Fig. 4.

Changes in residual calcium during stimulation. Shown is the increase of residual calcium, Δ[Ca²⁺]_res, in the nerve terminals of the neurohypophysis as a function of (A) time and (B) the stimulus number, n_Stim, during a train of 40 action potentials at the indicatedf_Stim. The open circlesrepresent the values of indicator fluorescence changes (ΔF/F₀) immediately before a transient Ca²⁺ rise. In B, the spacing between data points (○) at differentf_Stim represents an increment in time of Δt = (f_Stim)⁻¹sec, respectively. The solid lines represent single-exponential fits through the data. With increasingf_Stim, the range of data points conforming to this single-exponential rise rapidly decreases.C, Initial rise times (▪) of Δ[Ca²⁺]_res obtained from the single-exponential fits. The open squares represent the values of the plateau amplitudes of Δ[Ca²⁺]_res. Because the single-exponential fits did not reproduce plateau amplitudes properly, their values were instead set equal to Δ[Ca²⁺]_res atn_Stim = 25. The solid lines in C are only guides to the eye.