Extracellular Zn²⁺ Modulatory Effect On Inward Rectifing Potassium Channels Expressed On Carp Horizontal Cell

In vertebrate retina, Zn²⁺ is abundant in terminal of photoreceptors, free Zn²⁺ can be co-released with glutamate from synaptic terminal into synaptic clefts. In dark, horizontal cells receive the glutamatergic input from photoreceptors, given the possibility that Zn²⁺ is co-released with glutamate from photoreceptors, Zn²⁺ may play an important role in modulating postsynaptic activity of membrane ion channels. Horizontal cells express a large number of ion channels, inward rectifing potassium channel (Kir) is one of them. The present study examined the Zn²⁺ modulatory effect on Kir channel expressed on H1 type horizontal cell using whole-cell patch clamp technique.First, the Kir current on the isolated H1 horizontal cells was evoked under step or ramp protocol. Cells were held at -60 mV, command voltage were ranged from -120 mV to 20 mV. The characteristics of it were the same as previous descripted by others. Such as the properties of inward rectifying and being almost blocked by 10 mM Ba2+ (selective Kir blocker). All above confirm that what we had recorded was mediated by Kir. Second, the modulatory effect of Zn²⁺ on Kir current was examined. The result was that 2 mM Zn²⁺ had inhibitory effect on Kir when extracellular pH is 6.8. Then, different concentration of Zn²⁺ were also applied, they were 2μM, 20μM, 200μM, 1 mM, 5 mM, 10 mM and 20 mM. The results further show that: Zn²⁺ at micromolar concentration had no effect on Kir, while, Zn²⁺ at millimolar concentration had inhibitory effect on Kir when extracellular pH = 6.8, the higher concentration of Zn²⁺ had more inhibitory effect on Kir current. Zn²⁺ had no effect on Kir when extracellular pH was in acid environment (pH = 5.0) or physiological rang (pH = 7.2). These results suggest that the modulatory effect of Zn²⁺ on Kir channel is related to extracellular pH and dose-dependent.Part of this thesis also tested the effect of extracellular pH change on Kir current. The result show that Kir current was inhibited when extracellular pH = 4.0. It suggest that Kir expressed on carp retinal horizontal cell is modulated by extracellular H⁺ .Overall, The present work focused on the modulatoty effect of Zn²⁺ and H+ on Kir channel. The results show that: (1) in isolated H1 horizontal cells, Zn²⁺ at millimolar concentration has inhibitory effect on Kir when extracellular pH is 6.8, but micromolar concentration of Zn²⁺ has no effect; (2) Zn²⁺ has no effect on Kir when extracellular pH is in acid environment (pH = 5.0) or physiological rang (pH = 7.2). (3) extracellular pH variation modulate the response property of Kir channel.