Figure 6.

Ca²⁺ dependence of P_f. (A) Responses to 5 ms flashes (17,500 photons; arrow) recorded in different bath Ca²⁺ concentrations in trpl photoreceptors. Response kinetics became faster as Ca²⁺ was increased and resulted in a reduction of peak amplitude in most recordings. In addition to increasing in amplitude and area, the inward tail current was no longer well fit by an exponential at higher Ca²⁺ concentrations, but had an initial ramp-like phase indicative of saturation of the CalX exchanger. (B) Peak tail currents (mean ± SE) plotted as a function of [Ca²⁺]_o in WT, trp, and trpl mutants. (C) P_f values (symbols) at different Ca²⁺ concentrations compared with GHK-P_f predictions (continuous curves). Empirical P_f measurements increased in a Ca²⁺-dependent manner broadly as predicted by GHK theory, but values for trpl and WT deviated from the quantitative predictions.