Wittner M, Desfleurs E, Pajaud S, Moine G, de Rouffignac C, Di Stefano A
Service de Biologie Cellulaire, Département de Biologie Cellulaire et Moléculaire, CEA Saclay, 91911 Gif-sur-Yvette, France.
J Membr Biol. 1996 Sep;153(1):27-35. doi: 10.1007/s002329900106.
Recent studies from our laboratory have shown that in the mouse and rat nephron Ca2+ and Mg2+ are not reabsorbed in the medullary part of the thick ascending limb (mTAL) of Henle's loop. The aim of the present study was to investigate whether the absence of transepithelial Ca2+ and Mg2+ transport in the mouse mTAL is due to its relative low permeability to divalent cations. For this purpose, transepithelial ion net fluxes were measured by electron probe analysis in isolated perfused mouse mTAL segments, when the transepithelial potential difference (PDte.) was varied by chemical voltage clamp, during active NaCl transport inhibition by luminal furosemide. The results show that transepithelial Ca2+ and Mg2+ net fluxes in the mTAL are not driven by the transepithelial PDte. At zero voltage, a small but significant net secretion of Ca2+ into the tubular lumen was observed. With a high lumen-positive PDte generated by creating a transepithelial bath-to-lumen NaCl concentration gradient, no Ca2+ and Mg2+ reabsorption was noted; instead significant and sustained Ca2+ and Mg2+ net secretion occurred. When a lumen-positive PDte was generated in the absence of apical furosemide, but in the presence of a transepithelial bath-to-lumen NaCl concentration gradient, a huge Ca2+ net secretion and a lesser Mg2+ net secretion, not modified by ADH, were observed. Replacement of Na+ by K+ in the lumen perfusate induced, in the absence of PDte changes, important but reversible net secretions of Ca2+ and Mg2+. In conclusion, our results indicate that the passive permeability of the mouse mTAL to divalent cations is very low and not influenced by ADH. This nephron segment can secrete Ca2+ and Mg2+ into the luminal fluid under conditions which elicit large lumen-positive transepithelial potential differences. Given the impermeability of this epithelium to Ca2+ and Mg2+, the secretory processes would appear to be of cellular origin.
我们实验室最近的研究表明，在小鼠和大鼠肾单位中，钙（Ca2+）和镁（Mg2+）不会在亨氏袢髓质部的厚壁升支（mTAL）中被重吸收。本研究的目的是调查小鼠mTAL中不存在跨上皮钙和镁转运是否是由于其对二价阳离子的相对低通透性。为此，当通过化学电压钳改变跨上皮电位差（PDte.）时，在管腔应用呋塞米抑制主动氯化钠转运期间，通过电子探针分析测量分离灌注的小鼠mTAL节段中的跨上皮离子净通量。结果表明，mTAL中的跨上皮钙和镁净通量不受跨上皮PDte的驱动。在零电压时，观察到有少量但显著的钙向管腔的净分泌。通过建立跨上皮浴到管腔的氯化钠浓度梯度产生高的管腔正PDte时，未观察到钙和镁的重吸收；相反，出现了显著且持续的钙和镁净分泌。当在无顶端呋塞米但存在跨上皮浴到管腔的氯化钠浓度梯度的情况下产生管腔正PDte时，观察到大量的钙净分泌和较少的镁净分泌，且不受抗利尿激素（ADH）的影响。在管腔灌注液中用钾替代钠，在无PDte变化的情况下，诱导了重要但可逆的钙和镁净分泌。总之，我们的结果表明，小鼠mTAL对二价阳离子的被动通透性非常低且不受ADH影响。在引起大的管腔正跨上皮电位差的条件下，该肾单位节段可将钙和镁分泌到管腔液中。鉴于该上皮对钙和镁的不渗透性，分泌过程似乎起源于细胞。
