Regulation of inositol trisphosphate receptors by luminal Ca2+ contributes to quantal Ca2+ mobilization.
The quantal behaviour of inositol trisphosphate (InsP3) receptors allows rapid graded release of Ca2+ from intracellular stores, but the mechanisms are unknown. In Ca2+-depleted stores loaded with Fura 2, InsP3 caused concentration dependent increases in the rates of fluorescence quench by Mn2+ that were unaffected by prior incubation with InsP3, indicating that InsP3 binding did not cause desensitization. When Fura 2 was used to report the luminal free [Ca2+] after inhibition of further Ca2+ uptake, submaximal concentrations of InsP3 caused rapid, partial decreases in fluorescence ratios. Subsequent addition of a maximal InsP3 concentration caused the fluorescence to fall to within 5% of that recorded after ionomycin. Addition of all but the lowest concentrations of InsP3 to stores loaded with the lower affinity indicator, Calcium Green-5N, caused almost complete emptying of the stores at rates that increased with InsP3 concentration. The lowest concentration of InsP3 (10 nM) slowly emptied approximately 80% of the stores, but within 3 min the rate of Ca2+ release slowed leaving approximately 7 microM Ca2+ within the stores, which was then rapidly released by a maximal InsP3 concentration. In stores co-loaded with both indicators, InsP3-evoked Ca2+ release appeared quantal with Fura 2 and largely non-quantal with Calcium Green-5N; the discrepancy is not, therefore, a direct effect of the indicators. The fall in luminal [Ca2+] after activation of InsP3 receptors may, therefore, cause their inactivation, but only after the Ca2+ content of the stores has fallen by approximately 95% to < or = 10 microM.
Combettes
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Department of Pharmacology, Cambridge, UK.
Cheek
T R
TR
Taylor
C W
CW
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D005456:Fluorescent Dyes; D016257:Fura-2; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008099:Liver / Q000166:cytology / Q000378:metabolism; D008297:Male; D008345:Manganese / Q000378:metabolism; D009930:Organic Chemicals; D051381:Rats; D017208:Rats, Wistar; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D013050:Spectrometry, Fluorescence
D015220:Calcium Channels; D005456:Fluorescent Dyes; D053496:Inositol 1,4,5-Trisphosphate Receptors; D009930:Organic Chemicals; D018160:Receptors, Cytoplasmic and Nuclear; C071336:calcium green; D008345:Manganese; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium; D016257:Fura-2