Loss of hyperpolarization-activated Cl- current in salivary acinar cells from Clcn2 knockout mice

Abstract

ClC-2 is localized to the apical membranes of secretory epithelia where it has been hypothesized to play a role in fluid secretion. Although ClC-2 is clearly the inwardly rectifying anion channel in several tissues, the molecular identity of the hyperpolarization-activated Cl- current in other organs, including the salivary gland, is currently unknown. To determine the nature of the hyperpolarization-activated Cl- current and to examine the role of ClC-2 in salivary gland function, a mouse line containing a targeted disruption of the Clcn2 gene was generated. The resulting homozygous Clcn2-/- mice lacked detectable hyperpolarization-activated chloride currents in parotid acinar cells and, as described previously, displayed postnatal degeneration of the retina and testis. The magnitude and biophysical characteristics of the volume- and calcium-activated chloride currents in these cells were unaffected by the absence of ClC-2. Although ClC-2 appears to contribute to fluid secretion in some cell types, both the initial and sustained salivary flow rates were normal in Clcn2-/- mice following in vivo stimulation with pilocarpine, a cholinergic agonist. In addition, the electrolytes and protein contents of the mature secretions were normal. Because ClC-2 has been postulated to contribute to cell volume control, we also examined regulatory volume decrease following cell swelling. However, parotid acinar cells from Clcn2-/- mice recovered volume with similar efficiency to wild-type littermates. These data demonstrate that ClC-2 is the hyperpolarization-activated Cl- channel in salivary acinar cells but is not essential for maximum chloride flux during stimulated secretion of saliva or acinar cell volume regulation.