The extracellular zinc-sensing receptor mediates intercellular communication by inducing ATP release

Taste and salivary secretion disorders have been linked to zinc deficiency, indeed zinc is found in secretory granules in the salivary gland. The signaling role for the zinc release in this tissue, however, is poorly understood. Here, we address the signaling pathways and physiological role of the zinc-sensing receptor, ZnR, in the ductal salivary gland cell line, HSY. Exposure of these cells to zinc triggered intracellular Ca²⁺ release from thapsigargin-sensitive stores. The Gαq inhibitor, YM-254890 (1 μM), eliminated the Zn²⁺-dependent Ca²⁺ response, demonstrating that ZnR is a Gαq-coupled receptor. Dose-response curves yielded an apparent K_0.5 of 36 μM and a Hill coefficient of 7 in the absence of extracellular Ca²⁺, and K_0.5 of 55 μM with a Hill coefficient of 3 in its presence. This indicates that although Zn²⁺ is essential for ZnR activation, Ca²⁺ may affect the receptor co-operativity. The homologous desensitization pattern of ZnR was characterized by pre-exposure of cells to Zn²⁺ at concentrations found to activate the receptor. Re-exposure of cells to Zn²⁺ elicited an attenuated Zn²⁺-dependent Ca²⁺ response for at least 3 h, indicating that the ZnR is strongly desensitized by Zn²⁺. Finally, we studied the paracrine affects of ZnR using a co-culture consisting of the HSY cells and vascular smooth muscle cells (VSMCs). While no Zn²⁺-dependent Ca ²⁺ release was observed in VSMC alone, application of Zn²⁺ to the co-culture induced a Ca²⁺ rise in both HSY cells and VSMC. This Ca²⁺ rise was inhibited by the ATP scavenger, apyrase. Taken together, our results demonstrate that ZnR activity is monitored in salivary cells and is modulated by extracellular Ca²⁺. We further show that ZnR enhances secretion of ATP, thereby linking zinc to key signaling pathways involved in modification of salivary secretions by the ductal cells.