A steady-state mechanism can account for the properties of inositol 2,4,5-trisphosphate-stimulated Ca2+ release from permeabilized L1210 cells.
We have investigated the effects of sub-maximal Ins(2,4,5)P3 concentrations on the Ca2+ permeability of the residual undischarged Ca2+ stores in electroporated or digitonin-permeabilized L1210 cells by measuring Ca(2+)-efflux rate after addition of the ATPase inhibitor thapsigargin. Low concentrations of Ins(2,4,5)P3, causing rapid discharge of a small proportion of the releasable Ca2+, result in a substantial stimulation of Ca2+ efflux after thapsigargin addition. This indicates firstly that in the absence of thapsigargin there must have been a substantial, counterbalancing, increase in rate of Ca2+ pumping, and secondly that the increased Ca2+ permeability is more consistent with a steady state than with a quantal model of Ca2+ release. Similar increases in passive Ca2+ permeability are produced by addition of concentrations of ionomycin which produce equivalent changes in Ca2+ loading to those produced by Ins(2,4,5)P3, although the time course and initial rate of Ca2+ release are very much slower. In the presence of a Ca(2+)-buffering system, the time course of Ca2+ release by Ins(2,4,5)P3 becomes superimposable on that of ionomycin, indicating that the initial rapid phase of Ins(2,4,5)P3-stimulated Ca2+ is at least partially due to positive feedback from extravesicular Ca2+.
School of Biological Sciences, University of East Anglia, Norwich, U.K.
J W
JW
Loomis-Husselbee
A P
AP
Dawson
D000814:Aniline Compounds; D000818:Animals; D001693:Biological Transport, Active; D002021:Buffers; D002118:Calcium / Q000493:pharmacokinetics*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000502:physiology*; D002463:Cell Membrane Permeability; D005470:Fluorometry; D007295:Inositol Phosphates / Q000494:pharmacology*; D015759:Ionomycin / Q000494:pharmacology; D007939:Leukemia L1210 / Q000378:metabolism*; D008954:Models, Biological; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D014966:Xanthenes
D000814:Aniline Compounds; D002021:Buffers; D007295:Inositol Phosphates; D013729:Terpenes; D014966:Xanthenes; C059715:Fluo-3; D015759:Ionomycin; D019284:Thapsigargin; C062616:inositol 2,4,5-trisphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium