Frommer J P, Laski M E, Wesson D E, Kurtzman N A
J Clin Invest. 1984 Apr;73(4):1034-45. doi: 10.1172/JCI111288.
The present experiments were designed to localize the sites of carbonic anhydrase-independent bicarbonate reabsorption in the rat kidney and to examine some of its mechanisms. Young Munich-Wistar rats were studied using standard cortical and papillary free-flow micropuncture techniques. Total CO2 (tCO2) was determined using microcalorimetry. In control rats both superficial and juxtamedullary proximal nephrons reabsorbed approximately 95% of the filtered load of bicarbonate. The administration of acetazolamide (20 mg/kg body weight [bw]/h) decreased proximal reabsorption to 65.6% of the filtered load in superficial nephrons (32% was reabsorbed by the proximal convoluted tubule while 31.7% was reabsorbed by the loop segment), and to 38.4% in juxtamedullary nephrons. Absolute reabsorption of bicarbonate was also significantly higher in superficial than in juxtamedullary nephrons after administration of acetazolamide (727 +/- 82 vs. 346 +/- 126 pmol/min; P less than 0.05). The infusion of amiloride (2.5 mg/kg bw/h) to acetazolamide-treated rats increased the fractional excretion of bicarbonate as compared with animals treated with acetazolamide alone (34.9 +/- 1.9 vs. 42.9 +/- 2.1%; P less than 0.01), and induced net addition of bicarbonate between the superficial early distal tubule and the final urine (34.8 +/- 3.0 vs. 42.9 +/- 2.1%; P less than 0.05). Amiloride at this dose did not affect proximal water or bicarbonate transport; our studies localize its site of action to the terminal nephron. Vasa recta (VR) plasma and loop of Henle (LH) tubular fluid tCO2 were determined in control and acetazolamide-treated rats in order to identify possible driving forces for carbonic anhydrase-independent bicarbonate reabsorption in the rat papilla. Control animals showed a tCO2 gradient favoring secretion (LH tCO2, 7.4 +/- 1.7 mM vs. VR tCO2, 19.1 +/- 2.3 mM; P less than 0.005). Acetazolamide administration reversed this chemical concentration gradient, inducing a driving force favoring reabsorption of bicarbonate (LH tCO2, 27.0 +/- 1.4 mM vs. VR tCO2, 20.4 +/- 1.0 mM; P less than 0.005). Our study shows that in addition to the superficial proximal convoluted tubule, the loop segment and the collecting duct show acetazolamide-insensitive bicarbonate reabsorption. No internephron heterogeneity for bicarbonate transport was found in controls. The infusion of acetazolamide, however, induced significant internephron heterogeneity for bicarbonate reabsorption, with superficial nephrons reabsorbing a higher fractional and absolute load of bicarbonate than juxtamedullary nephrons. We think that the net addition of bicarbonate induced by amiloride is secondary to inhibition of voltage-dependent, carbonic anhydrase-independent bicarbonate reabsorption at the level of the collecting duct, which uncovers a greater delivery of carbonate from deeper nephrons to the collecting duct. Finally, our results suggest that carbonic anhydrase-independent bicarbonate reabsorption is partly passive, driven by favorable chemical gradients in the papillary tubular structures, and partly voltage-dependent, in the collecting duct.
本实验旨在确定大鼠肾脏中不依赖碳酸酐酶的碳酸氢盐重吸收位点，并研究其部分机制。采用标准的皮质和乳头游离流微穿刺技术对年轻的慕尼黑-威斯塔大鼠进行研究。使用微量量热法测定总二氧化碳（tCO2）。在对照大鼠中，浅表和近髓近端肾单位重吸收了约95%的滤过碳酸氢盐负荷。给予乙酰唑胺（20mg/kg体重[bw]/h）后，浅表肾单位近端重吸收降至滤过负荷的65.6%（近端曲管重吸收32%，而髓袢段重吸收31.7%），近髓肾单位则降至38.4%。给予乙酰唑胺后，浅表肾单位碳酸氢盐的绝对重吸收也显著高于近髓肾单位（727±82对346±126pmol/min；P<0.05）。与仅用乙酰唑胺治疗的动物相比，向用乙酰唑胺治疗的大鼠输注阿米洛利（2.5mg/kg bw/h）增加了碳酸氢盐的分数排泄（34.9±1.9对42.9±2.1%；P<0.01），并导致浅表早期远端小管与终尿之间碳酸氢盐的净增加（34.8±3.0对42.9±2.1%；P<0.05）。此剂量的阿米洛利不影响近端水或碳酸氢盐转运；我们的研究将其作用位点定位于终末肾单位。测定对照和用乙酰唑胺治疗的大鼠的直小血管（VR）血浆和髓袢（LH）管腔液tCO2，以确定大鼠乳头中不依赖碳酸酐酶的碳酸氢盐重吸收的可能驱动力。对照动物显示有利于分泌的tCO2梯度（LH tCO2，7.4±1.7mM对VR tCO2，19.1±2.3mM；P<0.005）。给予乙酰唑胺可逆转这种化学浓度梯度，产生有利于碳酸氢盐重吸收的驱动力（LH tCO2，27.0±1.4mM对VR tCO2，20.4±1.0mM；P<0.005）。我们的研究表明，除了浅表近端曲管外，髓袢段和集合管也显示出对乙酰唑胺不敏感的碳酸氢盐重吸收。在对照中未发现肾单位间碳酸氢盐转运的异质性。然而，给予乙酰唑胺可导致肾单位间碳酸氢盐重吸收的显著异质性，浅表肾单位重吸收的碳酸氢盐分数和绝对负荷高于近髓肾单位。我们认为，阿米洛利诱导的碳酸氢盐净增加是由于抑制了集合管水平上依赖电压的、不依赖碳酸酐酶的碳酸氢盐重吸收，从而使更多的碳酸盐从更深的肾单位输送到集合管。最后，我们的结果表明，不依赖碳酸酐酶的碳酸氢盐重吸收部分是被动的，由乳头管状结构中的有利化学梯度驱动，部分是依赖电压的，发生在集合管中。
