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.
Combettes
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L
Institut National de la Santé et de la Recherche Médicale, U. 274, Bât. 443 Université Paris-Sud, Orsay, France.
Berthon
B
B
Doucet
E
E
Erlinger
S
S
Claret
M
M
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