Sodium chloride and water transport in the descending limb of Henle

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 ²²Na from isosmolal perfusion solution. P_Na was surprisingly low, 1.61 ±0.27 × 10^-5 cm sec^-1, a figure which is significantly less than P_Na in the proximal convoluted tubule (PCT). Reflection coefficient for NaCl (σNaCl) 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 σNaCl of 0.96 ±0.01 is significantly greater than σNaCl 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 × 10^-4 ml cm^-2 sec^-1 atm^-1 and with NaCl 1.62 ±0.05 × 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.     

Urea transport in the proximal tubule and the descending limb of Henle

Urea transport in proximal convoluted tubule (PCT) and descending limb of Henle (DLH) was studied in perfused segments of rabbit nephrons in vitro.Active transport of urea was ruled out in a series of experiments in which net transport of fluid was zero. Under these conditions the collected urea concentration neither increased nor decreased when compared to the mean urea concentration in the perfusion fluid and the bath.Permeability coefficient for urea (P(urea)) was calculated from the disappearance of urea-(14)C added to perfusion fluid. Measurements were obtained under conditions of zero net fluid movement: DLH was perfused with isosmolal ultrafiltrate (UF) of the same rabbit serum as the bath, while PCT was perfused with equilibrium solution (UF diluted with raffinose solution for fluid [Na] = 127 mEq/liter). Under these conditions P(urea) per unit length was 3.3+/-0.4 x 10(-7) cm(2)/sec (5.3+/-0.6 x 10(-5) cm/sec assuming I.D. = 20mu) in PCT and 0.93+/-0.4 x 10(-7) cm(2)/sec (1.5+/-0.5 x 10(-5) cm/sec) in DLH. When compared to previously published results, these values show that the PCT is 2.5 times less permeable to urea than to Na, while the DLH is as impermeable to urea as to Na. These results further indicate that the DLH is less permeable to both Na and urea than the PCT.The reflection coefficient for urea, sigma(urea), was calculated as the ratio of induced solution efflux when 95 mOsm/liter of urea was added to the bath, as compared to net fluid movement induced by addition to the bath of equivalent amount of raffinose, sigma(urea) in DLH is 0.95+/-0.4 as compared to 0.91+/-0.05 in PCT. sigma(urea) in DLH is approximately equal to sigma(Na); however, sigma(urea) in PCT is higher than sigma(Na) (0.68).Several types of studies were conducted to examine the role of urea and urea plus sodium chloride in concentrating the fluid in the DLH. From the obtained results it was concluded that the intraluminal fluid of DLH is primarily concentrated by abstraction of water without significant net entry of solute. These results are discussed with respect to possible significance in the overall operation of the countercurrent system.     

Calcium transport in the pars recta and thin descending limb of Henle of the rabbit, perfused in vitro.

Unidirectional calcium flux (JCa) in the superficial pars recta and thin descending limb of Henle (DLH) was examined by the isolated tubule microperfusion technic using 45Ca as the isotopic tracer. In the pars recta sequential measurements of lumen-to-bath flux (JlbCa) and bath-to-lumen flux (JblCa) revealed: JlbCa 22.4 +/- 4.18, JblCa 7.97 +/- 1.95, and calculated net efflux of calcium (JnetCa 13.0 +/- 1.74 peq min-1 mm-u. To measure JnetCa directly, 45Ca of identical specific activity was used to bathe and perfuse the tubule. These studies revealed: JlbCa 14.1 +/- 1.33, JnetCa 11.2 +/- 1.15, and calculated JblCa 2.91 +/- 0.49 peq min-1 mm-1. The addition of ouabain (10 microM) resulted in a rise in potential difference and a fall in water absorption, but not a statistically significant change in JnetCa. Tubules studies at 25 degrees C bath temperature, showed no significant JnetCa, and upon heating the bath to 37 degrees C, showed JnetCa of 3.75--5.00 peg min-1 mm-1. Unidirectional and net efflux studies in six DLH showed no significant transport of calcium. These studies demonstrate substantial active absorption of calcium by the superficial pars recta, which is not inhibitable by ouabain but is inhibited by lowering bath temperature to 25 degrees C. No significant calcium transport was found in the DLH using identical technics.     

