Evidence for inositol tetrakisphosphate-activated Ca2+ influx pathway refilling inositol trisphosphate-sensitive Ca2+ stores in hamster eggs.
To identify the Ca2+ influx pathway responsible for maintaining Ca2+ oscillations in hamster eggs, changes in intracellular Ca2+ concentration ([Ca2+]i) were recorded using the Fura-2 fluorescent imaging technique during iontophoretic injection of inositol phosphates under voltage clamp. Both inositol 1,4,5-trisphosphate (InsP3) and 1,3,4,5-tetrakisphosphate (InsP4) caused repetitive Ca2+ transients when injected continuously into eggs, although the latter was much less effective. These Ca2+ transients were inhibited by the monoclonal antibody 18A10 to the InsP3 receptor/Ca2+ channel. In Ca(2+)-free medium, InsP4-induced Ca2+ transients were absent or much less frequent than in normal medium. A small but persistent increase in [Ca2+]i during InsP4 injection was revealed when Ca2+ uptake into InsP3-sensitive Ca2+ stores was suppressed by thapsigargin. This Ca2+ rise is due to Ca2+ entry, but not Ca2+ release, because it was: (i) increased by raising the extracellular Ca2+ concentration and abolished in Ca(2+)-free medium; (ii) larger at more negative membrane potentials which provide greater electrical driving force for Ca2+ entry; and (iii) not affected by 18A10. A moderate dose of InsP3 did not cause substantial Ca2+ entry, as tested in thapsigargin- and 18A10-treated eggs. InsP4 facilitated the restoration of Ca2+ stores after Ca2+ releases induced by pulsatile InsP3 injections. Thus, we obtained evidence for a Ca2+ influx pathway activated by InsP4 which provides Ca2+ to refill InsP3-sensitive Ca2+ stores in intact cells.
Shirakawa
H
H
Department of Physiology, Tokyo Women's Medical College, Japan.
Miyazaki
S
S
D000818:Animals; D000911:Antibodies, Monoclonal / Q000494:pharmacology; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D006224:Cricetinae; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism* / Q000494:pharmacology; D007295:Inositol Phosphates / Q000378:metabolism* / Q000494:pharmacology; D008647:Mesocricetus; D010063:Ovum / Q000187:drug effects / Q000378:metabolism*; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
D000911:Antibodies, Monoclonal; D004791:Enzyme Inhibitors; D007295:Inositol Phosphates; D013729:Terpenes; C081655:inositol-1,3,4,5-tetrakisphosphate; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium