Predominant expression of IP3R type 1 in sub-PM SR encourages SPCU. (A) Ca2+ stores visualized with fluo-3FF consisted of a sub-PM SR network and some central formation. (B) Immunolocalization of IP3R type 1 and RyRs using a double-staining protocol (see Section 2). Confocal image of Alexa Fluor 488 fluorescence (green), showing type 1 IP3R distribution (middle), and confocal image of Alexa Fluor 633 fluorescence (red), showing RyR distribution (right), are overlaid (left). (C) The fluo-4-loaded SMC was stimulated with 10 μM CCh and 5 mM caffeine (with 10-min interval). Two galleries of images (acquired at 42 Hz) highlight the difference in the initial phase of the responses. (D) The temporal profiles of the fluorescence at three regions outlined in red, green and magenta (left) are shown in corresponding colour.