Dissection of Calcium Signaling Events in Exocrine Secretion

The secretion of fluid and electrolytes by salivary gland acinar cells requires the coordinated regulation of multiple ion channel and transporter proteins, signaling components, and water transport. Importantly, neurotransmitter stimulated increase in the cytosolic free [Ca2+] ([Ca2+](i)) is critical for the regulation of salivary gland secretion as it regulates several major ion fluxes that together establish the sustained osmotic gradient to drive fluid secretion. The mechanisms that act to modulate these increases in [Ca2+] i are therefore central to the process of salivary fluid secretion. Such modulation involves membrane receptors for neurotransmitters, as well as mechanisms that mediate intracellular Ca2+ release, and Ca2+ entry, as well as those that maintain cellular Ca2+ homeostasis. Together, these mechanisms determine the spatial and temporal aspects of the [Ca2+] i signals that regulate fluid secretion. Molecular cloning of these transporters and channels as well as development of mice lacking these proteins has established the physiological significance of key components that are involved in regulating [Ca2+] i in salivary glands. This review will discuss these important studies and the findings which have led to resolution of the Ca2+ signaling mechanisms that determine salivary gland fluid secretion.