Zadunaisky J A, Kinne-Saffran E, Kinne R
Department of Physiology and Biophysics, New York University Medical Center, New York 10016.
Invest Ophthalmol Vis Sci. 1989 Nov;30(11):2332-40.
The retinal pigment epithelium (RPE) interposed between the vascular system of the choroid and the neural retina performs a variety of functions essential for vision. In order to further elucidate the transport functions of the RPE, apical membranes were isolated from the RPE of the dogfish (Squalus acanthias) by differential precipitation with calcium. Na-K-ATPase, an apical marker enzyme in this tissue, was enriched 15-fold in the final membrane fraction. About 50% of the membranes form right-side-out vesicles in which the membrane has retained its in vivo orientation. Sodium uptake into these vesicles as determined by a rapid filtration method was stimulated 37% by the presence of a proton gradient across the membrane (pHi = 6.1, pHo = 8.1). The stimulation was also observed in membrane vesicles "short-circuited" with valinomycin and K. The pH gradient-dependent sodium uptake but not the uptake in the absence of a pH gradient was completely inhibited by 5 X 10(-4) M amiloride, and 56% inhibition was found at 10(-5) M amiloride. The uptake of 22Na was also strongly decreased in the presence of nonradioactively labelled sodium and lithium; potassium was without effect. pH gradient dependence, amiloride sensitivity, saturability and cation specificity of the sodium flux indicate the presence of a Na/H exchanger in the apical membrane of the retinal pigment epithelium. The presence of the Na/H exchange process might have important implications for the control of pH in the subretinal space, optimum intracellular pH of the RPE and the triggering of other functions of the RPE.
视网膜色素上皮（RPE）介于脉络膜血管系统和神经视网膜之间，执行着多种对视觉至关重要的功能。为了进一步阐明RPE的转运功能，通过用钙进行差异沉淀从角鲨（Squalus acanthias）的RPE中分离出顶端膜。Na-K-ATP酶是该组织中的一种顶端标记酶，在最终的膜组分中富集了15倍。约50%的膜形成外翻小泡，其中膜保留了其体内取向。通过快速过滤法测定，跨膜质子梯度（细胞内pH = 6.1，细胞外pH = 8.1）的存在使这些小泡对钠的摄取增加了37%。在与缬氨霉素和钾“短路”的膜小泡中也观察到了这种刺激。5×10⁻⁴ M的氨氯吡脒完全抑制了pH梯度依赖性钠摄取，但不抑制无pH梯度时的摄取，在10⁻⁵ M氨氯吡脒时发现有56%的抑制作用。在存在非放射性标记的钠和锂时，²²Na的摄取也显著降低；钾则无影响。钠通量的pH梯度依赖性、氨氯吡脒敏感性、饱和性和阳离子特异性表明在视网膜色素上皮顶端膜中存在Na/H交换体。Na/H交换过程的存在可能对视网膜下间隙的pH控制、RPE的最佳细胞内pH以及RPE其他功能的触发具有重要意义。
