Bao Hong, Wong Weng Hoe, Goldberg Jay M, Eatock Ruth Anne
The Bobby R. Alford Department of Otorhinolaryngology and Communicative Sciences, Baylor College of Medicine, Houston, Texas 77030, USA.
J Neurophysiol. 2003 Jul;90(1):155-64. doi: 10.1152/jn.00244.2003.
When studied in vitro, type I hair cells in amniote vestibular organs have a large, negatively activating K+ conductance. In type II hair cells, as in nonvestibular hair cells, outwardly rectifying K+ conductances are smaller and more positively activating. As a result, type I cells have more negative resting potentials and smaller input resistances than do type II cells; large inward currents fail to depolarize type I cells above -60 mV. In nonvestibular hair cells, afferent transmission is mediated by voltage-gated Ca2+ channels that activate positive to -60 mV. We investigated whether Ca2+ channels in type I cells activate more negatively so that quantal transmission can occur near the reported resting potentials. We used the perforated patch method to record Ca2+ channel currents from type I and type II hair cells isolated from the rat anterior crista (postnatal days 4-20). The activation range of the Ca2+ currents of type I hair cells differed only slightly from that of type II cells or nonvestibular hair cells. In 5 mM external Ca2+, currents in type I and type II cells were half-maximal at -41.1 +/- 0.5 (SE) mV (n = 10) and -37.2 +/- 0.2 mV (n = 10), respectively. In physiological external Ca2+ (1.3 mM), currents in type I cells were half-maximal at -46 +/- 1 mV (n = 8) and just 1% of maximal at -72 mV. These results lend credence to suggestions that type I cells have more positive resting potentials in vivo, possibly through K+ accumulation in the synaptic cleft or inhibition of the large K+ conductance. Ca2+ channel kinetics were also unremarkable; in both type I and type II cells, the currents activated and deactivated rapidly and inactivated only slowly and modestly even at large depolarizations. The Ca2+ current included an L-type component with relatively low sensitivity to dihydropyridine antagonists, consistent with the alpha subunit being CaV1.3 (alpha1D). Rat vestibular epithelia and ganglia were probed for L-type alpha-subunit expression with the reverse transcription-polymerase chain reaction. The epithelia expressed CaV1.3 and the ganglia expressed CaV1.2 (alpha1C).
在体外研究时，羊膜动物前庭器官中的I型毛细胞具有较大的、负向激活的钾离子电导。在II型毛细胞中，与非前庭毛细胞一样，外向整流钾离子电导较小且正向激活程度更高。因此，I型细胞比II型细胞具有更负的静息电位和更小的输入电阻；大的内向电流无法使I型细胞去极化至-60 mV以上。在非前庭毛细胞中，传入神经传递由电压门控钙离子通道介导，这些通道在膜电位为-60 mV时激活。我们研究了I型细胞中的钙离子通道是否在更负的电位下激活，以便在报道的静息电位附近发生量子传递。我们使用穿孔膜片钳方法记录从大鼠前嵴（出生后4 - 20天）分离的I型和II型毛细胞的钙离子通道电流。I型毛细胞的钙离子电流激活范围与II型细胞或非前庭毛细胞的激活范围仅略有不同。在外部钙离子浓度为5 mM时，I型和II型细胞中的电流分别在-41.1±0.5（标准误）mV（n = 10）和-37.2±0.2 mV（n = 10）时达到最大值的一半。在生理外部钙离子浓度（1.3 mM）下，I型细胞中的电流在-46±1 mV（n = 8）时达到最大值的一半，在-72 mV时仅为最大值的1%。这些结果支持了I型细胞在体内具有更正的静息电位的观点，这可能是通过突触间隙中的钾离子积累或对大钾离子电导的抑制实现的。钙离子通道动力学也不显著；在I型和II型细胞中，电流激活和失活迅速，即使在大的去极化情况下，失活也仅缓慢且适度。钙离子电流包括一个对二氢吡啶拮抗剂敏感性相对较低的L型成分，这与α亚基为CaV1.3（α1D）一致。使用逆转录 - 聚合酶链反应检测大鼠前庭上皮和神经节中L型α亚基的表达。上皮表达CaV1.3，神经节表达CaV1.2（α1C）。
