Ion Channel Integration and Functional Coupling in Salivary Gland Fluid Secretion

Salivary glands produce saliva through precisely coordinated epithelial ion transport processes. Ion channels are essential components of the molecular machinery that convert neural and hormonal signals into targeted ion and water flux. This review focuses on the integrated molecular and cellular mechanisms by which ion channels cooperate to generate salivary fluid under physiological conditions. Saliva formation proceeds through two sequential stages: isotonic primary fluid secretion by acinar cells, followed by ionic modification within the ductal epithelium. Parasympathetic stimulation activates muscarinic M1/3 receptors, initiating intracellular calcium signaling through inositol 1,4,5-trisphosphate-dependent release from the endoplasmic reticulum and sustained calcium entry via Orai1/TRPC channels. Elevated cytosolic calcium activates apical ANO1/TMEM16A chloride channels, the rate-limiting step in acinar fluid secretion, together with basolateral calcium-activated potassium channels that preserve the electrochemical driving force for chloride efflux. Chloride accumulation is maintained by Na⁺/K⁺-ATPase and the Na⁺-K⁺-2Cl^- cotransporter, while osmotic gradients drive water movement through apical aquaporin-5 and basolateral aquaporin-1/3. As primary saliva traverses the ductal system, epithelial sodium channels, CFTR, and additional ion transport pathways reabsorb sodium and chloride and secrete potassium and bicarbonate, producing hypotonic final saliva. By synthesizing calcium signaling, chloride and potassium conductance, sodium handling, and epithelial polarity into a unified framework, this review establishes ion channel integration as the fundamental basis of salivary gland fluid secretion.