Potentiation of inositol trisphosphate-induced Ca2+ mobilization in Xenopus oocytes by cytosolic Ca2+.
1. The ability of cytosolic Ca2+ ions to modulate inositol 1,4,5-trisphosphate (Insp3)-induced Ca2+ liberation from intracellular stores was studied in Xenopus oocytes using light flash photolysis of caged InsP3. Changes in cytosolic free Ca2+ level were effected by inducing Ca2+ entry through ionophore and voltage-gated plasma membrane channels and by injection of Ca2+ through a micropipette. Their effects on Ca2+ liberation were monitored by video imaging of Fluo-3 fluorescence and by voltage clamp recording of Ca(2+)-activated membrane Cl- currents. 2. Treatment of oocytes with the Ca2+ ionophores A23187 and ionomycin caused a transient elevation of cytosolic Ca2+ level when cells were bathed in Ca(2+)-free solution, which probably arose because of release of Ca2+ from intracellular stores. 3. Membrane current and Fluo-3 Ca2+ signals evoked by photoreleased InsP3 in ionophore-treated oocytes were potentiated when the intracellular Ca2+ level was elevated by raising the Ca2+ level in the bathing solution. 4. Responses to photoreleased InsP3 were similarly potentiated following activation of Ca2+ entry through voltage-gated Ca2+ channels expressed in the plasma membrane. 5. Ca(2+)-activated membrane currents evoked by depolarization developed a delayed 'hump' component during sustained photorelease of InsP3, probably because Ca2+ ions entering through the membrane channels triggered liberation of Ca2+ from intracellular stores. 6. Ba2+ and Sr2+ ions were able to substitute for Ca2+ in potentiating InsP3-mediated Ca2+ liberation. 7. Gradual photorelease of InsP3 by weak photolysis light evoked Ca2+ liberation that began at particular foci and then propagated throughout, but not beyond that area of the oocyte exposed to the light. Local elevations of intracellular Ca2+ produced by microinjection of Ca2+ acted as new foci for the initiation of Ca2+ liberation by InsP3. 8. In resting oocytes, intracellular injections of Ca2+ resulted only in localized elevation of intracellular Ca2+, and did not evoke propagating waves. 9. The results show that cytosolic Ca2+ ions potentiate the ability of InsP3 to liberate Ca2+ from intracellular stores. This process may be important for the positive feedback mechanism underlying the generation of Ca2+ spikes and waves, and for interactions between the InsP3 pathway and Ca2+ ions entering cells through voltage- and ligand-gated channels.
Yao
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Y
Department of Psychobiology, University of California, Irvine 92717.
Parker
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I
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D015640:Ion Channel Gating / Q000187:drug effects; D007473:Ion Channels / Q000187:drug effects; D008027:Light; D008564:Membrane Potentials / Q000187:drug effects; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism*; D013268:Stimulation, Chemical; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors; D014982:Xenopus laevis
D007473:Ion Channels; D015544:Inositol 1,4,5-Trisphosphate; D010738:Type C Phospholipases; D002118:Calcium