Cell-specific Behavior of P2X7 Receptors in Mouse Parotid Acinar and Duct Cells

P2X7 receptors (P2X7Rs) affect many epithelial cell functions including transcellular ion transport, secretion, and cell death. Here we used parotid acinar and duct cells to reveal the unique cell-specific assembly and gating of the P2X7R channels. Immunolocalization indicated expression of P2X7Rs in the luminal membrane of both cell types. Stimulation with 5 mM ATP raised [Ca ²⁺]_i levels in a cell-specific manner and activated multiple currents. The current mediated by P2X7R was isolated by infusing the cells with high [EGTA]. The initial activation of acinar cell P2X7Rs by ATP was slow requiring ∼2.5 min. Subsequent removal and addition of ATP, however, resulted in rapid inhibition and activation (gating) of the P2X7Rs. By contrast, P2X7Rs in duct cells displayed only rapid gating by ATP. Activation of P2X7Rs in both cell types was verified by (a) low K_m for ATP, (b) sensitivity to external divalent ions, (c) lack of desensitization/inactivation, (d) permeability to Na⁺, and (e) inhibition by Brilliant Blue G, Cu²⁺, and pyridoxal phosphate-6-azophenyl-2′,4′ -disulfonic acid tetrasodium. The slow P2X7R activation in acinar cells was not affected by manipulation of exo-/endocytosis. Rather, disassembly or solidification of the actin cytoskeleton prior to incubation with ATP prevented channel assembly. Remarkably, after completion of the slow activation, manipulation of the actin cytoskeleton no longer affected gating by ATP. Accordingly, manipulation of the actin cytoskeleton had no effect on P2X7R gating by ATP in duct cells. We concluded that P2X7Rs are not active in resting acinar cells. On exposure to ATP, P2X7Rs are assembled into functional channels with the aid of the actin cytoskeleton. Once assembled, P2X7Rs are subject to rapid gating by ATP. Duct cell P2X7Rs are preassembled and therefore continually subject to rapid gating by ATP. This cell-specific behavior may reflect the specific function of P2X7Rs in the two cell types.