Steady-state coupling of ion-channel conformations to a transmembrane ion gradient.

Under stationary conditions, opening and closing of single Torpedo electroplax chloride channels show that the number of transitions per unit time between inactivated and conducting states are unequal in opposite directions. This asymmetry, which increases with transmembrane electrochemical gradient for the chloride ion, violates the principle of microscopic reversibility and thus demonstrates that the channel-gating process is not at thermodynamic equilibrium. The results imply that the channel's conformational states are coupled to the transmembrane electrochemical gradient of the chloride ion.