Calcium release and influx colocalize to the endoplasmic reticulum.
Intracellular Ca2+ is released from intracellular stores in the endoplasmic reticulum (ER) in response to the second messenger inositol (1,4,5) trisphosphate (InsP3) [1,2]. Then, a poorly understood cellular mechanism, termed capacitative Ca2+ entry, is activated [3,4]; this permits Ca2+ to enter cells through Ca(2+)-selective Ca(2+)-release-activated ion channels [5,6] as well as through less selective store-operated channels [7]. The level of stored Ca2+ is sensed by Ca(2+)-permeant channels in the plasma membrane, but the identity of these channels, and the link between them and Ca2+ stores, remain unknown. It has been argued that either a diffusible second messenger (Ca2+ influx factor; CIF) [8] or a physical link [9,10] connects the ER Ca(2+)-release channel and store-operated channels; strong evidence for either mechanism is lacking, however [7,10]. Petersen and Berridge [11] showed that activation of the lysophosphatidic acid receptor in a restricted region of the oocyte membrane results in stimulation of Ca2+ influx only in that region, and concluded that a diffusible messenger was unlikely. To investigate the relationship between ER stores and Ca2+ influx, we used centrifugation to redistribute into specific layers the organelles inside intact Xenopus laevis oocytes, and used laser scanning confocal microscopy with the two-photon technique to 'uncage' InsP3 while recording intracellular Ca2+ concentration. Ca2+ release was localized to the stratified ER layer and Ca2+ entry to regions of the membrane directly adjacent to this layer. We conclude that Ca2+ depletion and entry colocalize to the ER and that the mechanism linking Ca2+ stores to Ca2+ entry is similarly locally constrained.
Jaconi
M
M
Department of Pharmacology, Mayo Foundation, Rochester, Minnesota 55905, USA.
Pyle
J
J
Bortolon
R
R
Ou
J
J
Clapham
D
D
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005260:Female; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D015640:Ion Channel Gating; D017136:Ion Transport; D007700:Kinetics; D018613:Microscopy, Confocal; D009865:Oocytes / Q000378:metabolism; D015290:Second Messenger Systems; D014981:Xenopus
D015220:Calcium Channels; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium