Figure 10. A hypothetical schematic model of channel gating kinetics of the NSC channel for untreated (A) and terbutaline-treated (B) conditions.

The channel has two Cl⁻-binding sites and one Ca²⁺-binding site which regulate the channel activity. A, the cytosolic Cl⁻-binding sites are almost always occupied by Cl⁻ in untreated channels. The cytosolic Ca²⁺-binding site is almost never occupied by Ca²⁺. If the channel gate has a negative charge, two Cl⁻ ions, with their negative charges, bound to the channel promote channel closing. B, on average, the two binding sites for cytosolic Cl⁻ are usually occupied by cytosolic Cl⁻ for only about half the time in terbutaline-treated channels, i.e. on average only one of the two binding sites for cytosolic Cl⁻ is occupied by cytosolic Cl⁻ in terbutaline-treated channels. The cytosolic Ca²⁺-binding site is almost always occupied by Ca²⁺. In the terbutaline-stimulated case, one Cl⁻ ion and one Ca²⁺ ion, with one net positive charge, bound to the channel cause the channel to remain in the open state for a longer time, leading to a decrease in the closing rate.