Normann R A, Perlman I, Anderton P J
Department of Bioengineering, University of Utah, Salt Lake City 84112.
Brain Res. 1988 Mar 8;443(1-2):95-100. doi: 10.1016/0006-8993(88)91602-2.
The modulation by magnesium ions of cone-to-horizontal-cell signal transmission was studied in the turtle superfused everted eye-cup preparation using solutions which contained various concentrations of divalent cations and/or chelating agents (EDTA and EGTA). Removal of magnesium ions from 'normal' solutions had no apparent effect on the horizontal cells provided that 'normal' levels of calcium ions were maintained. Solutions which actively removed calcium but contained 'normal' levels of magnesium hyperpolarized horizontal cells, reduced the amplitude of their photoresponses but also changed the character of these photoresponses; dim stimuli evoked purely depolarizing responses while bright flashes elicited triphasic responses. Active removal of both calcium and magnesium from the superfusate and from retinal stores with chelators depolarized the horizontal cell and eliminated light-evoked responses. These solutions also depolarized the cones but augmented their light-evoked responses. We conclude that magnesium ions can modulate the membrane potential of horizontal cells, but this role can be revealed only in the absence of calcium ions. Further, the depolarizing responses seen in low calcium, 'normal' magnesium superfusion indicate that the ionic mechanisms underlying the horizontal cell photoresponse are more complex than has been previously described.
在龟的离体翻转眼杯标本中，使用含有不同浓度二价阳离子和/或螯合剂（乙二胺四乙酸和乙二醇双乙醚二胺四乙酸）的溶液，研究了镁离子对视锥细胞至水平细胞信号传递的调节作用。只要维持“正常”水平的钙离子，从“正常”溶液中去除镁离子对水平细胞没有明显影响。主动去除钙离子但含有“正常”水平镁离子的溶液使水平细胞超极化，降低其光反应的幅度，但也改变了这些光反应的特征；弱刺激引起纯粹的去极化反应，而强光闪光则引发三相反应。用螯合剂从灌流液和视网膜储存中主动去除钙离子和镁离子，使水平细胞去极化并消除光诱发反应。这些溶液也使视锥细胞去极化，但增强了它们的光诱发反应。我们得出结论，镁离子可以调节水平细胞的膜电位，但只有在没有钙离子的情况下，这种作用才能显现出来。此外，在低钙、“正常”镁离子灌流中看到的去极化反应表明，水平细胞光反应的离子机制比以前描述的更为复杂。
