New insights into the upregulation and function of the salivary Na+-K+-2Cl- cotransporter

In many exocrine epithelia, the Na+-K+-2Cl- cotransporter is the main provider of cellular chloride entry during transepithelial salt and water secretion. Because of its accessibility and hormonal responsiveness, the salivary gland has recently emerged as a convenient preparation in which to study the regulation and characteristics of this transport protein. In this review, we summarize recent findings from our laboratory which demonstrate that muscarinic, alpha1-adrenergic and peptidergic stimulation of rat parotid acinar cells induce a dramatic (up to twenty-fold) upregulation of Na+-K+-2Cl- cotransporter activity. Our results indicate that this effect is dependent on the rise in intracellular calcium concentration ([Ca2+]i) that accompanies stimulation, and is not a consequence of the KCl loss and the concomitant cell shrinkage associated with fluid secretion. In addition, we show that the effect of muscarinic stimulation on the cotransporter can be blocked by inhibitors of phospholipase A2, by a general inhibitor of arachidonic acid metabolism, and by specific inhibitors of the cytochrome P450 pathway. These data argue strongly for the involvement of a product of the cytochrome P450 pathway of arachidonic acid metabolism in upregulation of the salivary Na+-K+-2Cl- cotransporter.