Background & aims: We have previously shown that endocytosis at the apical plasma membrane in pancreatic acinar cells is coupled to ductal bicarbonate secretion into the lumen. We hypothesized that decreased bicarbonate secretion in cystic fibrosis (CF) inhibits apical endocytosis. The aim of this study was to determine in cftr(-/-) mice (1) if the pH of the pancreatic juice is acidic compared with wild-type (WT) controls, (2) if there is a selective block in endocytosis, and (3) if alkalinization of the luminal fluid reverses this defect.
Methods: Fluid secretion and pH of pancreatic juice were measured. Exocytosis, endocytosis, and morphology were compared in pancreatic lobules from cftr(-/-) and WT mice.
Results: Pancreatic juice pH was 8.12 +/- 0.06 in WT mice compared with 6.60 +/- 0.04 in cftr(-/-) mice. Although cholecystokinin-stimulated amylase secretion was not significantly different, endocytosis was markedly inhibited in cftr(-/-) compared with WT mice. Cleavage of GP2, a GPI-anchored protein tightly associated with activation of endocytosis, was also decreased. Incubation of lobules from cftr(-/-) mice at pH 8.3 reversed the luminal dilatation.
Conclusions: These data indicate that apical endocytosis is selectively impaired in cftr(-/-) mice, which explains, in part, the luminal dilatation observed at the apical plasma membrane. In vitro alkalinization of luminal fluid led to reversal of defects in membrane dynamics, restored coupled exocytosis and endocytosis, and abolished the luminal dilatation in this animal model of CF. Acidic pH changes in luminal secretions may play a role in the pancreatic membrane dysfunction observed in CF.