Chloride channels and salivary gland function.

Fluid and electrolyte transport is driven by transepithelial Cl- movement. The opening of Cl- channels in the apical membrane of salivary gland acinar cells initiates the fluid secretion process, whereas the activation of Cl- channels in both the apical and the basolateral membranes of ductal cells is thought to be necessary for NaCl re-absorption. Saliva formation can be evoked by sympathetic and parasympathetic stimulation. The composition and flow rate vary greatly, depending on the type of stimulation. As many as five classes of Cl- channels with distinct gating mechanisms have been identified in salivary cells. One of these Cl- channels is activated by intracellular Ca2+, while another is gated by cAMP. An increase in the intracellular free Ca2+ concentration is the dominant mechanism triggering fluid secretion from acinar cells, while cAMP may be required for efficient NaCl re-absorption in many ductal cells. In addition to cAMP- and Ca(2+)-gated Cl- channels, agonist-induced changes in membrane potential and cell volume activate different Cl- channels that likely play a role in modulating fluid and electrolyte movement. In this review, the properties of the different types of Cl- currents expressed in salivary gland cells are described, and functions are proposed based on the unique properties of these channels.