Rocha A S, Kokko J P
J Clin Invest. 1973 Mar;52(3):612-23. doi: 10.1172/JCI107223.
Transport of NaCl and water was examined in the rabbit medullary thick ascending limb of Henle (ALH) by perfusing isolated segments of these nephrons in vitro. Osmotic water permeability was evaluated by perfusing tubules against imposed osmotic gradients. In these experiments the net transport of fluid remained at zero when segments of thick ALH were perfused with isotonic ultrafiltrate in a bath of rabbit serum in which the serum osmolality was increased by the addition of either 239+/-8 mosmol/liter of raffinose or 232+/-17 mosmol of NaCl indicating that the thick ascending limb of Henle is impermeant to osmotic flow of water. When these tubules were perfused at slow rates with isosmolal ultrafiltrate of same rabbit serum as used for the bath, the effluent osmolality was consistently lowered to concentrations less than the perfusate and the bath. That this decrease in collected fluid osmolality represented salt transport was demonstrated in a separate set of experiments in which it was shown that the sodium and chloride concentrations decreased to 0.79+/-0.02 and 0.77+/-0.02 respectively when compared with the perfusion fluid concentrations. In each instance the simultaneously determined transtubular potential difference (PD) revealed the lumen to be positive with the magnitude dependent on the perfusion rate. At flow rates above 2 nl.min(-1), the mean transtubular PD was stable and equal to 6.70+/-0.34 mv. At stop-flow conditions this PD became more positive. Ouabain and cooling reversibly decreased the magnitude of this PD. The transtubular PD remained positive, 3.3+/-0.2 mV, when complete substitution of Na by choline was carried out in both the perfusion fluid and the bathing media. These results are interpreted to indicate that the active transport process is primarily an electrogenic chloride mechanism. The isotopic permeability coefficient for Na was 6.27+/-0.38 x 10(-5) cm.s(-1) indicating that the thick ALH is approximately as permeable to Na as the proximal convoluted tubule. The chloride permeability coefficient for the thick ALH was 1.06+/-0.12 x 10(-5) cm.s(-1) which is significantly less than the chloride permeability of the proximal tubule. These data demonstrate that the medullary thick ascending limb of Henle is water impermeable while having the capacity for active outward solute transport as a consequence of an electrogenic chloride pump. The combination of these characteristics allows this segment to generate a dilute tubular fluid and participate as the principal energy source for the overall operation of the countercurrent multiplication system.
通过在体外灌注兔肾髓质亨氏袢升支粗段（ALH）的分离节段，研究了NaCl和水的转运。通过在设定的渗透梯度下灌注肾小管来评估渗透水通透性。在这些实验中，当用等渗超滤液灌注粗ALH节段时，在添加了239±8 mosmol/升棉子糖或232±17 mosmol NaCl的兔血清浴中，液体的净转运保持为零，这表明亨氏袢升支粗段对水的渗透流是不通透的。当以缓慢速率用与浴液相同的兔血清等渗超滤液灌注这些肾小管时，流出液的渗透压持续降低至低于灌注液和浴液的浓度。在另一组实验中证明，收集的液体渗透压的这种降低代表盐转运，该实验表明，与灌注液浓度相比，钠和氯的浓度分别降至0.79±0.02和0.77±0.02。在每种情况下，同时测定的跨管电位差（PD）显示管腔为正，其大小取决于灌注速率。在流速高于2 nl·min⁻¹时，平均跨管PD稳定且等于6.70±0.34 mV。在停流条件下，该PD变得更正。哇巴因和冷却可逆地降低了该PD的大小。当在灌注液和浴液中都用胆碱完全替代Na时，跨管PD仍为正，为3.3±0.2 mV。这些结果被解释为表明主动转运过程主要是一种电致氯机制。Na的同位素渗透系数为6.27±0.38×10⁻⁵ cm·s⁻¹，表明粗ALH对Na的通透性与近端曲管大致相同。粗ALH的氯渗透系数为1.06±0.12×10⁻⁵ cm·s⁻¹，明显低于近端小管的氯通透性。这些数据表明，肾髓质亨氏袢升支粗段对水不通透，同时由于电致氯泵而具有主动向外溶质转运的能力。这些特性的组合使该节段能够产生稀释的小管液，并作为逆流倍增系统整体运作的主要能量来源发挥作用。
