Kokko J P
J Clin Invest. 1970 Oct;49(10):1838-46. doi: 10.1172/JCI106401.
The unique membrane characteristics of the thin descending limb of Henle (DLH) play an integral part in the operation of the countercurrent system. We examined these properties in vitro by perfusing isolated thin descending limbs of rabbits. Active transport of NaCl was ruled out by failure to demonstrate either net transport or transmembrane potential difference when perfusing with isosmolal ultrafiltrate of the same rabbit serum as the bath. Transmembrane potential was zero, and net fluid transport was -0.07 +/-0.06 nl mm(-1) min(-1), which also is not significantly different from zero. Passive permeability coefficient for Na(P(Na)) was determined from the disappearance rate of (22)Na from isosmolal perfusion solution. P(Na) was surprisingly low, 1.61 +/-0.27 x 10(-5) cm sec(-1), a figure which is significantly less than P(Na) in the proximal convoluted tubule (PCT). Reflection coefficient for NaCl (sigmaNaCl) was measured by perfusing the tubule with Na-free raffinose solution in a bath of rabbit serum to which sufficient NaCl was added to obtain conditions of zero net fluid movement. The measured sigmaNaCl of 0.96 +/-0.01 is significantly greater than sigmaNaCl in the PCT. Water permeability to osmotic gradients (L(p)) was determined by perfusing with ultrafiltrate of rabbit serum in a bath made hyperosmotic by addition of either 100 mOsm raffinose or NaCl. L(p) with raffinose was 1.71 +/-0.15 x 10(-4) ml cm(-2) sec(-1) atm(-1) and with NaCl 1.62 +/-0.05 x 10(-4) ml cm(-2) sec(-1) atm(-1), indicating much greater water permeability than in the PCT. In each case the measured increase in osmolality of the collected fluid was primarily due to water efflux without significant influx of solute. The finding of low permeability to sodium and high permeability to water is consonant with the hypotheses that high interstitial concentration of Na in the medulla generates an effective osmotic pressure which results in concentration of the fluid as it courses through the DLH primarily by abstraction of water without significant net entry of NaCl.
亨利袢细降支（DLH）独特的膜特性在逆流系统的运作中起着不可或缺的作用。我们通过灌注兔离体细降支在体外研究了这些特性。当用与浴液相同的兔血清等渗超滤液灌注时，未能证明存在净转运或跨膜电位差，从而排除了NaCl的主动转运。跨膜电位为零，净液体转运为-0.07±0.06 nl·mm⁻¹·min⁻¹，这与零也无显著差异。Na的被动通透系数（P(Na)）由等渗灌注溶液中²²Na的消失速率确定。P(Na)出奇地低，为1.61±0.27×10⁻⁵ cm·sec⁻¹，这一数值显著低于近端曲小管（PCT）中的P(Na)。通过在添加了足够NaCl以获得零净液体移动条件的兔血清浴中用无Na棉子糖溶液灌注小管来测量NaCl的反射系数（σNaCl）。测得的σNaCl为0.96±0.01，显著高于PCT中的σNaCl。通过在添加100 mOsm棉子糖或NaCl使其高渗的浴中用兔血清超滤液灌注来确定对渗透梯度的水通透性（L(p)）。用棉子糖时L(p)为1.71±0.15×10⁻⁴ ml·cm⁻²·sec⁻¹·atm⁻¹，用NaCl时为1.62±0.05×10⁻⁴ ml·cm⁻²·sec⁻¹·atm⁻¹，表明水通透性比PCT中高得多。在每种情况下，所收集液体渗透压的测量增加主要是由于水外流，而溶质无明显内流。对钠低通透性和对水高通透性的发现与以下假设一致，即髓质中高浓度的Na间质产生有效的渗透压，这导致液体在通过DLH时主要通过水的抽取而浓缩，而NaCl无明显净进入。
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