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.open박