Patch clamp recordings of H⁺ currents after recombinant expression of wild-type HVCN1, M91T HVCN1, and empty vector in COS-7 cells. (A) Typical overview recording for this set of experiments. Top trace shows current measured at holding potentials as shown in the bottom trace. Baseline voltage was −80 mV, pulsed every 40 s to +20 mV, and a voltage step protocol from −40 to +80 mV, step 20 mV, was applied before and after 10 µM ZnCl₂ was added to the bath; pH_i 6 and pH_o 7. Note the recovery of depolarization-activated currents after the first current–voltage steps. (B–D) Recording from cells expressing recombinant wild-type HVCN1. Current pulse to 0 mV is indicated in B, and voltage step protocol is depicted in the inset; recordings in B and C are from the same cell, and the y-axis scaling is the same. (E–G) Recording from a cell expressing recombinant M91T HVCN1. Current pulse at 20 mV is indicated in E; recordings in E and F are from the same cell, and the y-axis scaling is the same. (H–J) Mock-transfected cells expressed no significant currents. The y axis in I is the same as in H. The time bar in B is the same for all current recordings (excluding A). Recordings in H and I are from the same cell, and the y-axis scaling is the same. D and G are reported in percent of maximal current, and the x axis is the same as in J. The number of experiments is given in parentheses.