Expression of Ca²⁺-dependent synaptotagmin isoforms in mouse and rat parotid acinar cells

Synaptotagmin is a Ca²⁺ sensing protein, which triggers a fusion of synaptic vesicles in neuronal transmission. Little is known regarding the expression of Ca²⁺-dependent synaptotagmin isoforms and their contribution to the release of secretory vesicles in mouse and rat parotid acinar cells. We investigated a type of Ca²⁺-dependent synaptotagmin and Ca²⁺ signaling in both rat and mouse parotid acinar cells using RT-PCR, micro-fluorometry, and amylase assay. Mouse parotid acinar cells exhibited much more sensitive amylase release in response to muscarinic stimulation than did rat parotid acinar cells. However, transient [Ca²⁺]i increases and Ca²⁺ influx in response to muscarinic stimulation in both cells were identical, suggesting that the expression or activity of the Ca^2- sensing proteins is different. Seven Ca²⁺- dependent synaptotagmins, from 1 to 7, were expressed in the mouse parotid acinar cells. However, in the rat parotid acinar cells, only synaptotagmins 1, 3, 4 and 7 were expressed. These results indicate that the expression of Ca²⁺-dependent synaptotagmins may contribute to the release of secretory vesicles in parotid acinar cells.