Do submaximal InsP3 concentrations only induce the partial discharge of permeabilized hepatocyte calcium pools because of the concomitant reduction of intraluminal Ca2+ concentration?
In several types of cells whose cytoplasmic Ca2+ is regulated by inositol phosphate derivatives, low concentrations of InsP3 added to permeabilized cell suspensions induce the rapid discharge of part of the InsPs-sensitive Ca2+ pool instead of slow monophasic release of Ca2+ from the entire pool. As a tentative explanation for this puzzling observation, sometimes called 'quantal release', it was suggested that the reduced intraluminal Ca2+ concentration remaining in the Ca2+ pool after a certain amount of Ca2+ had been released might allosterically reduce the channels' affinity for InsP3 and the corresponding InsP3-dependent Ca2+ efflux, and thus result in partial pool discharge (Irvine, R.F. (1990) FEBS Lett. 263, 5-9). We have tested this hypothesis by manipulating the Ca2+ pool contents with ionophore, and found that the rate of InsP3-dependent Ca2+ efflux after ionophore-induced partial discharge of the Ca2+ pools was much faster than what was predicted on the basis of this hypothesis. Heterogeneity of the Ca2+ pools appears to be a more likely reason for the 'quantal release' behavior.
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Unité de Recherche U274, Institut National de la Santé et de la Recherche Médicale, Université Paris Sud, Orsay, France.
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D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002413:Cations, Divalent; D002478:Cells, Cultured; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D015759:Ionomycin / Q000494:pharmacology; D008099:Liver / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats
D002413:Cations, Divalent; D015759:Ionomycin; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium