Open-channel subconductance state of K⁺ channel from cardiac sarcoplasmic reticulum

We have characterized the K⁺ channel of canine cardiac sarcoplasmic reticulum in terms of its gating kinetics and conductance states. We demonstrate that the open channel dwells in two states, O₁ and O₂, where O₁ is a true subconductance state of O₂. The two open states are linked with a closed state by a cyclic gating scheme. Under certain circumstances, however, important information can be derived using a binary model. Each open state separately exhibited an ohmic current-voltage relation with unitary conductance values of 105 (O₁) and 189 (O₂) pS in 0.1 M K⁺. Gating between closed and open states was weakly voltage dependent, and we derive reaction rate constants for the state transitions. Finally, we postulate three models to explain the existence of a subconductance state (blockade, stenosis, flutter). We argue that a flutter model best accounts for our observations of O₁.