ACIDIFICATION AND BICARBONATE TRANSPORT BY RENAL TUBULES

氨和碳酸氢盐运输的研究正在进行， 从大鼠和兔子分离的灌流小管。 家兔研究 近端直小管显示：1）氨分泌发生 自发地通过NH3向下扩散产生的浓度梯度 通过管腔酸化; 2）近端直肌的S-3部分 肾小管产生自发的不平衡pH， 分泌; 3）S-2近端直小管的渗透性为2x 10 E-2 cm/s，足以说明氨分泌 测得的NH3浓度梯度; 4）NH 4+渗透率为5 × 10 ~（-5） cm/s，这与NH 4+的显著内腔-浴回流一致 in vivo. 家兔外髓集合管实验表明， 1)外条部分产生管腔不平衡pH， 通过增加跨上皮浓度增强氨分泌 差异驱动NH3扩散到管腔中; 2）内部条纹 部分不产生自发不平衡pH，尽管快速 质子分泌率表明存在内源性碳 顶端膜中的脱水酶; 3）外纹和内纹 条纹段分泌质子的速度比 皮质集合管 家兔皮质集合管实验 显示：1）该片段缺乏内源性管腔碳酸酐酶， 可以产生自发的管腔不平衡pH;和2）NH3 渗透率为7 × 10-3 cm/s，该值高到足以解释氨 通过NH3沿着跨上皮NH3被动扩散的分泌 通过管腔酸化产生的浓度梯度。

Studies of ammonia and bicarbonate transport are being conducted in isolated, perfused tubules from rats and rabbits. Studies in rabbit proximal straight tubules showed that 1) ammonia secretion occurs spontaneously by diffusion of NH3 down a concentration gradient generated by luminal acidification; 2) the S-3 portion of the proximal straight tubule generates a spontaneous disequilibrium pH which enhances ammonia secretion; 3) the permeability of the S-2 proximal straight tubule is 2 x 10E-2 cm/s which is adequate to account for ammonia secretion down the measured NH3 concentration gradient; 4) the NH4+ permeability is 5 x 10E-5 cm/s which is consistent with a significant lumen-to-bath backflux of NH4+ in vivo. Experiments in outer medullary collecting ducts from rabbits show 1) the outer stripe portion generates a luminal disequilibrium pH which enhances ammonia secretion by increasing the transepithelial concentration difference driving NH3 diffusion into the lumen; 2) the inner stripe portion does not generate a spontaneous disequilibrium pH despite a rapid rate of proton secretion implying the presence of endogeneous carbonic anhydrase in the apical membrane; 3) both the outer stripe and the inner stripe segments secrete protons at rates that are rapid compared with the cortical collecting duct. Experiments in rabbit cortical collecting ducts show: 1) that this segment lacks endogenous luminal carbonic anhydrase and can generate a spontaneous luminal disequilibrium pH; and 2) that the NH3 permeability is 7 x 10-3 cm/s, a value high enough to account for ammonia secretion by passive diffusion of NH3 down a transepithelial NH3 concentration gradient generated by luminal acidification.