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Journal of Experimental Biology 150,381-394 (1990)
Published by Company of Biologists 1990

Bicarbonate Transport Systems in the Intestine of the Seawater EEL

MASAAKI ANDO ¹ and M. V. SUBRAMANYAM ¹

¹ Laboratory of Physiology, Faculty of Integrated Arts and Sciences, Hiroshima University Hiroshima 730, Japan Department of Sericulture, Bangalore University Bangalore 560 001, India

Utilizing a pH-stat method, the rates of mucosal and serosal alkalinization were measured separately in the seawater eel intestine. These two rates were dependent on contralateral HCO₃^- concentration and were inhibited by contralateral application of DIDS, an inhibitor of HCO₃^- transport, indicating that the mucosal and serosal alkalinization are due to HCO₃^- secretion and absorption, respectively. The mucosal alkalinization was enhanced after inhibiting Na⁺/K⁺/Cl^- cotransport by treatment with bumetanide, furosemide or Ba²⁺, with a latent period of more than lOmin, suggesting that HCO₃^- absorption from mucosa to serosa depends on Na⁺/K⁺/Cl^- cotransport. The serosal alkalinization caused by HCO₃^- absorption was completely abolished after mucosal application of bumetanide. After pretreatment with bumetanide, mucosal omission of Cl^- halved the enhanced rate of mucosal alkalinization, and Na⁺ omission had no effect on it; this indicates that the exit of HCO₃^- into the lumen depends on luminal Cl^-, i.e. on the existence of the usual C1^-/HCO₃^- exchange on the brushborder membrane. When serosal Na+ was removed under the same conditions, mucosal alkalinization was reduced, indicating that HCO₃^- entry from the serosal fluid depends on Na⁺. Serosal omission of Cl^- did not reduce mucosal alkalinization. In addition, serosal alkalinization was enhanced by serosal removal of Na⁺ but not of Cl^-. These results suggest that there is a Na⁺/HCO₃^- cotransport on the basolateral membrane. A possible model for HCO₃^- transport systems in the seawater eel intestine is proposed, and a possible role for these transport systems is discussed in relation to Na⁺, Cl^- and water transport.

Key words: HCO₃^-, Cl^-/HCO₃^- exchange, Na⁺/HCO₃^- cotransport, Na⁺/K⁺/Cl^- cotransport, pH-stat, eel intestine

Accepted on February 1, 1990