Highly localized intracellular Ca2+ signals promote optimal salivary gland fluid secretion

Abstract

Salivary fluid secretion involves an intricate choreography of membrane transporters to result in the trans-epithelial movement of NaCl and water into the acinus lumen. Current models are largely based on experimental observations in enzymatically isolated cells where the Ca2+ signal invariably propagates globally and thus appears ideally suited to activate spatially separated Cl and K channels, present on the apical and basolateral plasma membrane, respectively. We monitored Ca2+ signals and salivary secretion in live mice expressing GCamp6F, following stimulation of the nerves innervating the submandibular gland. Consistent with in vitro studies, Ca2+ signals were initiated in the apical endoplasmic reticulum. In marked contrast to in vitro data, highly localized trains of Ca2+ transients that failed to fully propagate from the apical region were observed. Following stimuli optimum for secretion, large apical-basal gradients were elicited. A new mathematical model, incorporating these data was constructed to probe how salivary secretion can be optimally stimulated by apical Ca2+ signals.

A Mathematical Model of Salivary Gland Duct Cells

“…The duct was injected with 20 µl of solution containing 50 µM sulphorrhodamine over 3-5 min. The mice were then imaged by multiphoton microscopy as previously described (Takano et al 2021). 49 images of 1 µm spacing and 512 × 512 pixels resolution are obtained.…”

“…We modified the acinar cell model of Takano et al (2021) to generate the primary saliva, while our duct model is a modified version of the minipig salivary gland duct model of Fong et al (2017). The major changes from the model of Fong et al (2017) are in the types and densities of ion transporters in both SD and ID cells.…”

“…The previous 3D spatial temporal acinar cell model of Takano et al (2021) describes how the calcium oscillations inside an acinar cell lead to primary saliva secretion. Upon neuronal stimulation, the acinar cell releases Ca 2+ from an internal store (the endoplasmic reticulum) near the apical membrane, which activates apical Cl − channels thus allowing a flux of Cl − into the lumen.…”

“…However, the model of Takano et al (2021) produces primary saliva that is not exactly compatible with the input requirement of the duct model; it contains only 3 ion species, Na + , K + and Cl − , whereas the duct model also requires HCO − 3 and H + . In order for the two to be compatible, we modified the acinar cell model to produce HCO − 3 and H + in the primary saliva.…”

“…We develop a model of the mouse salivary gland that includes several acini and the ID and SD extending from them, where the duct structure is based on the 3D reconstruction of a confocal image stack of the mouse submandibular gland. Acinar cell models are reasonably well developed (Vera-Sigüenza et al 2018, 2020Takano et al 2021) and so we utilise these older models, with minor adjustments, to generate dynamic primary saliva flow to be used as input to the duct model. The acinar cell model output is the primary saliva flow rate and ionic concentrations, all given as functions of time.…”