Cotransport in Normal and Cystic Fibrosis Intestine

In a search for the HCO3 supply mechanisms to the enterocyte we cloned and sequenced an intestinal subtype of the NaHCO3 cotransporter isoform I (dNBC1), which turned out to be identical to the pancreatic NBC1 subtype (pNBC1). Within the intestine, we found particularly high NBC1 expression levels in the duodenum and proximal colon. Experiments with stripped rabbit duodenum in Ussing-chambers revealed that NaHCO3 cotransport (NBC) and CO2 hydration/Na/H exchange were equally important duodenal HCO3 supply pathways and were both upregulated during cAMP-mediated secretion. In the proximal colon, however, HCO3 secretion was low but NBC1 expression even higher than in the duodenum. Ussing-chamber experiments with an NBC-specific inhibitor revealed that NBC, coupled to basolateral Cl /HCO3 exchange, was an important alternative Cl supply pathway to NaK2Cl cotransport (NKCC) during cAMP-stimulated colonic Cl secretion. To investigate the functional integrity of anion uptake pathways in the absence of cystic fibrosis transmembrane conductance regulator (CFTR), we fluorometrically assessed NBC and NKCC transport rates and cell volume before and during forskolin-stimulation in isolated colonic crypts from normal and CFTR (-/-) mice. Although forskolin stimulation decreased cell volume only in normal, not in CFTR (-/-) crypts, it activated NBC and NKCC to a similar degree in both normal and CFTR (-/-) crypts. We conclude that, depending on the intestinal segment, NBC1 plays an important role in basolateral HCO3 or Cl uptake. Expression and activation by cAMP is preserved in CFTR (-/-) intestine.