Figure 4.

DCRR versus [Ca_T]_SR and release permeability versus [Ca²⁺]_SR. (A) DCRR versus [Ca_T]_SR obtained from the DCRR and [Ca_T]_SR versus time signals in Fig. 3 A. (B) Release permeability versus [Ca²⁺]_SR also determined from the corresponding signals in Fig. 3 A. Details for the experiment in A and B are given in the legend of Fig. 3. Details for the experiment in C and D are given at the end of this legend. (C) The red circles plot versus [Ca_T]_SR the values at the peaks of DCRR signals in response to voltage-clamp steps to −45 mV. Each point in this case is from a different stimulation done 5 min apart. The variation of [Ca_T]_SR in this case was achieved by varying the resting SR Ca load, [Ca_T]_SR,R. Each black circle plots the steady level of DCRR reached during a 300-ms voltage-clamp step to −60 mV measured 400 ms before the pulse to −45 mV. (D) Release permeability versus [Ca²⁺]_SR obtained from the data in C. Values for [Ca²⁺]_SR on the abscissa were obtained by converting the values of [Ca_T]_SR in A with Eq. 13 using the average values for K_CB, n, and [CaIB]_max for case 1 in Table 1. Each release permeability value was obtained by multiplying the corresponding DCRR value in C by the ratio [Ca_T]_SR/[Ca²⁺]_SR for that point. The fiber reference for the experiment in C and D is 510971; details for this experiment are given in the legends of Fig. 2 in Pape and Carrier (1998) and Fig. 5 of Pape et al. (2002). The internal and external solutions were the same as those given in Materials and methods for the two points with the highest concentrations of Ca. After these two points were obtained, the internal solution in the end pools was exchanged for the same Cs internal solution except that there was no Ca present and all of the other points were obtained with this Ca-free internal solution (see Pape and Carrier, 1998).