Modulation of voltage-gated Ca2+ current in vestibular hair cells by nitric oxide

The structural elements of the nitric oxide-cyclic guanosine monophosphate (NO-cGMP) signaling pathway have been described in the vestibular peripheral system. However, the functions of NO in the vestibular endorgans are still not clear. We evaluated the action of NO on the Ca(2+) currents in hair cells isolated from the semicircular canal crista ampullaris of the rat (P14-P18) by using the whole cell and perforated-cell patch-clamp technique. The NO donors 3-morpholinosydnonimine (SIN-1), sodium nitroprusside (SNP), and (+/-)-(E)-4-ethyl-2-[(Z)-hydroxyimino]-5-nitro-3-hexen-1-yl-nicotinamide (NOR-4) inhibited the Ca(2+) current in hair cells in a voltage-independent manner. The NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (CPTIO) prevented the inhibitory effect of SNP on the Ca(2+) current. The selective inhibitor of the soluble form of the enzyme guanylate cyclase (sGC), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), also decreased the SNP-induced inhibition of the Ca(2+) current. The membrane-permeant cGMP analogue 8-Br-cGMP mimicked the SNP effect. KT-5823, a specific inhibitor of cGMP-dependent protein kinase (PGK), prevented the inhibition of the Ca(2+) current by SNP and 8-Br-cGMP. In the presence of N-ethylmaleimide (NEM), a sulfhydryl alkylating agent that prevents the S-nitrosylation reaction, the SNP effect on the Ca(2+) current was significantly diminished. These results demonstrated that NO inhibits in a voltage-independent manner the voltage-activated Ca(2+) current in rat vestibular hair cells by the activation of a cGMP-signaling pathway and through a direct action on the channel protein by a S-nitrosylation reaction. The inhibition of the Ca(2+) current by NO may contribute to the regulation of the intracellular Ca(2+) concentration and hair-cell synaptic transmission.