Evidence That Calcium Entry Into Calcium-Transporting Dental Enamel Cells Is Regulated by Cholecystokinin, Acetylcholine and ATP

Dental enamel is formed by specialized epithelial cells which handle large quantities of Ca(2+) while producing the most highly mineralized tissue. However, the mechanisms used by enamel cells to handle bulk Ca(2+) safely remain unclear. Our previous work contradicted the dogma that Ca(2+) is ferried through the cytosol of Ca(2+)-transporting cells and instead suggested an organelle-based route across enamel cells. This new paradigm involves endoplasmic reticulum (ER)-associated Ca(2+) stores and their concomitant refilling by store-operated Ca(2+) entry (SOCE) mediated by Ca(2+) release activated Ca(2+) (CRAC) channels. Given that Ca(2+) handling is maximal during the enamel-mineralization stage (maturation), we anticipated that SOCE would also be elevated then. Confirmation was obtained here using single-cell recordings of cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in rat ameloblasts. A candidate SOCE agonist, cholecystokinin (CCK), was found to be upregulated during maturation, with Cck transcript abundance reaching 30% of that in brain. CCK-receptor transcripts were also detected and Ca(2+) imaging showed that CCK stimulation increased [Ca(2+)](cyt) in a dose-responsive manner that was sensitive to CRAC-channel inhibitors. Similar effects were observed with two other SOCE activators, acetylcholine and ATP, whose receptors were also found in enamel cells. These results provide the first evidence of a potential regulatory system for SOCE in enamel cells and so strengthen the Ca(2+) transcytosis paradigm for ER-based transport of bulk Ca(2+). Our findings also implicate enamel cells as a new physiological target of CCK and raise the possibility of an auto/paracrine system for regulating Ca(2+) transport.