Characteristics of bile acid-mediated Ca2+ release from permeabilized liver cells and liver microsomes.
Saponin-treated liver cells and a microsomal fraction were used to characterize the mechanism of the Ca2+ release induced by different bile acids. The saponin-treated cells accumulated 0.8-1 nmol/mg of protein of the medium Ca2+ in a nonmitochondrial, high affinity, and inositol (1,4,5)-trisphosphate (Ins(1,4,5)P3)-sensitive Ca2+ pool. Three of five bile acids tested, lithocholate and the conjugates taurolithocholate and taurolithocholate sulfate, released 85% of the Ca2+ pool within 45-60 s and with ED50 from 16 to 28 microM. Ins(1,4,5)P3 released 80% from the same Ca2+ pool with an ED50 of 0.3 microM. The Ca2+-Mg2+-ATPase inhibitor vanadate (1 mM) had no effect on the Ca2+ released by the bile acids and Ins(1,4,5)P3. The Ins(1,4,5)P3-binding antibiotic neomycin (1 mM) and the receptor competitor heparin (16 micrograms/ml) abolished the releasing effect of Ins(1,4,5)P3 but had no effect on the bile acid-mediated Ca2+ release. The 45Ca2+ accumulated by the microsomal fraction (8 nmol of 45Ca2+/mg of protein) was released by the bile acids within 45-90 s and with an ED50 of 17 microM. In contrast, the bile acids had no effect on the Ca2+ permeability of other natural and artificial membranes. The resting 45Ca2+ influx of intact cells (0.45 nmol/mg of protein/min), the 45Ca2+ accumulated by mitochondria (2-13 nmol of 45Ca2+/mg of protein), and the 45Ca2+ trapped in sonicated phosphatidylcholine vesicles (5 mM 45Ca2+) were not altered by the different bile acids. These results suggest that the Ca2+ release initiated by lithocholate and its conjugates results from a direct action on the Ca2+ permeability of the Ins(1,4,5)P3-sensitive pool. It is not mediated by Ins(1,4,5)P3 or via activation of the Ins(1,4,5)P3 receptor, and it is specific for the membrane of the internal pool.
Institut National de la Santé et de la Recherche Médicale, U. 274, Bât. 443 Université Paris-Sud, Orsay, France.
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Combettes
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Berthon
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Doucet
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Erlinger
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Claret
D000818:Animals; D001647:Bile Acids and Salts / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002463:Cell Membrane Permeability; D004987:Ethers / Q000494:pharmacology; D006493:Heparin / Q000494:pharmacology; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000494:pharmacology; D015759:Ionomycin; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D009355:Neomycin / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012503:Saponins; D014638:Vanadates / Q000494:pharmacology
D001647:Bile Acids and Salts; D004987:Ethers; D007295:Inositol Phosphates; D012503:Saponins; D014638:Vanadates; D015759:Ionomycin; D015544:Inositol 1,4,5-Trisphosphate; D006493:Heparin; D009355:Neomycin; D002118:Calcium