CFTR drives Na+- n HCO 3 − cotransport in pancreatic duct cells: a basis for defective HCO 3 − secretion in CF

Pancreatic dysfunction in patients with cystic fibrosis (CF) is felt to result primarily from impairment of ductal[Formula: see text] secretion. We provide molecular evidence for the expression of NBC-1, an electrogenic Na+-[Formula: see text]cotransporter (NBC) in cultured human pancreatic duct cells exhibiting physiological features prototypical of CF duct fragments (CFPAC-1 cells) or normal duct fragments [CAPAN-1 cells and CFPAC-1 cells transfected with wild-type CF transmembrane conductance regulator (CFTR)]. We further demonstrate that 1)[Formula: see text] uptake across the basolateral membranes of pancreatic duct cells is mediated via NBC and 2) cAMP potentiates NBC activity through activation of CFTR-mediated Cl− secretion. We propose that the defect in agonist-stimulated ductal[Formula: see text] secretion in patients with CF is predominantly due to decreased NBC-driven[Formula: see text] entry at the basolateral membrane, secondary to the lack of sufficient electrogenic driving force in the absence of functional CFTR.

Localization of Cl- conductance in normal and Cl- impermeability in cystic fibrosis sweat duct epithelium

We studied the Cl- permeability properties of apical and basolateral membranes of human reabsorptive sweat duct (RSD) from normal and cystic fibrosis (CF) subjects. In normal ducts, Cl- substitution by impermeant anion gluconate in the lumen increased the voltage divider ratio (VDR) from 4.8 +/- 0.9 to 7.0 +/- 1.1 (n = 8, P less than 0.05), whereas Cl- substitution in the contraluminal bath decreased the VDR from 3.2 +/- 0.7 to 1.9 +/- 0.4 (n = 7, P less than 0.05). These results are consistent with a significant Cl- permeability in both apical and basolateral membranes of normal ducts. Amiloride (10(-4) M) in the lumen of normal ducts resulted in a small increase in VDR from 4.2 +/- 0.6 to 5.0 +/- 0.8 (n = 10, P less than 0.05), whereas the current-induced basolateral membrane voltage deflections (delta Vb) increased from 6.9 +/- 1.3 to 7.7 +/- 1.2 mV, suggesting that inhibition of Na+ permeability decreased basolateral membrane Cl- permeability. In the absence of luminal Cl-, amiloride decreased delta Vb and induced much greater effect on VDR (from 5.2 +/- 1.1 to 10.8 +/- 2.3, n = 9, P less than 0.05) than in the presence of Cl-. Likewise, in the presence of amiloride, Cl- substitution in the lumen had greater effect on VDR (increased from 3.5 +/- 0.5 0.5 to 10.0 +/- 1.5, n = 15, P less than 0.05) than in the absence of amiloride. These results indicate that Na+ conductance in the apical membrane of the normal duct is significantly smaller than Cl- conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered electrical potential profile of human reabsorptive sweat duct cells in cystic fibrosis

The electrophysiological properties of reabsorptive sweat duct (RSD) cells from normal and cystic fibrosis (CF) subjects were studied using intracellular microelectrodes. The apical membrane potential (Va) of CF duct cells was reversed in "polarity" (+28.0 +/- 2.4 mV, n = 46) compared with normal duct cells (-24.9 +/- 0.4 mV, n = 145), and the basolateral membrane potential (Vb) of CF cells was hyperpolarized significantly (-50.1 +/- 1.2 mV, n = 46) in comparison to normal cells (-34.6 +/- 0.4 mV, n = 145). The substitution of the impermeant anion gluconate for Cl- in the lumen of the normal duct depolarized Va from -24.9 +/- 1.1 to 8.9 +/- 3.1 mV (n = 18) and hyperpolarized Vb from -34.3 +/- 1.1 to -55.6 +/- 3.7 mV (n = 18), which mimicked the cell electrical potential profile of CF ducts even in the presence of Cl-. Cl- substitution in the bath depolarized Vb of normal ducts by 22.5 +/- 2.6 mV (n = 24), while hyperpolarizing Va by -3.4 +/- 1.6 mV (n = 24). The response of the electrical profiles of CF cells to Cl- substitution in either the lumen or the bath was significantly reduced compared with normal cells. The effect of the Na+ conductance blocker amiloride (10(-4) M) on Vb was not significantly different in CF (delta Vb = -26.4 +/- 3.8 mV, n = 9) vs. normal (delta Vb = -27.6 +/- 2.5 mV, n = 30) cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of abnormal Cl- impermeability on sweating in cystic fibrosis

Parameters of electrolyte transport in single sweat glands in normal subjects and cystic fibrosis (CF) patients were monitored and compared. Results indicate that in both normal and CF sweat ducts, Na+ is reabsorbed by an active process in which Cl- follows passively while K+ is secreted. However, while net NaCl reabsorption is markedly lower, the electrical potential associated with sweat emerging from the sweat duct is significantly more negative in CF than in normal subjects. Comparison of the differences in apparent electrochemical potential experienced by ions in the sweat duct during secretion indicates that Na+ is held out of the lumen of both groups of ducts against a large but similar gradient, but that Cl- is held in the CF duct against a much larger gradient than in the normal duct. These results indicate that the mechanism for Na+ reabsorption is not inhibited in the CF duct, but that the decreased NaCl transport in the defective duct is due to an abnormally low permeability to Cl-. Analysis of the electrical potential as a function of the Cl- gradient in the sweat suggests that the normal and defective route of Cl- uptake may be transcellular.