Skatchkov S N, Krusek J, Reichenbach A, Orkand R K
CMBN, Department of Biochemistry, School of Medicine, Central Caribbean University, Bayamon, Puerto Rico, USA.
Glia. 1999 Aug;27(2):171-80.
Müller (radial glial) cells span the retina from the outer to the inner limiting membranes. They are the only glial cells found in the amphibian retina. The thickness of the frog (Rana pipiens) retina decreases by a factor of about four from the center to the periphery. Thus, Müller cells were isolated, by enzymatic dissociation, with stalk lengths from 20 to 140 microm. Their ability to transfer K(+) via the stalk between soma and endfoot was studied. Membrane currents were recorded using the whole-cell voltage-clamp technique with the pipette sealed to either the endfoot or the soma. Inward (I(KIN)) or outward (I(KO)) currents were elicited by rapid increases (3 to 10 mM) or decreases (3 to 1 mM) of the extracellular K(+) concentration (K(+)) either by local application (close or distant to the recording pipette) or around the entire cell (whole cell perfusion). For the long central cells, the ratio I(KIN)/I(KO) was 4.6 +/- 0.6 SE (n = 9) at the endfoot and 1.7 +/- 0.1 SE (n = 8) at the soma. In cells from the retinal periphery, the ratio I(KIN)/I(KO) was higher, 7.0 +/- 0.27 (n = 8) at the endfoot and 3.2 +/- 0.1 (n = 10) at the soma. The results suggest that there is less inward rectification in the somatic than in the endfoot membrane. As expected from previous studies, the sensitivity of the cells to K(+) was higher at the endfoot than at the soma. The amplitude of I(KIN) at the endfoot compared to the soma was about 8-fold for the long central cells but only about 1.5-fold for the short peripheral cells. Currents spread readily from endfoot to soma in the peripheral cells. In the long central Müller cells the soma and endfoot appeared electrotonically isolated. The "functional length constant", lambda, of cell stalk processes was about 70 microm. The relative decrement of large inward currents was stronger than that of smaller outward currents; this difference ("artificial rectification") is explained by a simple model, where larger currents (inward) are attenuated more than smaller (outward) currents. The data support the hypothesis that in the retinal periphery, Müller cells provide extensive spatial K(+) buffering from both plexiform layers into the vitreous body. In the central retina, however, such currents are limited within a short (interlaminar) range.
米勒（放射状胶质）细胞从视网膜外边界膜延伸至内边界膜。它们是两栖动物视网膜中唯一发现的胶质细胞。青蛙（豹蛙）视网膜的厚度从中心到周边大约减小四倍。因此，通过酶解分离出了米勒细胞，其柄的长度在20至140微米之间。研究了它们通过柄在胞体和终足之间转运钾离子（K⁺）的能力。使用全细胞电压钳技术记录膜电流，将微电极封接在终足或胞体上。通过局部施加（靠近或远离记录微电极）或围绕整个细胞（全细胞灌流）快速增加（3至10毫摩尔）或降低（3至1毫摩尔）细胞外钾离子浓度（[K⁺]ₒ）来引发内向（I(KIN)）或外向（I(KO)）电流。对于长的中央细胞，终足处的I(KIN)/I(KO)比值为4.6±0.6标准误（n = 9），胞体处为1.7±0.1标准误（n = 8）。在视网膜周边的细胞中，I(KIN)/I(KO)比值更高，终足处为7.0±0.27（n = 8），胞体处为3.2±0.1（n = 10）。结果表明，胞体膜的内向整流比终足膜少。正如先前研究所预期的，细胞对钾离子的敏感性在终足处高于胞体。对于长的中央细胞，终足处I(KIN)的幅度与胞体处相比约为8倍，而对于短的周边细胞仅约为1.5倍。在外周细胞中，电流很容易从终足扩散到胞体。在长的中央米勒细胞中，胞体和终足在电紧张方面似乎是隔离的。细胞柄状突起的“功能长度常数”λ约为70微米。大的内向电流的相对衰减比小的外向电流更强；这种差异（“人为整流”）由一个简单模型解释，即较大电流（内向）比较小电流（外向）衰减得更多。这些数据支持这样的假说，即在视网膜周边，米勒细胞从两个神经丛层向玻璃体提供广泛的空间钾离子缓冲。然而，在中央视网膜中，这种电流被限制在短的（层间）范围内。
