Salivary acinar cells from aquaporin 5-deficient mice have decreased membrane water permeability and altered cell volume regulation

Aquaporins (AQPs) are channel proteins that regulate the movement of water through the plasma membrane of secretory and absorptive cells in response t o osmotic gradients. In the salivary gland, AQP5 is the major aquaporin exp ressed on the apical membrane of acinar cells. Previous studies have shown that the volume of saliva secreted by AQP5-deficient mice is decreased, ind icating a role for AQP5 in saliva secretion; however, the mechanism by whic h AQP5 regulates water transport in salivary acinar cells remains to be det ermined. Here we show that the decreased salivary flow rate and increased t onicity of the saliva secreted by Aqp5(-/-) mice in response to pilocarpine stimulation are not caused by changes in whole body fluid homeostasis, ind icated by similar blood gas and electrolyte concentrations in urine and blo od in wild-type and AQPB-deficient mice. In contrast, the water permeabilit y in parotid and sublingual acinar cells isolated from Aqp5(-/-) mice is de creased significantly. Water permeability decreased by 65% in parotid and 7 7% in sublingual acinar cells from Aqp5(-/-) mice in response to hypertonic ity-induced cell shrinkage and hypotonicity-induced cell swelling. These da ta show that AQP5 is the major pathway for regulating the water permeabilit y in acinar cells, a critical property of the plasma membrane which determi nes the flow rate and ionic composition of secreted saliva.