Ca²⁺ transients in response to trains of depolarizing steps in VSP-negative fibers and Ci-VSP–expressing fibers. (A) Control and test voltage-pulse protocols. (B and D) Rhod-2 F/F₀ traces recorded in response to the protocols shown in A in a VSP-negative fiber and in a Ci-VSP–expressing fiber, respectively. The graphs on the right show the ratio of the successive peak F/F₀ values during the test record to the corresponding values during the control record, plotted versus the pulse number during the protocols; values were normalized to the ratio value corresponding to the first Ca²⁺ transient. (C and E) Ca²⁺ release traces calculated from the above-corresponding rhod-2 transients. The graphs on the right show the ratio of the successive peak Ca²⁺ release values during the test record to the corresponding values during the control record, plotted versus the pulse number; values were normalized to the ratio value corresponding to the first Ca²⁺ transient. (F; left) Mean values for the normalized ratio of peak F/F₀ during the test record to the corresponding peak F/F₀ during the control record, versus the pulse number, in VSP-negative and in Ci-VSP–positive fibers. (Right) Corresponding mean values for the normalized ratio of peak Ca²⁺ release during the test record to the corresponding peak Ca²⁺ release during the control record. Error bars represent ± SEM.