title: myo-Inositol 1,4,5-trisphosphate mobilizes Ca2+ from isolated adipocyte endoplasmic reticulum but not from plasma membranes.
abstract: The effects of myo-inositol 1,4,5-trisphosphate (IP3) on Ca2+ uptake and release from isolated adipocyte endoplasmic reticulum and plasma membrane vesicles were investigated. Effects of IP3 were initially characterized using an endoplasmic reticulum preparation with cytosol present (S1-ER). Maximal and half-maximal effects of IP3 on Ca2+ release from S1-ER vesicles occurred at 20 microM- and 7 microM-IP3, respectively, in the presence of vanadate which prevents the re-uptake of released Ca2+ via the endoplasmic reticulum Ca2+ pump. At saturating IP3 concentrations, Ca2+ release in the presence of vanadate was 20% of the exchangeable Ca2+ pool. IP3-induced release of Ca2+ from S1-ER was dependent on extravesicular free Ca2+ concentration with maximal release occurring at 0.13 microM free Ca2+. At 20 microM-IP3 there was no effect on the initial rate of Ca2+ uptake by S1-ER. IP3 promoted Ca2+ release from isolated endoplasmic reticulum vesicles (cytosol not present) to a similar level as compared with S1-ER. Addition of cytosol to isolated endoplasmic reticulum vesicles did not affect IP3-induced Ca2+ release. The endoplasmic reticulum preparation was further fractionated into heavy and light vesicles by differential centrifugation. Interestingly, the heavy fraction, but not the light fraction, released Ca2+ when challenged with IP3. IP3 (20 microM) did not promote Ca2+ release from plasma membrane vesicles and had no effect on the (Ca2+ + Mg2+)-ATPase activity or on the initial rate of ATP-dependent Ca2+ uptake by these vesicles. These results support the concept that IP3 acts exclusively at the endoplasmic reticulum to promote Ca2+ release.
mesh_terms: D000273:Adipose Tissue / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000494:pharmacology*; D007700:Kinetics; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D013403:Sugar Phosphates / Q000494:pharmacology*
chemical_list: D007295:Inositol Phosphates; D013403:Sugar Phosphates; D015544:Inositol 1,4,5-Trisphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium