Miniature Ag−AgCl electrode for voltage clamping of theAmbystoma collecting duct

Summary

We have developed a miniature silver-silver chloride electrode. The outer diameter of the electrodes averaged 22 μm and the input resistance 8.8 kΩ. Since the core of the electrode is a glass fiber, the problem of the extreme malleability of a small diameter silver fiber is circumvented. The properties of the electrode permit us to insert it into short (600 μm) fragments of the amphibian collecting duct while they are being perfusedin vitro. The passage of currents in the range of 0 to 6×10⁻⁸ amperes allowed us to voltage clamp the nephron fragment between +20 and −20 mV. The current-voltage plots are linear over this range. Two lines of evidence suggest that the voltage clamp is homogeneous. First, the voltage measured at the perfusion end during a voltage-clamp experiment of the tubule is not significantly different from that measured at the collecting end. Secondly, the specific resistance of collecting ducts estimated from the “core conductor analysis” is 3.3±0.8×10⁴ Ω cm, a value not significantly different from that computed from the current-voltage plots as determined with the Ag−AgCl electrode, 3.0±0.5×10⁴ Ω cm. This method permits precise control of both the ionic and electrical gradients across fragments of the amphibian collecting duct.