Peak magnitude of cytosolic ΔF/F₀ transients as a function of calcium channel activity.

<p><b>A</b>: Representative traces of whole-cell <i>I</i>_Ca recordings with 2 mM [Ca²⁺]_e. Currents were elicited using 300-ms depolarizations ranging between -50 and +10 mV in 5-mV steps. Pulse interval was 10 s. <b>B</b>: Peak <i>I</i>_Ca−voltage relationship. Current amplitudes were normalized to cell capacitance and plotted as mean values. Error bars represent SEM (n = 11 cells). Dashed lines indicate voltages generated by each [K⁺]_e in 2-mM [Ca²⁺]_e bath solutions. <b>C</b>: Voltage-dependence of steady-state <i>I</i>_Ca activation, <i>P</i>_act. Values are mean ± SEM for 11 cells. The solid line represents the mean of the best fit to each cell by a Boltzmann distribution, with <i>V</i>_<i>1/2</i> and <i>k</i> values of -31.2 mV and 3.4 mV, respectively. <b>D</b>: Voltage-dependence of steady-state <i>I</i>_Ca inactivation, (1 –<i>P</i>_inact). For measuring voltage-dependence of inactivation, a paired-pulse voltage protocol was used consisting of a 300-ms conditioning prepulse to voltages from -90 to 40 mV followed, after a 20-ms gap at -90 mV, by a 300-ms test pulse to 0 mV. Holding potential was -90 mV and the interval between conditioning prepulses as 10 s. For generating inactivation curve, the peak amplitudes of currents evoked by the test pulse were normalized to the current evoked during each prepulse and plotted as a function of prepulse potential. Solid line is the mean of the best fit of the descending portion of the inactivation-voltage relationship (i.e., between -111.9 and 18.1 mV) to a Boltzmann function, with <i>V</i>_<i>1/2</i> and <i>k</i> values of 55.3 mV and 16.3 mV, respectively. Values are mean ± SEM (n = 8 cells). <b>E</b>: Calculated voltage-dependence of steady-state <i>P</i>_act of high voltage-gated Ca²⁺ channels in sympathetic ganglion neurons. The curve shows the theoretical steady-state <i>P</i>_act at any potential, using the Boltzmann values for the amount of available current and the amount of current inactivation. The maximum available current was set to 1. Circles denote values for steady-state <i>P</i>_act at voltages generated by each [K⁺]_e in Ca²⁺-free bath solutions.