Rates of Ion Movement from Plasma to Endolymph in the Dogfish

The rates of movement of Na⁺, Rb⁺, Cl⁻ and HCO₃⁻ from plasma to endolymph were studied in the elasmobranch fish, Squalus acanthias, by use of the appropriate isotopes. Rb⁺ was used as a marker for K⁺. The half-times to equilibrium for Na⁺, Rb⁺ and Cl⁻ were about 100 hours; for HCO₃⁻ it was 6 hours. The equilibrium ratios, endolymph/plasma, are Na⁺ 0.87, K⁺ 26, Cl⁻ 1.37, HCO₃⁻ 1.47. Carbonic anhydrase inhibition decreased the rate of HCO₃⁻ accumulation, suggesting that the process is actually the formation of endolymphatic HCO₃⁻ from plasma or tissue CO₂. Increase in plasma pCO₂ elevates endolymph HCO₃⁻ concentration. The secretory tissue contains carbonic anhydrase and Na-K-ATPase. These and other data suggest that a dominant feature of endolymph chemistry may be HCO₃⁻ formation linked in some fashion with K⁺ transport, through rates catalyzed by these two enzymes.