Figure 5. Coupling fidelity under different conditions.

Coupling fidelity estimated at different membrane potentials under control conditions, based on published estimates of amplitude and duration of calcium fluxes and of calcium release gain (black triangles, Zhou et al. 1999), based on the measurement of the extent of I_Ca inactivation by solitary DHPR openings and the published estimates of n_DHPR and DHPR P_O (grey inverted triangles, Zahradnikova et al. 2004), directly measured in the presence of the calcium channel agonist FPL 64176 and at 10 mm extracellular Ca²⁺ (open squares, Wang et al. 2001), and obtained in this work by fitting the distributions of calcium spike latencies in response to solitary DHPR openings under control conditions (filled circle) and in the presence of Bay K 8644 (open circle). Theoretical values calculated using data from Table 1 and eqns (5) and (6) are shown as a continuous line. Values calculated using data from Tables 3 and 4 of Zahradnikova et al. (2004) and their eqns (12) and (14), and using two different estimates of n_DHPR and DHPR P_O (25 and 0.05, Zahradnikova et al. 2004; and 32 and 0.29, this work) are shown as a dashed and a dotted line, respectively. Values calculated using data from Table 1 and (5) and eqns (6), assuming τ_O= 5 ms in the presence of FPL 64176 and a 50% increase of i_Ca at 10 mm external Ca²⁺ (cf. data in Sham et al. 1998; Wang et al. 2001) are shown as a dashed–dotted line.