Requirement of aquaporin-1 for NaCl-driven water transport across descending vasa recta

Deletion of AQP1 in mice results in diminished urinary concentrating ability, possibly related to reduced NaCl- and urea gradient-driven water transport across the outer medullary descending vasa recta (OMDVR). To quantify the role of AQP1 in OMDVR water transport, we measured osmotically driven water permeability in vitro in microperfused OMDVR from wild-type, AQP1 heterozygous, and AQP1 knockout mice. OMDVR diameters in AQP1(-/-) mice were 1.9-fold greater than in AQP1(+/+) mice. Osmotic water permeability (P(f)) in response to a 200 mM NaCl gradient (bath > lumen) was reduced about 2-fold in AQP1(+/-) mice and by more than 50-fold in AQP1(-/-) mice. P(f) increased from 1015 to 2527 microm/s in AQP1(+/+) mice and from 22 to 1104 microm/s in AQP1(-/-) mice when a raffinose rather than an NaCl gradient was used. This information, together with p-chloromercuribenzenesulfonate inhibition measurements, suggests that nearly all NaCl-driven water transport occurs by a transcellular route through AQP1, whereas raffinose-driven water transport also involves a parallel, AQP1-independent, mercurial-insensitive pathway. Interestingly, urea was also able to drive water movement across the AQP1-independent pathway. Diffusional permeabilities to small hydrophilic solutes were comparable in AQP1(+/+) and AQP1(-/-) mice but higher than those previously measured in rats. In a mathematical model of the medullary microcirculation, deletion of AQP1 resulted in diminished concentrating ability due to enhancement of medullary blood flow, partially accounting for the observed urine-concentrating defect.     

Mechanism of sodium and chloride transport in the thin ascending limb of Henle.

Our previous in vitro studies have disclosed that the thin ascending limb of Henle (tALH) possesses some unique membrane characteristics. In those studies we failed to demonstrated active transport of sodium chloride by the tALH, although it was shown that the isotopic permeability to sodium and chloride was unusually high. However, we did not examine the mechanisms by which the apparent high permeation of sodium chloride occurs. Thus the purpose of the present studies was to elucidate the mechanism of sodium chloride transport across the isolated tALH of the rabbit by conducting four different types of studies: (1) comparison of the observed chloride and sodium flux ratios to those predicted by Ussing's equation under imposed salt concentration gradients; (2) kinetic evaluation of chloride and sodium fluxes; (3) examination of the effect of bromide on the kinetics of chloride transport; and (4) experiments to test for the existence of exchange diffusion of chloride. In the first set of studies the predicted and the theoretical flux ratios of sodium were identical in those experiments in which sodium chloride was added either to the perfusate or to the bath. However, the observed chloride flux ratio, lumen-to-bath/bath-to-lumen, was significantly lower than that predicted from Ussing's equation when 100 mM sodium chloride was added to the bath. In the second set of experiments the apparent isotopic permeability for sodium and for chloride was measured under varying perfusate and bath NaCl concentrations. There was no statistical change in the apparent sodium permeability coefficient when the NaCl concentration was raised by varying increments from 85.5 to 309.5 mM. However, permeation of 36Cl decrease significantly with an increase in Cl from 73.6 to 598.6 mM. These events could be explained by a two component chloride transport process consisting of simple diffusion and a saturable facilitated diffusion process with a Vmax = 3.71 neq mm-1 min-1. In the third set of studies it was shown that bromide inhibits transport of chloride and that the magnitude of inhibition is dependent on chloride concentrations. The fourth set of studies ruled out the existence of exchange diffusion. In conclusion, these studies indicate that sodium transport across tALH is by simple passive diffusion, while chloride transport across tALH involves at least two mechanisms: (1) simple passive diffusion; and (2) a specific membrane interaction process (carrier-mediated) which is competitively inhibited by bromide.     

