Co-activation of inositol trisphosphate-induced Ca2+ release by cytosolic Ca2+ is loading-dependent.
Luminal and cytosolic Ca2+ control the sensitivity of the intracellular Ca2+ stores to inositol 1,4,5-trisphosphate (InsP3). In this work, we have characterized how luminal Ca2+ interfered with the stimulation of the InsP3 receptor by cytosolic Ca2+ in permeabilized A7r5 smooth muscle cells. InsP3-induced Ca2+ release from stores containing 34 pmol of Ca2+/10(6) cells absolutely depended on a simultaneous rise in cytosolic Ca2+ concentration in the submicromolar range. This stimulation by cytosolic Ca2+ was more pronounced when the stores were preincubated with Ca2+. In contrast, fully loaded stores containing 3400 pmol of Ca2+/10(6) cells already responded to InsP3 in the absence of cytosolic Ca2+ and the release from these stores was much less stimulated by increasing the cytosolic Ca2+ concentration. The loading dependence of the release was not due to a higher cytosolic Ca2+ concentration around more loaded stores sensitizing the InsP3 receptor by binding at a cytoplasmic site. Luminal Ca2+ was therefore found to functionally substitute for cytosolic Ca2+ in triggering Ca2+ release in the presence of a constant InsP3 concentration. These findings are relevant for explaining base-line Ca2+ spiking in non-excitable cells.
Missiaen
L
L
Laboratorium voor Fysiologie, K.U. Leuven Campus Gasthuisberg, Belgium.
De Smedt
H
H
Parys
J B
JB
Casteels
R
R
D000818:Animals; D001011:Aorta / Q000166:cytology / Q000196:embryology; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism*; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D051381:Rats
D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium