Effects of D<inf>2</inf>O on permeation and gating in the Ca²⁺-activated potassium channel from Chara

Abstract

We studied the effects of H2O/D2O substitution on the permeation and gating of the large conductance Ca2+-activated K+ channels in Chara gymnophylla droplet membrane using the patchclamp technique. The selectivity sequence of the channel was: K+>Rb+≫Li+, Na+, Cs+ and Cl-. The conductance of this channel in symmetric 100 mm KCl was found to be 130 pS. The single channel conductance was decreased by 15% in D2O as compared to H2O. The blockade of channel conductance by cytosolic Ca2+ weakened in D2O as a result of a decrease in zero voltage Ca2+ binding affinity by a factor of 1.4. Voltage-dependent channel gating was affected by D2O primarily due to the change in Ca2+ binding to the channel during the activation step. The Hill coefficient for Ca2+ binding was 3 in D2O and around 1 in H2O. The values of the Ca2+ binding constant in the open channel conformation were 0.6 and 6 μm in H2O and D2O, respectively, while the binding in the closed conformation was much less affected by D2O. The H2O/D2O substitution did not produce a significant change in the slope of channel voltage dependence but caused a shift as large as 60 mV with 1 mm internal Ca2+. © 1993 Springer-Verlag New York Inc.