Severe impairment of salivation in Na+/K+/2Cl(-) cotransporter (NKCC1)-deficient mice

The salivary fluid secretory mechanism is thought to require Na+/K+/2Cl(-) cotransporter-mediated Cl- uptake. To directly test this possibility we stu died the in vivo and in vitro functioning of acinar cells from the parotid glands of mice with targeted disruption of Na+/R+/2Cl(-) cotransporter isof orm 1 (Nkcc1), the gene encoding the salivary Na+/K+/2Cl(-) cotransporter. In wildtype mice NKCC1 was localized to the basolateral membranes of paroti d acinar cells, whereas expression was not detected in duct cells. The lack of functional NKCC1 resulted in a dramatic reduction (>60%) in the volume of saliva secreted in response to a muscarinic agonist, the primary in situ salivation signal, Consistent with defective Cl- uptake, a loss of bumetan ide-sensitive Cl- influx was observed in parotid acinar cells from mice lac king NKCC1. Cl-/HCO3- exchanger activity was increased in parotid acinar ce lls isolated from knockout mice suggesting that the residual saliva secrete d by mice lacking NKCC1 is associated with anion exchanger-dependent Cl- up take. Indeed, expression of the Cl-/HCO3- exchanger AE2 was enhanced sugges ting that this transporter compensates for the loss of functional Na+/K+/2C l(-) cotransporter. Furthermore, the ability of the parotid gland to conser ve NaCl was abolished in NKCC1-deficient mice. This deficit was not associa ted with changes in the morphology of the ducts, but transcript levels for the alpha-, beta-, and gamma-subunits of the epithelial Na+ channel were re duced. These data directly demonstrate that NKCC1 is the major Cl- uptake m echanism across the basolateral membrane of acinar cells and is critical fo r driving saliva secretion in vivo.