The presence of NHE1 and NHE3 Na+-H+ exchangers and an apical cAMP-independent Cl- channel indicate that both absorptive and secretory functions are present in calf gall bladder epithelium

We investigated the transport systems that can sustain Na+ and Cl- movement s across bovine gall bladder epithelium, focusing on the Na+ -H+ exchanger (NHE) family and chloride conductive pathways. Experiments conducted using the fluorescent probe acridine orange (AO) with brush-border membrane vesic les (BBMV) or vesicles obtained from the total epithelium (EMV) demonstrate d the presence of a Na+-H+ exchange in both preparations. The use of specif ic inhibitors indicated the presence of an apical NHE3 exchanger and a NHE1 isoform which should reside in the basolateral membrane. Using reverse tra nscriptase (RT) PCR, we identified cDNA fragments corresponding to the NHE1 , NHE3, Cl--HCO3- (AE2a) transporters and to the CFTR channel. Using the pa tch-clamp technique, we investigated Cl- conductances on cultured epithelia l cells. We found a 5 pS Cl- channel with a voltage-independent open probab ility, insensitive to stilbenes (SITS), Zn2+ and cANW. The results suggest that absorption and secretion coexist in calf gall bladder epithelium. A Na +-H+-Cl--HCO3- double exchange may, at least partially, sustain the absorpt ive function, and a cr apical conductive pathway may be involved in secreti on. The conductance we observed does not seem to be cAMP-regulated, unlike other mammalian gall bladders.