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 studied the in vivo and in vitro functioning of acinar cells from the parotid glands of mice with targeted disruption of Na(+)/K(+)/2Cl(-) cotransporter isoform 1 (Nkcc1), the gene encoding the salivary Na(+)/K(+)/2Cl(-) cotransporter. In wild-type mice NKCC1 was localized to the basolateral membranes of parotid 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 bumetanide-sensitive Cl(-) influx was observed in parotid acinar cells from mice lacking NKCC1. Cl(-)/ HCO(3)(-) exchanger activity was increased in parotid acinar cells isolated from knockout mice suggesting that the residual saliva secreted by mice lacking NKCC1 is associated with anion exchanger-dependent Cl(-) uptake. Indeed, expression of the Cl(-)/ HCO(3)(-) exchanger AE2 was enhanced suggesting that this transporter compensates for the loss of functional Na(+)/K(+)/2Cl(-) cotransporter. Furthermore, the ability of the parotid gland to conserve NaCl was abolished in NKCC1-deficient mice. This deficit was not associated with changes in the morphology of the ducts, but transcript levels for the alpha-, beta-, and gamma-subunits of the epithelial Na(+) channel were reduced. These data directly demonstrate that NKCC1 is the major Cl(-) uptake mechanism across the basolateral membrane of acinar cells and is critical for driving saliva secretion in vivo.