P2X2 and P2X4 receptor-mediated cation absorption in utricular transitional cells and macula

Abstract

Adenosine 5’-triphosphate (ATP) regulates inner ear function by modulating ion transport through purinergic receptors in inner ear epithelial cells. This study was designed to investigate purinergic receptor-mediated cation transport by mouse utricular macula and the surrounding transitional cells (TCs), where linear acceleration stimuli are sensed. Among ATP, adenosine 5’-diphosphate (ADP), uridine 5’-triphosphate (UTP), and uridine 5’-diphosphate (UDP), only ATP (100 μM) induced cation absorption currents in TCs and macula. The current was almost completely inhibited by the application of gadolinium (100 μM). The order of agonist potencies for the cation absorption current was ATP > 3’-O-(4-benzoyl-benzoyl) adenosine 5’-triphosphate (bzATP) >> α,β-methyleneadenosine 5’ -triphosphate (αβmeATP) in both TCs and macula, and the EC50 (concentration that produces a half-maximal effect) values for ATP, bzATP, and αβmeATP were 27.2 μM, 43.9 μM, and 34.5 μM in the TCs and 20.7 μM, 63.4 μM, and 2014.1 μM in the macula, respectively (EC50 values of αβmeATP were not definitively identified due to the low potency of αβmeATP). The ATP-induced current was partially blocked by suramin (100 μM), pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS) (10 μM), and 5-(3-bromophenyl)-1,3-dihydro-2H-benzofuro[3,2-e]-1,4-diazepin-2-one (5-BDBD) (5 μM) and was almost completely blocked by PPADS + 5-BDBD in both areas. Immunocytochemistry revealed that P2X2 receptors were distributed in TCs and supporting cells in the macula; however, only P2X4 receptor was not detected in the macula by immunocytochemistry, but only its mRNA expression was detected there. These results indicate that ATP induces cation absorption through P2X2 and P2X4 receptors in utricular TCs and the macula. P2X2 and P2X4-mediated cation transport likely provides a cation shunt under conditions of excessive linear acceleration, thereby protecting hair cells by reducing their cation burden.