Lung fluid transport in aquaporin-1 and aquaporin-4 knockout mice

The mammalian lung expresses water channel aquaporin-1 (AQP1) in microvascular endothelia and aquaporin-4 (AQP4) in airway epithelia. To test whether these water channels facilitate fluid movement between airspace, interstitial, and capillary compartments, we measured passive and active fluid transport in AQP1 and AQP4 knockout mice. Airspace-capillary osmotic water permeability (Pf) was measured in isolated perfused lungs by a pleural surface fluorescence method. Pf was remarkably reduced in AQP1 (-/-) mice (measured in cm/s x 0.001, SE, n = 5-10: 17 +/- 2 [+/+]; 6.6 +/- 0.6 AQP1 [+/-]; 1.7 +/- 0.3 AQP1 [-/-]; 12 +/- 1 AQP4 [-/-]). Microvascular endothelial water permeability, measured by a related pleural surface fluorescence method in which the airspace was filled with inert perfluorocarbon, was reduced more than 10-fold in AQP1 (-/-) vs. (+/+) mice. Hydrostatically induced lung interstitial and alveolar edema was measured by a gravimetric method and by direct measurement of extravascular lung water. Both approaches indicated a more than twofold reduction in lung water accumulation in AQP1 (-/-) vs. (+/+) mice in response to a 5- to 10-cm H2O increase in pulmonary artery pressure for five minutes. Active, near-isosmolar alveolar fluid absorption (Jv) was measured in in situ perfused lungs using 125I-albumin as an airspace fluid volume marker. Jv (measured in percent fluid uptake at 30 min, n = 5) in (+/+) mice was 6.0 +/- 0.6 (37 degrees C), increased to 16 +/- 1 by beta-agonists, and inhibited to less than 2.0 by amiloride, ouabain, or cooling to 23 degrees C. Jv (with isoproterenol) was not affected by aquaporin deletion (18.9 +/- 2.2 [+/+]; 16.4 +/- 1.5 AQP1 [-/-]; 16.3 +/- 1.7 AQP4 [-/-]). These results indicate that osmotically driven water transport across microvessels in adult lung occurs by a transcellular route through AQP1 water channels and that the microvascular endothelium is a significant barrier for airspace-capillary osmotic water transport. AQP1 facilitates hydrostatically driven lung edema but is not required for active near-isosmolar absorption of alveolar fluid.     

Sodium chloride, urea, and water transport in the thin ascending limb of Henle. Generation of osmotic gradients by passive diffusion of solutes

Studies were designed to examine whether the thin ascending limb of Henle (tALH) decreases its luminal solute concentration by an active or a passive transport process. In all experiments isolated segments of rabbit tALH were perfused in vitro. When tubules were perfused with solutions identical to the bath, active transport of NaCl was excluded by the following: (a) osmolality of the collected fluid remained unchanged and the same as the bath. (b) net water reabsorption could not be demonstrated, and (c) transtubular potential difference was zero. Isotopic permeability coefficients (x 10(-5) cm s-1) were calculated from the disappearance rate of the respective isotope added to the perfusate. These values indicate that tALH is moderately permeable to [14C]urea (6.97 +/- 1.95) while having a higher permeability to 22Na (25.5 +/- 1.8) and [not readable: see text]Cl (117 +/- 9.1) than any other segment similarly studied. The influx (bath-to-lumen) isotopic permeabilities were not statistically different from the above efflux permeabilities. Osmotic water permeability was immeasurably small. When tALH were perfused with a 600 mosmol/liter solution predominantly of NaCl against a 600 mosmol/liter bath in which 50% of osmolality was NaCl and 50% urea (to simulate in vivo papillary interstitium), the collected fluid osmolality was decreased significantly below that of the bath (300 mosmol/liter/mm of tubule). The decrease in osmolality was due to greater efflux of NaCl as compared to influx of urea. We conclude that active transport of salt by the tALH was not detected by the experimental protocol of the current studies, and that the unique membrane characteristics of tALH allows for generation of osmotic gradients (lumen less concentrated than adjacent surroundings) on purely passive mechanisms when perfused with isosmolal salt solutions in a bath with appropriate salt and urea concentrations. These findings are consistent with the passive counter-current model previously proposed from this laboratory.     

Furosemide acts on short loop of descending thin limb, but not on long loop

In order to elucidate the tubular sites of action of loop diuretics such as furosemide, bumetanide and ethacrynic-cysteine complex within isolated rat descending thin limbs, cellular ATP was measured by luciferin-luciferase technique. When short descending thin limbs of Henle's loop (SDL) were incubated in the absence of exogenous substrate at 37 degrees C, cellular ATP content was decreased in a time-dependent manner (up to 49% after 60 min). This ATP decrease, however, was retarded significantly in the presence of loop diuretics at 60 min. The mean percentage of change in ATP compared with the control for each loop diuretic in SDL was as follows: 10(-5) M furosemide, 178%; 10(-5) M bumetanide, 189%; and 10(-7) M ethacrynic-cysteine complex, 154%; respectively. To the contrary, cellular ATP in long descending thin limb of Henle's loop (LDL) was not changed by loop diuretics compared with the control. A similar protection against ATP depletion was observed in the medullary thick ascending limb of Henle's loop, in which the mean percentage was as follows: 10(-5) M furosemide, 163%; 10(-5) M bumetanide, 187%; and 10(-7) M ethacrynic-cysteine complex, 134%. Similarly to LDL, the cellular ATP did not change in outer medullary collecting tubule. From these results, we conclude that loop diuretics act on the isolated rat SDL, but not on LDL.     

