Di Stefano A, Wittner M, Gebler B, Greger R
Départment de Biologie, Centre d'Etudes Nucléaires de Saclay, Gif-sur-Yvette, France.
Ren Physiol Biochem. 1988 Jan-Feb;11(1-2):70-9. doi: 10.1159/000173151.
We have tested whether increased Ca++ and Mg++ concentrations have an effect on transepithelial voltage (PDte) and transepithelial resistance (Rte) in isolated perfused cortical thick ascending limbs (cTAL) of rabbit kidney. The divalent cations added at 2.5, 5.0 and 10.0 mmol.l-1 to the lumen or peritubular bath perfusate led to a concentration-dependent increase in Rte. The maximal response in Rte was observed between 5 and 10 mmol.l-1. No significant change in active transepithelial potential difference (PDte) was observed. The increase in Rte still occurred when the transcellular current was reduced by Ba++ (3 mmol.l-1) added to the lumen perfusate. This suggests that the increase in Rte caused by Ca++ and Mg++ is due to a modification of the paracellular shunt pathway. In the absence of active transport, i.e. when furosemide (5.10(-5) mol.l-1) was added to the lumen perfusate. Ca++ and Mg++ reduced the transepithelial diffusion potential generated by a NaCl gradient established across the epithelium, and thus produced a reduction of the relative permeability for Na+ over Cl- (PNa+/PCl-) of the paracellular shunt pathway. This indicates that divalent cations increase Rte by reducing the sodium permeability of the tight junctions. The observed Ca++ and Mg++ induced reduction of the sodium permeability of the paracellular pathway corresponds to a decrease in net Na+ reabsorption by 5-10%. Since it has been demonstrated that peptide hormones such as parathyrin (PTH) modulate divalent cation and NaCl reabsorptions, in a second series of experiments we tested the effects of PTH (2-20 USP.l-1) and dbcAMP (10(-3) mol.l-1) on PDte and Rte of isolated perfused cTAL segments of rabbit nephron. Neither Rte nor PDte were affected by PTH or dbcAMP.
我们已经测试了增加钙离子（Ca++）和镁离子（Mg++）浓度是否会对兔肾离体灌注皮质厚升支（cTAL）的跨上皮电压（PDte）和跨上皮电阻（Rte）产生影响。向管腔或肾小管周浴灌流液中添加浓度为2.5、5.0和10.0 mmol·l-1的二价阳离子会导致Rte呈浓度依赖性增加。在5至10 mmol·l-1之间观察到Rte的最大反应。未观察到主动跨上皮电位差（PDte）有显著变化。当向管腔灌流液中添加钡离子（Ba++，3 mmol·l-1）使跨细胞电流降低时，Rte仍会增加。这表明由Ca++和Mg++引起的Rte增加是由于细胞旁分流途径的改变。在无主动转运的情况下，即向管腔灌流液中添加呋塞米（5×10-5 mol·l-1）时，Ca++和Mg++降低了由跨上皮建立的氯化钠梯度产生的跨上皮扩散电位，从而使细胞旁分流途径对钠离子（Na+）相对于氯离子（Cl-）的相对通透性（PNa+/PCl-）降低。这表明二价阳离子通过降低紧密连接的钠通透性来增加Rte。观察到的Ca++和Mg++诱导的细胞旁途径钠通透性降低对应于净Na+重吸收减少5%至10%。由于已经证明甲状旁腺素（PTH）等肽类激素可调节二价阳离子和NaCl的重吸收，在第二系列实验中，我们测试了PTH（2 - 20 USP·l-1）和二丁酰环磷腺苷（dbcAMP，10-3 mol·l-1）对兔肾单位离体灌注cTAL节段的PDte和Rte的影响。Rte和PDte均不受PTH或dbcAMP的影响。
