Figure 2.

Physiology of saliva secretion and role of AQP5. During the first secretory step, acinar cells secrete NaCl which creates a transepithelial osmotic gradient driving consequently water flow through AQP5 and paracellular pathway, leading to a primary isotonic fluid secretion. During the second secretory step, ductal cells, impermeable to water, reabsorb most NaCl and secrete K⁺ and HCO₃⁻. This leads to the formation a final hypotonic saliva flowing to the mouth cavity.