Effects of spermine on water absorption, polyethylene glycol 4000 permeability and collagenase activity in rat descending colon in vivo.

1. The effects of spermine in the concentration range 0-10 mmol/l on (a) the fluid absorption, (b) the polyethylene glycol permeability, (c) the release of collagenase activity activity into the lumen and (d) the histological appearance of rat descending colon were examined. 2. Spermine (5 mmol/l) decreased fluid absorption from 48.83 +/- 2.98 (n = 7) to 23.98 +/- 2.32 (n = 6) microliters h-1 cm-2 (P < 0.01); polyethylene glycol 4000 permeability was increased from 0.030 +/- 0.001 (n = 7) to 0.047 +/- 0.003 (n = 6) cm/h (P < 0.01) and luminal collagenase activity increased from a negligible control value to 250 +/- 39 (n = 6) units/ml (P < 0.001). Spermine also caused oedema formation within the mucosal interstitial fluid, without inducing an overt breakdown of the mucosa at the luminal surface. 3. Polyamine-free dialysed seminal plasma had no effect on polyethylene glycol 4000 permeability, although it still caused a significant decrease in colonic fluid absorption from 48.83 +/- 2.98 (n = 7) (control) to 31.41 +/- 2.08 (n = 5) microliters h-1 cm-2 (P < 0.01). 4. Low-molecular-mass heparin (600 units/ml) prevented the spermine (5 mmol/l)- and whole-semen-induced increase in colonic polyethylene glycol 4000 permeability and reduced the effect of semen on fluid absorption by 63% (P < 0.001) and that of spermine by 56% (P < 0.01). 5. The Zn2+ chelator and collagenase inhibitor o-phenanthroline reduced the effect of spermine on fluid absorption and polyethylene glycol 4000 permeability by 100% (P < 0.001) and on interstitial oedema formation. o-Phenanthroline also reduced the effects of whole semen on fluid absorption (by 70%, P < 0.01) and on polyethylene glycol 4000 permeability by 95%, P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypoxia induces severe right ventricular dilatation and infarction in heme oxygenase-1 null mice

Heme oxygenase (HO) catalyzes the oxidation of heme to generate carbon monoxide (CO) and bilirubin. CO increases cellular levels of cGMP, which regulates vascular tone and smooth muscle development. Bilirubin is a potent antioxidant. Hypoxia increases expression of the inducible HO isoform (HO-1) but not the constitutive isoform (HO-2). To determine whether HO-1 affects cellular adaptation to chronic hypoxia in vivo, we generated HO-1 null (HO-1(-/-)) mice and subjected them to hypoxia (10% oxygen) for five to seven weeks. Hypoxia caused similar increases in right ventricular systolic pressure in wild-type and HO-1(-/-) mice. Although ventricular weight increased in wild-type mice, the increase was greater in HO-1(-/-) mice. Similarly, the right ventricles were more dilated in HO-1(-/-) mice. After seven weeks of hypoxia, only HO-1(-/-) mice developed right ventricular infarcts with organized mural thrombi. No left ventricular infarcts were observed. Lipid peroxidation and oxidative damage occurred in right ventricular cardiomyocytes in HO-1(-/-), but not wild-type, mice. We also detected apoptotic cardiomyocytes surrounding areas of infarcted myocardium by terminal deoxynucleotide transferase-mediated dUTP nick end-labeling (TUNEL) assays. Our data suggest that in the absence of HO-1, cardiomyocytes have a maladaptive response to hypoxia and subsequent pulmonary hypertension. J.Clin. Invest. 103:R23-R29 (1999).     

A null deficiency allele of alpha 1-antitrypsin, QOludwigshafen, with altered tertiary structure.

