Spermidine release from xenopus oocytes. Electrodiffusion through a membrane channel.
The mechanism of spermidine release from Xenopus oocytes was examined by measuring release of radioactive [3H]spermidine under different ionic conditions, and under voltage-clamp. In normal solution (2 mM K+), the efflux rate is less than 1% per hour, and is stimulated approximately 2-fold by inclusion of Ca2+ (1 mm) in the incubation medium. Spermidine efflux is stimulated approximately 10-fold in high [K+] (KD98) solution. In KD98 solution, efflux is strongly inhibited by divalent cations (Ki for Ba2+ block of spermidine efflux is approximately 0.1 mM), but not by tetraethylammonium ions or verapamil. Spermidine efflux rates were not different between control oocytes and those expressing HRK1 inward rectifier K+ (Kir) channels. When the membrane potential was clamped, either by changing external [K+] in oocytes expressing HRK1, or by 2-microelectrode voltage-clamp, spermidine efflux was shown to be strongly dependent on voltage, as expected for a simple electrodiffusive process, where spermidine3+ is the effluxing species. This result argues against spermidine diffusing out as an uncharged species, or in exchange for similarly charged counterions. These results are the first conclusive demonstration of a simple electrodiffusive pathway for spermidine efflux from cells.
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Department of Cell Biology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Romano
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Lopatin
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Nichols
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D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002478:Cells, Cultured; D004058:Diffusion; D005260:Female; D007700:Kinetics; D009865:Oocytes / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology; D015221:Potassium Channels / Q000096:biosynthesis / Q000187:drug effects / Q000502:physiology*; D011994:Recombinant Proteins / Q000378:metabolism; D013095:Spermidine / Q000378:metabolism*; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D014316:Tritium; D014700:Verapamil / Q000494:pharmacology; D014982:Xenopus laevis
D015221:Potassium Channels; D011994:Recombinant Proteins; D013757:Tetraethylammonium Compounds; D014316:Tritium; D019789:Tetraethylammonium; D014700:Verapamil; D011188:Potassium; D002118:Calcium; D013095:Spermidine