A novel Na+/HCO3--codependent choline transporter in the syncytial epithelium of the cestode Hymenolepis diminuta.
Absorption of exogenous choline by the cestode Hymenolepis diminuta was found to be both Na+- and HCO3--dependent and, at pH 6 to 7, accounted for up to 65% of the total choline uptake. Na+/HCO3- dependent choline uptake was activated at approximately 6 mM HCO3- (EC50 approximately 9 mM), and, above 100 mM Na+, the rate of uptake was directly proportional to the Na+ concentration. Atempts to uncouple Na+-dependent uptake from HCO3--dependent uptake were not successful: K+-depolarization was without effect on HCO3--dependent choline uptake, and use of valinoomycin to hyperpolarize the brush-border membrane resulted in inhibition of uptake. Na-/HCO3--dependent choline uptake was not associated with solvent drag. The Na+/HCO3--dependent choline uptake displayed a Q10 of 6.4 (27 degrees to 37 degrees) and a relatively high activation energy of 126 kJ x mol(-1). At pH 6.0 and 7.0, Na-/HCO3--dependent choline uptake rates were similar, but Na+/HCO3--dependent choline uptake was reduced at pH 5.0. The Na+/HCO3--dependent choline uptake, at pH 7.0, displayed a Kt of approximately 500 microM and a Vmax of 4.01 pmol x mg wet weight(-1) x min(-1). The Na+/HCO3--dependent choline uptake was hemicholinium-3 sensitive, but not significantly inhibited by 200 microM bumetanide, 100 microM amiloride, benzamil, or EIPA or by 1 mM 4,4'-diisothiocyano-2,2'-stilbene disulfonate (DIDS) or 4-acetamido-4'-isothiocvanostilbene-2,2'-disulfonic acid (SITS). Although it remains to be shown that HCO3- uptake is coupled directly to both choline and Na+ uptake, the data suggest that choline up take occurs via choline/Na+/HCO3--co-trans porter.
Webb
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Department of Biology, York University, North York, Ontario, Canada. raw@yorku.ca
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D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D002794:Choline / Q000378:metabolism*; D004232:Diuretics / Q000494:pharmacology; D004847:Epithelial Cells / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D006925:Hymenolepiasis / Q000378:metabolism; D006926:Hymenolepis / Q000378:metabolism*; D007422:Intestines / Q000469:parasitology; D008297:Male; D051381:Rats; D017208:Rats, Wistar / Q000469:parasitology*; D012964:Sodium / Q000378:metabolism*; D014700:Verapamil / Q000494:pharmacology
D001639:Bicarbonates; D002121:Calcium Channel Blockers; D002352:Carrier Proteins; D004232:Diuretics; D000584:Amiloride; D012964:Sodium; D014700:Verapamil; D002794:Choline; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid