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Fig. 5. Ins(1,4,5)P₃-sensitive Ca²⁺ release at different SR Ca²⁺ contents. At –70 mV, photolysed Ins(1,4,5)P₃ increased [Ca²⁺]_c (, A). A second photolysis of Ins(1,4,5)P₃ 90 seconds later at the same site generated an approximately comparable [Ca²⁺]_c increase (A). In a Ca²⁺-free bath solution (containing 1 mM EGTA and 3 mM MgCl₂), this [Ca²⁺]_c increase declined in amplitude and rate of rise as the store was depleted of Ca²⁺ (A). The velocity of release increased during the release process, as revealed by the increasing steepness of the slope during release (B,C), and acceleration increased (D). C and D are the first and second derivatives, respectively, of the upstroke of the transients numbered 1-4 in A. As the increase in velocity is also evident when the first derivative of the upstroke is plotted against [Ca²⁺]_c (E) rather than time (C), nonlinear Ca²⁺ buffering does not provide an explanation for these results. Had it done so, the velocities derived from each transient would have been similar when examined as a function of [Ca²⁺]_c. The numbered traces in B-E correspond to the Ca²⁺ transients numbered in A. The amplitudes of the transient (B) have been scaled and normalized to facilitate comparison. One explanation for these results is that Ins(1,4,5)P₃-mediated Ca²⁺ release is itself facilitated by Ca²⁺ released via the channel in a positive-feedback process. As lumenal [Ca²⁺] declines (in the Ca²⁺-free solution) and with it Ca²⁺ release, so does the extent of the Ca²⁺-dependent positive feedback. (F) The peak velocity of release (a measure of the extent of positive feedback) (C) determines the peak [Ca²⁺]_c achieved after Ins(1,4,5)P₃-mediated Ca²⁺ release. Here, the peak velocity is plotted against the peak [Ca²⁺]_c obtained from the same cell. In this figure, the results from three separate cells, each indicated by the different-coloured symbols, are shown and the [Ca²⁺]_c was calibrated as described in Materials and Methods. (G) Ins(1,4,5)P₃-evoked Ca²⁺ release from the transients numbered 1-4 in A that have been scaled to facilitate comparison of their time-course. As the peak [Ca²⁺]_c achieved decreases (see A), the time required to reach their peak increased (G). This result would suggest that Ins(1,4,5)P₃-mediated inactivation of Ins(1,4,5)P₃R is unlikely to explain the termination of release as the [Ins(1,4,5)P₃] is similar in each case.

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