Figure 10.

A four-state model can be used to fit the activation and deactivation kinetics for KCa3.1. (A) Gating scheme used to describe the activation and deactivation kinetics of KCa3.1. The six-state model is comprised of four closed states and two open states, with forward transitions between closed states being Ca²⁺ dependent and all other transitions being Ca²⁺ independent. The box outline represents the states that are not necessary to fit the activation and deactivation kinetics as determined through sensitivity analysis (Table S1). (B) Representative activation and deactivation records fit with two variations of the model shown in A. Activation and deactivation currents were recorded and fit according to the protocol described in the Materials and methods sections. The colors refer to the different Ca²⁺ concentrations, with red representing 0.5, blue 0.7, green 1.0, and black 10 µM Ca²⁺. The solid line represents the fit assuming Ca²⁺-dependent rate constants have a linear dependence on Ca²⁺ concentration, k = A · [Ca]. The dashed line represents the fit assuming Ca²⁺-dependent rate constants have a nonlinear dependence on Ca²⁺ concentration, k = A · [Ca]/(B + [Ca]). Rate constants derived from the fit are summarized in Table II. (C) The model can be used to predict the shift in apparent Ca²⁺ affinity with PCMBS. Plot of normalized 〈I〉 current against the corresponding Ca²⁺_i for KCa3.1 and KCa3.1+PCMBS. The symbols represent the experimental data (KCa3.1, ◆, and KCa3.1+PCMBS, ■), which is plotted against the predicted apparent Ca²⁺ affinity (KCa3.1, solid line) and (KCa3.1+PCMBS, dashed line). Model prediction: KCa3.1, EC₅₀ = 540 nM, and KCa3.1+PCMBS, EC₅₀ = 280 nM. Experimental observation: KCa3.1, EC₅₀ = 508 nM, and KCa3.1+PCMBS, EC₅₀ = 235 nM.