The effect of phenylalanine on the electrical properties of proximal tubule cells in the frog kidney

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Abstract
The present study was designed to elucidate the effects of sodium-coupled transport on the electrical properties of proximal tubule cells in the isolated perfused frog kidney. Cable analysis techniques have been employed to determine the resistance of the luminal and peritubular cell membranes in parallel (R m) and the apparent ratio of the luminal over the peritubular cell membrane resistance (VDR). Furthermore, the sensitivity of the potential difference across the peritubular cell membrane (PDpt) to 6-fold increases of peritubular potassium concentration (ΔPDk) was taken as a measure of the relative potassium conductance of this membrane. In the absence of luminal phenylalanine, PDpt amounts to −60±1 mV (n=90),R m to 36±3 kΩ cm (n=22), VDR to 1.81±0.14 (n=20), and ΔPDk to 15.0±0.9 mV (n=25). The application of 10 mmol/l phenylalanine replacing 10 mmol/l raffinose leads to a rapid (within 30 s) depolarisation of PDpt to 50±5% of its control value and to a delayed (within 12 min) recovery to 95±5% of control. The rapid depolarisation is associated with a decline ofR m and VDR, indicating a decrease mainly of the luminal cell membrane resistance. During recovery of PDpt there is a parallel increase of VDR and a further decline ofR m pointing to a decline of the basolateral cell membrane resistance. ΔPDk is decreased during rapid depolarisation but increases again during the recovery phase. Thus, phenylalanine initially decreases but then increases above control the apparent potassium conductance. Removal of phenylalanine leads to a transient hyperpolarisation and increased apparent potassium conductance. If a cell is depolarised by current injection into a neighbouring cell, a similar decrease of ΔPDk is observed which shows also a similar recovery (partial repolarisation) despite continued injection of constant current. The data point to a potential-dependent peritubular K+-conductance (of the inwardly rectifying type) and to a regulatory increase within some ten minutes, when the cell is depolarised either by sodium entry across the luminal cell membrane or by current injection into a neighbouring cell.