The Acid-Test for SLC4A11: Is it Feasible that Protons can Support the Microsiemens Conductances Exhibited by SLC4A11-expressing<i>Xenopus</i>Oocytes?

RATIONALE: SLC4A11 is a pH-sensitive H + transporter expressed in the corneal endothelium; mutations result in vision loss . Xenopus oocytes expressing SLC4A11 exhibit conductances &gt;10 µS: akin to cells expressing ENaC, despite [H + ] being 5-million times lower than [Na + ] in physiological solutions. The feasibility of H + supporting µS conductance is often questioned, but not addressed in existing SLC4A11 oocyte-studies because intracellular pH (pH i ) changes are monitored over minutes deep within the cell, while current-voltage ( I - V ) relationships are gathered within seconds. We set out to compare SLC4A11-mediated H + and elementary-charge ( e ) fluxes to determine whether such conductances are feasible. Approach: We expressed human SLC4A11 in oocytes from injected cRNA and activated H + conductance by incubation in bicine buffered solution, pH 8.50, such that membrane potential, V m equals E H (the equilibrium potential for H + ). From spontaneous V m (‑63 ± 1 mV, n=4) we used two electrode voltage-clamp to drive H + into or out of the cell (-120 mV or 0 mV) for 8 s. To monitor H + flux, we recorded surface pH (pH s ) using a blunted H + -selective microelectrode pressed against the cell surface for faster response. To monitor the flux of e, we recorded current. Results: The -120 mV clamp drove a negative current (mean peak amplitude -2.6 µA, decaying to ‑1.0 µA over 8 s) and a pH s alkalinization (from mean 8.47 to a plateau at 8.70). The 0 mV clamp drove a positive current (mean peak 2.5 µA, decaying to 1.3 µA) and a pH s acidification (mean 8.46 to plateau 8.24). Returning to original V m at t=8 from either clamp direction revealed a tail current with mean magnitude ~2 µA that resolved within 7 s. Analysis: We calculated subsurface pH (pH i,sub) from E H and pH s using Nernst’s equation. E H(t=8) was recreated from an interpolation of I-V points immediately before and after the return of clamp to original V m at t=8. We converted ΔpH i,sub into ΔH + assuming an intrinsic buffering power of 16 mM (Ref: PMC3471386), Avogadro’s number, oocyte diameter (mean 588 µm), and the volume of a hypothetical submembranous cytosolic ‘shell’ of 10 µm thickness (estimated penetration depth of H + over 8 s. Refs: PMC4216939, PMC2290307). To calculate the concomitant flux of e, we integrated the current trace by trapezoids. Discussion: We estimate passage of 15 ± 2 x 10 13 H + independent of current direction. This is a two-fold excess over the 7 ± 2 x 10 13 e moved the clamp period, demonstrating the feasibility of large H + currents. Concordance can be achieved by assuming a 5 µm shell thickness. This work is the first step to implementing SLC4A11 in a mathematical model of the interplay among acid-base transport processes in corneal endothelial cells. R21-EY036218 to MDP &amp; RO This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.