Ducts isolated from the pancreas of CFTR-null mice secrete fluid.

The pancreatic pathology in cystic fibrosis (CF) is normally attributed to the failure of ductal fluid secretion resulting from the lack of functional CF transmembrane conductance regulator (CFTR). However, murine models of CF show little or no pancreatic pathology. To resolve this dichotomy we analysed the transport mechanisms involved in fluid and electrolyte secretion by pancreatic ducts isolated from CFTR-null mice. Experiments were performed on cultured interlobular duct segments isolated from the pancreas of the Cftr(tm1Cam) strain of CFTR-null mouse. Fluid secretion to the closed luminal space was measured by video microscopy. The secretory response of ducts isolated from CF mice to cAMP-elevating agonists forskolin and secretin was significantly reduced compared with wild type but not abolished. The Cl(-)- and HCO(3) (-) -dependent components of the ductal secretion were affected equally by the absence of CFTR. The secretory response to carbachol stimulation was unaltered in CF ducts. Loading the ductal cells with the Ca2+ chelator BAPTA completely abolished carbachol-evoked secretion, but did not affect forskolin-evoked secretion in CF or wild-type ducts. We conclude that pancreatic duct cells from CF mice can secrete a significant amount of water and electrolytes by a cAMP-stimulated mechanism that is independent of CFTR and cannot be ascribed to the activation of calcium-activated chloride channels.

Basolateral anion transport mechanisms underlying fluid secretion by mouse, rat and guinea-pig pancreatic ducts.

Fluid secretion by interlobular pancreatic ducts was determined by using video microscopy to measure the rate of swelling of isolated duct segments that had sealed following overnight culture. The aim was to compare the HCO(3)(-) requirement for secretin-evoked secretion in mouse, rat and guinea-pig pancreas. In mouse and rat ducts, fluid secretion could be evoked by 10 nm secretin and 5 microm forskolin in the absence of extracellular HCO(3)(-). In guinea-pig ducts, however, fluid secretion was totally dependent on HCO(3)(-). Forskolin-stimulated fluid secretion by mouse and rat ducts in the absence of HCO(3)(-) was dependent on extracellular Cl(-) and was completely inhibited by bumetanide (30 microm). It was therefore probably mediated by a basolateral Na(+)-K(+)-2Cl(-) cotransporter. In the presence of HCO(3)(-), forskolin-stimulated fluid secretion was reduced approximately 40% by bumetanide, approximately 50% by inhibitors of basolateral HCO(3)(-) uptake (3 microm EIPA and 500 microm H(2)DIDS), and was totally abolished by simultaneous application of all three inhibitors. We conclude that the driving force for secretin-evoked fluid secretion by mouse and rat ducts is provided by parallel basolateral mechanisms: Na(+)-H(+) exchange and Na(+)-HCO(3)(-) cotransport mediating HCO(3)(-) uptake, and Na(+)-K(+)-2Cl(-) cotransport mediating Cl(-) uptake. The absence or inactivity of the Cl(-) uptake pathway in the guinea-pig pancreatic ducts may help to account for the much higher concentrations of HCO(3)(-) secreted in this species.