Basal nitric oxide production contributes to membrane potential and vasotone regulation of guinea pig in vitro spiral modiolar artery

Nitric oxide (NO) is a potent vasodilating agent implicated in cochlear blood flow regulation. We recently demonstrated that exogenously applied NO donor DPTA-NONOate hyperpolarizes both endothelial and smooth muscle cells of in vitro spiral modiolar artery (SMA) via activation of ATP-sensitive K ⁺ channels (K_ATP). Also, NO was detected in the SMA cells by NO indicator dye in the in vitro basal condition. Using intracellular recording techniques, electrochemical NO-sensing measurement, and a vaso-diameter video tracking method, we investigated the basal release of NO from the in vitro SMA and its role in the vascular function. We found that (1) 300 μM L-NAME, a NO synthase inhibitor, and 3 μM glipizide caused a depolarization of ∼4.5 and ∼3.2 mV, respectively, in cells with a resting potential less negative than -60 mV; (2) NO sensor in the close vicinity of the SMA detected a NO concentration of ∼50 nM that was suppressed by L-NAME and enhanced by L-arginine (1-1000 μM); (3) NO donor DPTA-NONOate (0.1-30 μM) applications produced about 8-245 nM of NO in the recording bath. These data indicate a NO concentration-hyperpolarization relation, with an EC₅₀ of 22 nM. (4) Finally, L-NAME but not glipizide produced a 4.8% reduction in SMA diameter (∼50 μm) in the majority of SMAs, whereas NONOate (10 μM) always caused a dilation. Both the induced constriction and dilation were not significantly affected by 3 μM glipizide. We conclude that a significant amount of NO (>50 nM) is tonically released from the in vitro SMA, which is above the EC₅₀ for activation of K_ATP, and thus contributes to the membrane polarization. The basal release of NO also contributes to vasotone relaxation, but the K_ATP activation appears to play little role in the relaxation of the in vitro SMA.