Rates of Ion Movement from Plasma to Endolymph in the Dogfish

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Abstract
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.