Functional interactions between P2X4 and P2X7 receptors from mouse salivary epithelia

Mouse parotid acinar cells express P2X(4) and P2X(7) receptors (mP2X(4)R and mP2X(7)R) whose physiological function remains undetermined. Here we show that mP2X(4)R expressed in HEK-293 cells do not allow the passage of tetraethylammonium (TEA(+)) and promote little, if any, ethidium bromide (EtBr) uptake when stimulated with ATP or BzATP. In contrast, mP2X(7)R generates slowly decaying TEA(+) current, sustained Na+ current and promotes robust EtBr uptake. However, ATP-activated TEA(+) current from acinar cells was unlike that generated by mP2X(7)R or mP2X(4)R. Functional interactions between mP2X(4)R and mP2X(7)R were investigated in HEK cells co-transfected with different mP2X(4) : mP2X(7) cDNA ratios and using solutions containing either TEA(+) or Na+ ions. Co-expressed channels generated a TEA(+) current that displayed faster decay during ATP stimulation than mP2X(7)R alone. Moreover, cells transfected with a 2 : 1 cDNA ratio displayed decaying kinetics similar to those observed in acinar cells. Concentration-response curves in Na+-containing solutions were constructed for heterologously expressed mP2X(4)R, mP2X(7)R and mP2X(4)R: mP2X(7)R co-expressions as well as acinar cells. The EC50 values determined were 11, 220, 434 and 442 mu M, respectively. Na+ currents generated by expressing mP2X(4)R or mP2X(7)R alone were potentiated by ivermectin (IVM). In contrast, IVM potentiation in acinar cells and HEK cells co-expressing P2X(4) and P2X(7) (1 : 1 or 2 : 1 cDNA ratios) was seen only when the ATP concentration was lowered from 5 to 0.03 mM. Taken together our observations indicate a functional interaction between murine P2X(7) and P2X(4) receptors. Such interaction might occur in acinar cells to shape the response to extracellular ATP in salivary epithelia.