Ca2+ release induced by inositol 1,4,5-trisphosphate is a steady-state phenomenon controlled by luminal Ca2+ in permeabilized cells.
Low concentrations of inositol 1,4,5-trisphosphate (InsP3) evoke a very rapid mobilization of intracellular Ca2+ stores in many cell types, which can be followed by a further, much slower efflux. Two explanations have been suggested for this biphasic release. The first proposes that the Ca2+ stores vary in their sensitivity to InsP3, and each store releases either its entire contents or nothing (all-or-none release); the second proposes instead that the stores are uniformly sensitive to the effects of InsP3, but that they can release only a fraction of their Ca2+ before their sensitivity is somehow attenuated (steady-state release). Experiments using purified InsP3 receptor molecules reconstituted into lipid vesicles have shown heterogeneity of the receptors in their response to InsP3 under conditions in which the total Ca2+ level at both sides of the receptor is held constant. We now report that in permeabilized A7r5 smooth-muscle cells incubated in Ca(2+)-free medium, the amount of 45Ca2+ remaining in the stores after the rapid transient phase of release is independent of their initial Ca2+ levels, indicating that partially depleted stores are less sensitive to InsP3. Moreover, if the stores are reloaded with 40Ca2+ after the first stimulus, reapplication of the same low concentration of InsP3 will release further 45Ca2+. This recovery of InsP3 sensitivity is almost complete. Under these conditions, Ca2+ release must thus occur by a steady-state mechanism, in which the decreasing Ca2+ content of the stores slows down further release.
Missiaen
L
L
Laboratorium voor Fysiologie, KUL Campus Gasthuisberg, Leuven, Belgium.
De Smedt
H
H
Droogmans
G
G
Casteels
R
R
D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D002460:Cell Line; D002463:Cell Membrane Permeability / Q000502:physiology*; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D007700:Kinetics; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
D002132:Calcium Radioisotopes; D013729:Terpenes; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium