Figure 7.

Effects of thapsigargin and ryanodine on the spikes and AHP of APs and the accompanying Ca²⁺ transient. (A) Effects of thapsigargin on single APs and Ca²⁺ transients. (a) and (b); single AP-induced Ca²⁺ transients measured at 0–1 μm (a) and 5–6 μm (b) from the plasma membrane. They consist of averaged pixel values over the regions obtained from five ratio images taken at 20-s intervals. Each Ca²⁺ transient is shown by the net change in [Ca²⁺]_i. Thapsigargin increased the basal [Ca²⁺]_i by 23 nM in this cell. (c) The AHPs of APs in the same time scale as the Ca²⁺ transients. (d) Spikes and their rates of fall of the APs. Gray and black traces indicate the records before and 10 min after the application of thapsigargin (1 μM), respectively. The derivatives of the spikes during current stimulation were omitted. Vertical arrows indicate the points at which the rates of spike repolarization were compared. (B) Effects of thapsigargin on repetitive APs and Ca²⁺ transients. Explanations are essentially the same as those in A except for the following points. The spikes of the first and fifth APs and their derivatives are shown in d. The spikes of shorter duration and their greater derivatives in gray and black traces correspond to those of the first APs. Smooth lines superimposed on the Ca²⁺ traces in a show the results of single exponential fittings over a period beginning at 400 ms after the first of stimuli, at which the spatial gradient of [Ca²⁺]_i disappeared. The inset in a shows the same decay time course of the [Ca²⁺]_i in semi-logarithmic scale. Black horizontal arrows in a note progressive decreases in the amplitude of [Ca²⁺]_i rises induced by individual APs. (C) Effects of ryanodine (10 μM) on the spike and AHP of a single AP and the accompanying Ca²⁺ transient. Ryanodine increased the basal [Ca²⁺]_i by 85 nM from the control level in this cell. Explanations are the same as those in A.