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.