Hyposecretion, Not Hyperabsorption, Is the Basic Defect of Cystic Fibrosis Airway Glands

Abstract

Human airways and glands express the anion channel cystic fibrosis transmembrane conductance regulator, CFTR, and the epithelial Na ؉ channel, ENaC. Cystic fibrosis (CF) airway glands fail to secrete mucus in response to vasoactive intestinal peptide or forskolin; the failure was attributed to loss of CFTR-mediated anion and fluid secretion. Alternatively, CF glands might secrete acinar fluid via CFTR-independent pathways, but the exit of mucus from the glands could be blocked by hyperabsorption of fluid in the gland ducts. This could occur because CFTR loss can disinhibit ENaC, and ENaC activity can drive absorption. To test these two hypotheses, we measured single gland mucus secretion optically and applied ENaC inhibitors to determine whether they augmented secretion. Human CF glands were pretreated with benzamil and then stimulated with forskolin in the continued presence of benzamil. Benzamil did not rescue the lack of secretion to forskolin (50 glands, 6 CF subjects) nor did it increase the rate of cholinergically mediated mucus secretion from CF glands. Finally, neither benzamil nor amiloride increased forskolin-stimulated mucus secretion from porcine submucosal glands (75 glands, 7 pigs). One possible explanation for these results is that ENaC within the gland ducts was not active in our experiments. Consistent with that possibility, we discovered that human airway glands express Kunitz-type and nonKunitz serine protease inhibitors, which might prevent proteolytic activation of ENaC. Our results suggest that CF glands do not display excessive, ENaC-mediated fluid absorption, leaving defective, anion-mediated fluid secretion as the most likely mechanism for defective mucus secretion from CF glands.

Human cystic fibrosis airway epithelia have reduced Cl − conductance but not increased Na + conductance

“…Thus, our data correspond to studies of newborn CF pigs done at a time when they manifest a defect in host defense against bacteria but not secondary manifestations of the disease (21,22). Our results also are consistent with studies of CF sweat gland ducts and airway submucosal glands, which show reduced anion transport but not Na + hyperabsorption (2,48,49). Although our studies do not address the possibility that increased Na + conductance might be important at some stage of the disease or that inhibiting ENaC might benefit patients with CF, these results focus attention on defective anion transport in CF lung disease.…”

Interregulation of Proton-gated Na+ Channel 3 and Cystic Fibrosis Transmembrane Conductance Regulator

“…To examine the role of endogenous ASIC3 in the absence of ENaC, we chose Calu-3 cells, because only ␣ ENaC is detected using RT-PCR (43). Amiloride-sensitive Na ϩ current has not been documented in Calu-3 cells (52), supportive of the fact that even heterologously overexpressed ␣ ENaC alone in oocytes only has onehundredth the channel activity of ␣␤␥ ENaC (23).…”

Proteases and cystic fibrosis

“…Obstruction of submucosal glands is an early pathologic feature on airway histology (Sturgess, 1982) and submucosal gland hyposecretion has recently been demonstrated in intact submucosal glands ex vivo (Joo et al, 2006). CF ASL has altered pH and increased viscosity (Verkman, 2003).…”