Developmental aspects of fluid and electrolyte secretion in salivary glands

The salivary glands of rodents undergo considerable cytodifferentiation after birth and are useful models for the study of functional development, including the mechanisms of fluid and electrolyte secretion. In the rat submandibular gland, secretion of salivary fluid cannot be elicited until approximately 2 weeks of age. The currently accepted model of salivary fluid secretion indicates that this process depends on the activation, on stimulation of cholinergic receptors, of several ion transport systems, resulting in a net transport of osmotically active ions (primarily Cl- and Na+) across the acinar epithelium. This creates the necessary osmotic gradient for the transacinar movement of water. The process is associated with a signal transduction pathway involving the formation of phosphoinositide products (primarily inositol triphosphate or IP3) and the mobilization of Ca2+. The latter regulates monovalent ion conductances (K+, Cl-), which are critical for the secretory process. Immature submandibular glands and cells of early postnatal rats have a lower density of cholinergic receptors and release less K+ and Cl- than mature cells and gradually develop other ion transport systems (such as a Na, K, 2Cl cotransport system) involved in the secretory process. Surprisingly, they form more IP3 and show a larger increase in cytosolic Ca2+ when stimulated with maximal or supramaximal concentrations of agonist. Therefore, they show some interesting dissociations in the signal transduction mechanism that suggest differences in the coupling between receptors and membrane phosphoinositides, between IP3 and IP3-dependent Ca2+ stores, and between the Ca2+ signal and the monovalent ion transport systems which are critical for secretion.