane of the eel enterocyte most likely via a Na+ (HCO3-) or (OH-) cotransport system. (C) 1996 Wiley-Liss, Inc.