Abstract
Taurocholate efflux was studied in Xenopus laevis oocytes and is consistent with a carrier-mediated process. This carrier can be competitively inhibited and trans stimulated by glycocholate. Transport is also trans stimulated by taurochenodeoxycholate and S-hexylglutathione, but not taurolithocholate or daunomycin, reflecting a range of specificity including substrates of both the hepatic canalicular bile acid transporter and the multispecific organic anion transporter. In addition, ATP added to the outside of the oocyte results in an increase in the maximal velocity of this transport process. The physiologic function of this endogenous carrier is not known, but it may act as a generalized system for the efflux of potentially toxic organic anions.
Highlights
ATP addedto the outside of the oocyte solution in EcoscintTM(National Diagnostics, Manville NJ)
Taurocholate Efflux-Following 36-72 h of incubation X . laevis oocytes were microinjected with 60 nl of a 475 p~ [3H]taurocholate solution, yielding a calculated intraoocyte concentration of 40 p ~
The remaining oocyte has been especially usefulin studyingprocesses whichinvolve was dissolved in 10%sodium dodecyl sulfate. [3H]Taurocholate that transport of compounds from outside itnoside the oocyte, but effluxed into the incubation media and that which was retained in to date oocytes have not been used to study carrier proteins the oocyte were measured by liquid scintillation
Summary
Characterization of the System-X. laevis oocytes could be injected with up to 2 mM taurocholate without avisible effect on the morphology of the oocyte. The taurocholate could be reproduciblyinjected into theoocyte, there was some variability in the total amount injected (Table I). Taurocholate effluxwas linear over the initial 15 min of incubation (Fig. 1).Over a wide range of concentrations 70% of the injected taurocholate was free to efflux from the oocytes (Fig. 2). Efflux was temperature dependent with significantly less efflux when the oocytes were incubated in ice-cold MBS -+ 2.2 versus 1.8f 1.1%efflux/l5 min,p< 0.05). oocytes were injected and washed in cold MBS, to limit efflux prior to theinitiation of the study. The percentage of taurocholate effluxed was variable from oocyte preparation to oocyte preparation,and inhibitionresults werealwayscompared to a control set of oocytes from the same preparation
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