Fig. 3. Ca2+ can move through the SR to replenish a site previously depleted of the ion. (A-C) At –70 mV, locally photolysed Ins(1,4,5)P3 (
, C) at a 10-µm-diameter region [bright spot in A, left-hand panel; see also whole-cell electrode (left side) and the Ca2+-containing shadow of the electrode (right side, `Ca2+ electrode')] increased [Ca2+]c (B,C). The [Ca2+]c images (B) are derived from the time-points indicated by the corresponding roman numerals in C. [Ca2+]c changes in B are represented by colour; blue low and red high [Ca2+]c (i-xii). A second photolysis of Ins(1,4,5)P3 
90 seconds later at the same site (C) generated an approximately comparable [Ca2+]c increase. In a Ca2+-free solution (containing 1 mM EGTA and 3 mM MgCl2), the [Ca2+]c increase evoked by Ins(1,4,5)P3 declined and was abolished as the store became depleted of Ca2+. [Ca2+]c changes as before in B are represented by colour: (v-vii; note the cell position was moved by the solution exchange, A middle panel). When the Ca2+-containing electrode (`Ca2+ electrode') was subsequently sealed onto the cell (A, right-hand panel; C, blue bar) the local [Ca2+]c, at the site of the Ca2+ electrode attachment, increased, as indicated by the colour changes as before (B, right-hand panels, ix-xii), presumably as a consequence of store-operated Ca2+ entry. [Ca2+]c was at basal levels by 30 µm from the patch pipette, the photolysis site was 77 µm from the pipette. [Ca2+]c at the photolysis site remained low (B,C). The Ca2+ increase to Ins(1,4,5)P3 at the photolysis region (A) was subsequently increased towards that of the control (C). The position of the region of measurement is shown as a white line in B (i,v,ix), left-hand corner. Measurements were made from a 1-pixel line; the line is drawn at a 2-pixel width to facilitate its visualization.