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 Ca2+-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 Ca2+-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+.