cAMP-mediated regulation of murine intestinal/pancreatic Na+/HCO3- cotransporter subtype pNBC1

Basolateral Na+-HCO3- cotransport is essential for intestinal anion secretion, and indirect evidence suggests that it may be stimulated by a rise of intracellular cAMP. We therefore investigated the expression, activity, and regulation by cAMP of the Na+-HCO3- cotransporter isoforms NBC1 and NBCn1 in isolated murine colonic crypts. Na+-HCO3- transport rates were measured fluorometrically in BCECF-loaded crypts, and mRNA expression levels and localization were determined by semiquantitative PCR and in situ hybridization. Acid-activated Na+-HCO3- cotransport rates were 5.07 +/- 0.7 mM/min and increased by 62% after forskolin stimulation. NBC1 mRNA was more abundant in colonic crypts than in surface cells, and crypts expressed far more NBC1 than NBCn1. To investigate whether the cAMP-induced Na+-HCO3- cotransport activation was secondary to secretion- associated changes in HCO3- or cell volume, we measured potential forskolin-induced changes in intracellular pH and assessed Na+-HCO3- transport activity in CFTR -/- crypts (in which no forskolin-induced cell shrinkage occurs). We found 30% reduced Na+-HCO3- transport rates in CFTR -/- compared with CFTR +/+ crypts but similar Na+-HCO3- cotransport activation by forskolin. These studies establish the existence of an intracellular HCO3- concentration- and cell volume-independent activation of colonic NBC by an increase in intracellular cAMP.