Seawater fish use an electrogenic boric acid transporter, Slc4a11A, for boric acid excretion by the kidney

Boric acid transport activity of marine teleost aquaporins expressed in Xenopus oocytes

“…To determine the tissue distribution of TrAqps, RT‐PCR analyses (28 and 33 cycles) in 14 tissues of Takifugu rubripes were performed (Figure 1 ). The intestine and kidney are the sites of boric acid absorption and excretion, respectively (Kato et al, 2022 ). In Takifugu rubripes , TrAqp1aa, 1ab, 3a, 7, and 10bb were highly expressed in the intestine, and TrAqp1aa, 1ab, 3a, 4a, 7, and 8bb were highly expressed in the kidney.…”

“…Boric acid (B(OH) 3 ) is in equilibrium with borate (B(OH) 4 − ) in aqueous solutions. To determine which form is transported in oocytes expressing TrAqps, B(OH) 3 or B(OH) 4 − , we analyzed changes in intracellular pH (pH i ) of oocytes expressing TrAqps in a solution containing boric acid because B(OH) 3 influx elicits intracellular acidification, but the influx of B(OH) 4 − elicits intracellular alkalization and membrane hyperpolarization (Kato et al, 2022 ; Ushio et al, 2022 ). Changes in pH i were analyzed in oocytes expressing TrAqp3a, 7, 8bb, 9a, and 9b in an iso‐osmotic solution containing boric acid.…”

“…We recently showed that pufferfishes acclimated to SW absorb boric acid in the intestine and excrete it through the kidney (Kato et al, 2022 ). Among the six boric acid permeable TrAqps shown in the present study, TrAqp3a and 7 were expressed in the intestine, and TrAqp3a, 7, and 8bb were expressed in the kidney, suggesting that these TrAqps may have potential roles in intestinal and renal handling of boric acid in pufferfish.…”

“…Therefore, SW fishes are exposed to a continuous influx of boric acid, which must be excreted. Recently, we analyzed boric acid and boron concentrations in the body fluids of the river pufferfish ( Takifugu obscurus , euryhaline) and the Japanese pufferfish ( Takifugu rubripes , marine) acclimated in FW, brackish water (BW), and SW (Kato et al, 2022 ). In SW, pufferfishes maintained serum boric acid concentration at 0.02–0.06 mM (the boric acid concentration of the intestinal fluid was 0.04–0.15 mM).…”

Borate Transporters and SLC4 Bicarbonate Transporters Share Key Functional Properties

“…When SLC4A11 was first characterized it was initially proposed to transport borate [ 49 ], but this claim proved controversial and evidence has since emerged that human SLC4A11 cannot transport borate but rather is likely to transport H + /OH − [ 26 , 50 , 51 , 52 ]. A recent report, however, shows that seawater fish use their SLC4A11 ortholog to excrete boric acid in the kidneys, suggesting that, during the history of vertebrate evolution, either mammalian SLC4A11 lost boric acid transport activity or saltwater fish acquired boric acid transport activity [ 53 ]. Further studies are likely to establish more evolutionary, structural, and mechanistic connections between borate transporters and the SLC4 family.…”