The Port of Dakar

Structural and functional aspects of calcium homeostasis in eukaryotic cells

“…The first reports of Ins(1 ,4,5)P3-induced Ca2+ release from skeletal muscle SR appeared in 1985 [234, 2351. The evidence in favour of Ins(1,4,5)P3 may be summarized as follows: (a) Ins(1,4,5)P3 releases Ca2 + both from isolated SR terminal cisternae [234] and from skinned fibre preparations [234, 235, 241 -2441; (b) the entire machinery for the synthesis of the Ins(1,4,5)P3 precursor PtdIns(4,5)P2, Ins( 1 ,4,5)P3 formation (G-proteindependent phospholipase C) and Ins(1 ,4,5)P3 degradation [Ins(l ,4,5)P3 5-phosphatase] is present in the plasma membrane of striated muscles [245][246][247][248] and in a few cases, a selective enrichment of these synthetic and metabolic pathways has been found in T-tubules [245, 2481; (c) Ins(1,4,5)P3 levels increase dramatically upon electrical stimulation of skeletal muscles [235]; (d) Ins(1 ,4,5)P3, at micromolar concentrations, increases the opening probability of the Ca2 +-release channel of the SR after vesicle fusion with lipid bilayers [249]. A number of objections have been raised against the physiological role of Ins(1,4,5)P3 as a mediator in EC coupling in striated muscle, the most important being: (a) the release of Ca2 + from skinned skeletal muscle fibers activated by photohydrolyzed Ins(1 ,4,5)P3 is orders of magnitude slower than that observed under physiological conditions [250]; (b) the formation of Ins(1,4,5)P3 in electrically stimulated cells is observed only after tetanus [235]; (c) fiber contraction is not blocked by heparin, a known inhibitor preventing Ins(1 ,4,5)P3 binding to its receptor in smooth muscle and in non-muscle cells [251].…”

Review Structural and functional aspects of calcium homeostasis in eukaryotic cells

“…The first reports of Ins(1 ,4,5)P3-induced Ca2+ release from skeletal muscle SR appeared in 1985 [234, 2351. The evidence in favour of Ins(1,4,5)P3 may be summarized as follows: (a) Ins(1,4,5)P3 releases Ca2 + both from isolated SR terminal cisternae [234] and from skinned fibre preparations [234, 235, 241 -2441; (b) the entire machinery for the synthesis of the Ins(1,4,5)P3 precursor PtdIns(4,5)P2, Ins( 1 ,4,5)P3 formation (G-proteindependent phospholipase C) and Ins(1 ,4,5)P3 degradation [Ins(l ,4,5)P3 5-phosphatase] is present in the plasma membrane of striated muscles [245][246][247][248] and in a few cases, a selective enrichment of these synthetic and metabolic pathways has been found in T-tubules [245, 2481; (c) Ins(1,4,5)P3 levels increase dramatically upon electrical stimulation of skeletal muscles [235]; (d) Ins (1 ,4,5)P3, at micromolar concentrations, increases the opening probability of the Ca2 +-release channel of the SR after vesicle fusion with lipid bilayers [249]. A number of objections have been raised against the physiological role of Ins(1,4,5)P3 as a mediator in EC coupling in striated muscle, the most important being: (a) the release of Ca2 + from skinned skeletal muscle fibers activated by photohydrolyzed Ins (1 ,4,5)P3 is orders of magnitude slower than that observed under physiological conditions [250]; (b) the formation of Ins(1,4,5)P3 in electrically stimulated cells is observed only after tetanus [235]; (c) fiber contraction is not blocked by heparin, a known inhibitor preventing Ins(1 ,4,5)P3 binding to its receptor in smooth muscle and in non-muscle cells [251].…”