The most common deficiency allele of the plasma protease inhibitor alpha 1-antitrypsin (alpha 1AT) is PI*Z. Some rare deficiency alleles of alpha 1AT produce low but detectable amounts of plasma alpha 1AT (1-20% of normal), which can be differentiated by isoelectric focusing. Others, designated null (QO) alleles, produce no alpha 1AT detectable by routine quantitative methods. We have previously described a method using DNA polymorphisms, haplotypes, and polyacrylamide isoelectric focusing gels, to differentiate various deficiency alleles. Based on haplotypes, we previously identified, in eight patients, five different null alleles, four of which had been previously sequenced. We have now analyzed all 12 null alleles in these eight patients, using allele-specific oligonucleotide probes, and have identified six different null alleles. We have cloned and sequenced one of these, PI*QOludwigshafen, which has a base substitution in exon II, replacing isoleucine 92 in the normal sequence with an asparagine. This substitution of a polar for a nonpolar amino acid occurs in one of the alpha-helices and is predicted to disrupt the tertiary structure. A total of 13 different alpha 1AT deficiency alleles, 6 of them null alleles, have been sequenced to date.     

高压氧对急性脑缺血再灌注小鼠AQP-4表达及血脑屏障通透性的影响

目的:探讨高压氧(HBO)对缺血再灌注小鼠脑组织中水通道蛋白-4 mRNA(AQP-4)的表达及血脑屏障通透性的影响。方法:复制清醒小鼠脑缺血再灌注模型,并于再灌注期间行0.25MPa HBO治疗5次,采用逆转录多聚酶链反应(RT-PCR)方法、比色法及干湿法分别检测脑组织中AQP-4 mRNA的表达、伊文思蓝(EB)的含量及脑组织含水量。结果:脑缺血再灌注组AQP-4 mRNA的表达、EB的含量及脑组织含水量明显高于假手术组,差异有显著性意义(P<0.01)。高压氧组与假手术组相比,脑组织中AQP-4 mRNA的表达、EB的含量及脑组织含水量变化无显著性差异。HBO 脑缺血再灌注组脑组织中AQP-4 mRNA的表达、EB的含量及脑组织含水量较脑缺血再灌注组明显降低,差异有非常显著性意义(P<0.01)。结论:高压氧可通过降低脑缺血再灌注中AQP-4 mRNA的表达,降低血脑屏障的通透性,减轻脑水肿。     

Randall's plaque of patients with nephrolithiasis begins in basement membranes of thin loops of Henle

Our purpose here is to test the hypothesis that Randall's plaques, calcium phosphate deposits in kidneys of patients with calcium renal stones, arise in unique anatomical regions of the kidney, their formation conditioned by specific stone-forming pathophysiologies. To test this hypothesis, we performed intraoperative biopsies of plaques in kidneys of idiopathic-calcium-stone formers and patients with stones due to obesity-related bypass procedures and obtained papillary specimens from non-stone formers after nephrectomy. Plaque originates in the basement membranes of the thin loops of Henle and spreads from there through the interstitium to beneath the urothelium. Patients who have undergone bypass surgery do not produce such plaque but instead form intratubular hydroxyapatite crystals in collecting ducts. Non-stone formers also do not form plaque. Plaque is specific to certain kinds of stone-forming patients and is initiated specifically in thin-limb basement membranes by mechanisms that remain to be elucidated.     

水通道蛋白1、5在肺出血新生大鼠肺组织中的表达

目的研究新生大鼠肺出血发生的过程中水通道蛋白1、5在肺中表达的变化和意义。方法将80只4～5 d龄新生SD大鼠分为4组,对照组1组和实验组3组,对照组为常温常氧6 h;实验组分别为低温缺氧1 h复温供氧2 h、低温缺氧2 h复温供氧2 h和低温缺氧4 h复温供氧2 h,用断头法处死大鼠后开胸取肺做病理切片和免疫组化染色。结果观察到随着低温缺氧时间的延长病理改变可从正常肺组织向着肺水肿、点状肺出血、局灶性肺出血至弥漫性肺出血的方向发展。水通道蛋白1(AQP 1)主要表达于胸膜脏层周围毛细血管内皮细胞腔膜面和基侧膜面以及脏层胸膜的间皮细胞,肺泡Ⅱ型上皮细胞顶膜面也有少量表达,AQP 1还存在于气管腔上皮细胞顶膜面和支气管黏膜下腺上皮细胞。水通道蛋白5(AQP 5)表达于Ⅰ型上皮细胞的顶膜。随着低温缺氧时间的延长,复温供氧后AQP 1在肺毛细血管内皮细胞表达明显下降,这种减少全肺均能观察到,肺泡Ⅰ型上皮细胞AQP 5的表达也一致减少。对应的病理改变为细胞炎症反应,肺水肿、肺泡腔及间质出血。结论 AQP 1、AQP 5与新生大鼠肺出血关系密切,AQP 1、AQP 5可能与肺水肿、肺出血的形成有关。通过调节AQP 1及AQP 5的表达可能预防新生儿肺出血的发生。     

