Abstract
The cystic fibrosis transmembrane conductance regulator (CFTR) is central to anion secretion in both the possum and eutherian small intestine. Here, we investigated its role in the possum proximal colon, which has novel transport properties compared with the eutherian proximal colon. Despite considerable CFTR expression, high doses of the CFTR activator forskolin (EC50 approximate to 10 mu moll(-1)) were required for a modest, CFTR-dependent increase in short-circuit current (I-sc) in the proximal colon. Presumably, this is because CFTR is restricted to the apical membrane of a small population of CFTR high expresser (CHE) cells in the surface and upper crypt epithelium. Furthermore, although the forskolin-stimulated I-sc was dependent on serosal Na+, Cl- and HCO3-, consistent with anion secretion, inhibition of the basolateral Na-K-2Cl(-) (NKCC1) or Na-HCO3 (pNBCe1) cotransporters did not prevent it. Therefore, although NKCC1 and pNBCe1 are expressed in the colonic epithelium they do not appear to be expressed in CHE cells. At low doses (IC50 approximate to 1 mu moll(-1)), forskolin also decreased the transepithelial conductance (G(T)) of the colon through inhibition of a 4,4 '-diisothiocyano-2,2 '-stilbenedisulphonic acid-sensitive anion conductance in the basolateral membrane of the CHE cells. This conductance is arranged in series with CFTR in the CHE cells and, therefore, the CHE cells provide a transepithelial Cl- conductance for passive Cl- absorption across the epithelium. Inhibition of the basolateral Cl- conductance of the CHE cells by forskolin will inhibit Na+ absorption by restricting the movement of its counter-ion Cl-, assisting in the conversion of the tissue from an absorptive to a secretory state.