Fujimoto M, Naito K, Kubota T
Membr Biochem. 1980;3(1-2):67-97. doi: 10.3109/09687688009063879.
A micropuncture study was performed on the bullfrog kidney proximal tubular cells utilizing double-barreled ion-selective microelectrodes. The intracellular of Na+, K+, Cl-, HCO3(-) and pH were determined to be 21.6 mEq/L, 67.4 mEq/L, 9.9 mEq/L, 20.2 mEq/L, and 7.49 pH units, respectively. In the extracellular fluid the following activities were found: Na+, 87.4 mEq/L; K+, 2.64 mEq/L; Cl-, 72.5 mEq/L; HCO3(-), 17.9 mEq/L; and pH, 7.66. The membrane potential difference was 68.4 mV and 60.4 mV across the peritubular and brush borders, respectively. The electrochemical potential differences across the individual borders of the proximal tubule cells were separately calculated by setting the intracellular level of both electrical and chemical potentials at zero for convenience. From these analyses, the following interpretations are made. (1) In the net reabsorption of Na+, luminal Na+ enters the cell along a 95-mV gradient across the luminal border and is pumped out to the interstitium against a 104 mV gradient. In the reabsorption of bicarbonate, an uphill pump of about 69 mV (about 70% of the Na+ entry gradient) must exist on the luminal border, of which about 55 mV (80% of the bicarbonate gradient) is accounted for by the H+ secretory pump. (2) In the net reabsorption of K+, a significant K+ uptake pump must exist on the luminal border in addition to the powerful peritubular Na+-K+ exchange pump. The reabsorption of Cl- by the epithelium may take place in two ways: (a) transmembrane transport involving an uphill step of several millivolts, and (b) paracellular leakage through the tight junction. It is thought that the Na+ pump located on the basolateral border of the proximal tubule cell plays a primary role in the regulation of the movement of other ions and water. The regulatory mechanisms of these substances may involve some electrochemical feedback mechanism that works across the proximal tubular epithelium.
利用双管离子选择性微电极对牛蛙肾近端肾小管细胞进行了微穿刺研究。测定细胞内Na⁺、K⁺、Cl⁻、HCO₃⁻和pH值分别为21.6 mEq/L、67.4 mEq/L、9.9 mEq/L、20.2 mEq/L和7.49 pH单位。在细胞外液中发现以下离子活性：Na⁺为87.4 mEq/L；K⁺为2.64 mEq/L；Cl⁻为72.5 mEq/L；HCO₃⁻为17.9 mEq/L；pH为7.66。跨肾小管周围和刷状缘的膜电位差分别为68.4 mV和60.4 mV。为方便起见，将近端肾小管细胞各边界处的电化学电位差分别计算，将细胞内的电势和化学势水平都设为零。通过这些分析，得出以下解释：(1) 在Na⁺的净重吸收过程中，管腔Na⁺沿跨管腔边界95 mV的梯度进入细胞，并逆着104 mV的梯度被泵入间质。在碳酸氢盐的重吸收过程中，管腔边界上必定存在一个约69 mV（约占Na⁺进入梯度的70%）的上坡泵，其中约55 mV（占碳酸氢盐梯度的80%）由H⁺分泌泵提供。(2) 在K⁺的净重吸收过程中，除了强大的肾小管周围Na⁺-K⁺交换泵外，管腔边界上必定存在一个显著的K⁺摄取泵。上皮细胞对Cl⁻的重吸收可能通过两种方式进行：(a) 跨膜转运，涉及几个毫伏的上坡步骤；(b) 通过紧密连接的细胞旁渗漏。据认为，位于近端肾小管细胞基底外侧边界的Na⁺泵在调节其他离子和水的移动中起主要作用。这些物质的调节机制可能涉及某种跨近端肾小管上皮起作用的电化学反馈机制。
