The Role of P2X4 Receptors in Calcium-Mediated Exocytosis in Parotid Acinar Cells

Abstract

Saliva is largely generated as a result of both parasympathetic and sympathetic neural input to the parotid salivary gland. Muscarinic and adrenergic receptor activations are the dominant signals that regulate fluid and protein secretion during salivation. In addition to these pathways, recent studies have suggested a noncholinergic, nonadrenergic pathway that contributes to salivation. One candidate for this pathway is the P2X class of Ca2+ permeable ion channels that evoke Ca2+ signals in response to extracellular adenosine triphosphate (ATP). P2X4 and P2X7 are the only two purinergic ionotropic receptors identified in the mouse parotid acinar cells. P2X7 receptors have been shown to be an important regulator of fluid secretion in the salivary gland. However, the physiological role of P2X4 receptors in parotid acinar cells remains ill defined. In this study, multi-photon and live-cell imaging of acinar clumps and organotypic slices were used to quantify changes in intracellular Ca2+ and monitor secretory events. We now demonstrate that activation of P2X4 purinergic receptors in mouse parotid acinar cells by extracellular ATP evoked Ca2+ influx and protein secretion in a concentration-dependent manner. Pre-treatment with ivermectin (a specific positive allosteric modulator for P2X4), induced a 3-fold leftward shift in the EC50 of the Ca2+ peak amplitude induced by ATP. Moreover, there was 8-fold enhancement in the number of Ca2+-evoked secretory events. These data identify P2X4 receptors as a novel regulator of fluid and protein secretion in the parotid gland and have implications for treatment of salivary hypofunction diseases like Sjogren's syndrome.