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Sun-Sook Paik, In-Beom Kim; The relationship between Ca2+ channel types and Ca2+-activated Cl− channels properties in mouse retinal on-bipolar cell axon terminals . Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3224. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Two types of voltage-gated Ca2+ channels, L-type and T-type, are known to be localized in axon terminals of mammalian bipolar cells. We previously characterized a Ca2+-activated Cl− channel (CaCC) in rod bipolar cell axon terminals of mouse retina and found that L-type Ca2+ channels acted as its major Ca2+ influx source. However, the role of T-type Ca2+ channel and its relationship with the CaCC remains unknown. This study was aimed to clarify the contribution of two types of voltage-gated Ca2+ channels to CaCC properties in on-type bipolar cell axon terminals of mouse retina.
Mice (7~8week-age, C57BL/6J) were deeply anaesthetized with 4% chloral hydrate and solitary on-type bipolar cells were obtained with enzymatic dissociation. The membrane currents of these cells were recorded by the patch-clamp technique in the whole-cell recording configuration. Statistical analysis was conducted by ANOVA and Student’s t-test. Data are represented as mean ± s.d. The differences were considered significant at P < 0.05.
According to the pharmacological profile and biophysical property, on-bipolar cells of mouse retina were divided into three groups. First, on-bipolar axon terminals (7 out of 15) showed prominent L-type activated Ca2+-activated Cl− current (ICl(Ca)) with a high sensitivity to nifedipine (10 µM) and a peak (20 ± 3 pA) at -30 mV. Second type (5 out of 15) possessed ICl(Ca) that attributed to the combination of two types of Ca2+ channels with a high sensitivity to nifedipine and a peak amplitude (13 ± 4 pA) at -40 mV. Third type of on-bipolar axon terminals (3 out of 15) showed prominent T-type mediated ICl(Ca) with a high sensitivity to mibefradil (10 µM) and a peak amplitude (5 ± 4 pA) at -55 mV. Interestingly, when the cells were depolarized by repeated train pulses, only L-type activated ICl(Ca) was highly enhanced in amplitude (330 ± 26 %, n=6).
These results demonstrate that both L-type and T-type Ca2+ channels contribute to ICl(Ca) in on-bipolar cell axon terminals of mouse retina and suggest that a dominant Ca2+ source of ICl(Ca) is L-type Ca2+ channel rather than T-type Ca2+ channel.
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