血管生成素-1对脓毒症小鼠肺血管通透性的影响

目的 探讨外源性血管生成素-1(Ang-1)对脓毒症小鼠肺血管通透性的影响.方法 腹腔注射脂多糖(LPS)制作脓毒症小鼠模型.64只BALB/c小鼠随机分为NS组、Ang-1组、LPS组和LPS+ Ang-1组(n=16).各组经处理12 h后,分别收集血浆、肺泡灌洗液及肺组织标本.ELISA法测定血浆Ang-1、Ang-2浓度并计算Ang-2/Ang-1比值,测定血浆、肺泡灌洗液的总蛋白浓度并计算肺通透指数(LPI),测定肺组织湿干比及观察肺组织病理变化.结果 Ang-1组血浆Ang-1浓度较NS组明显升高(P＜0.05).LPS组和LPS+ Ang-1组血浆Ang-1浓度较NS组均明显降低(P＜0.01),而血浆Ang-2浓度及Ang-2/Ang-1比值较NS组均明显升高(P＜0.01).LPS+ Ang-1组血浆Ang-1浓度较LPS组明显升高(P＜0.01),且血浆Ang-2浓度及Ang-2/Ang-1比值较LPS组明显降低(P＜0.01).Ang-2/Ang-1比值与小鼠肺湿干比呈明显正相关(r=0.76,P＜0.01).LPS组肺湿干比、肺通透指数较NS组明显升高(P＜0.01),LPS+ Ang-1组肺湿干比、肺通透指数较LPS组明显降低(P＜0.01),且肺组织病理渗出水肿较LPS组明显好转.结论 外源性Ang-1通过调节脓毒症小鼠Ang-2/Ang-1失平衡而发挥降低肺血管通透性及改善肺水肿作用.     

Effect of acidosis on chloride transport in the cortical thick ascending limb of Henle perfused in vitro.

The present studies examined the effect of acute in vitro acidosis on chloride reabsorption in the rabbit cortical thick ascending limb of Henle (cTALH). Four protocols were used: hypercapnic acidosis; "isocapnic" peritubular acidosis (bath bicarbonate reduction to 10 mM); isocapnic luminal acidosis (luminal bicarbonate reduction to 10 mM); isocapnic peritubular acidosis in the absence of luminal potassium. Transepithelial voltage (VT) decreased during hypercapnic acidosis and increased with recovery. Chloride reabsorption (pmol X mm-1 X min-1) decreased from 50.3 +/- 8.4 to 15.7 +/- 5.6, then increased to 45.6 +/- 11.1 with recovery. Likewise, VT was decreased reversibly during isocapnic peritubular acidosis, and chloride reabsorption decreased by 60%. Chloride reabsorption was greater (28.3 +/- 3.6) when tubules were perfused at normal luminal pH than at an acidotic luminal pH (11.4 +/- 4.5; P less than 0.05). Luminal potassium removal reduced chloride transport, and acidosis had no significant additional effect. Decreased chloride reabsorption in the cTALH during acidosis could contribute to the chloruresis associated with systemic acidosis. The symmetrical nature of this effect suggests that acidosis inhibits chloride reabsorption through an effect on cytosolic pH.     

Calcium transport in the thick ascending limb of Henle. Heterogeneity of function in the medullary and cortical segments.

Calcium transport was studied in medullary and cortical segments of the thick ascending limb of Henle perfused in vitro. 45Ca was added to the perfusate for measuring lumen-to-bath flux (JlbCa), to the bath for measuring bath-to-lumen flux (JblCa), or to both perfusate and bath for measuring net flux (JnetCa). In the medullary segment JlbCa exceeding JblCa and the efflux:influx coefficient ratio was not different from the value predicted from the observed potential difference (PD). In the cortical segments, however, efflux:influx coefficient ratio was greater than the value predicted from the PD, suggesting that calcium transport in this segment may be active, while it is passive in the medullary segment. Furosemide, which reversibly decreases PD in both cortical and medullary segments, inhibited JlbCa only in the medullary segment. Parathyroid hormone (PTH), on the other hand, had no effect on JnetCa in the medullary segment, but it significantly augmented JnetCa in the cortical segment. These results indicate that calcium transport in the thick ascending limb is heterogeneous. In the medullary segment it is passive, inhibited by furosemide and not influenced by PTH. In the cortical segment, however, calcium transport appears to be active, not inhibited by furosemide and stimulated by PTH.