Barium Inhibition of the Collapse of the Shaker K+ Conductance in Zero K+

AbstractIn the absence of K+ on both sides of the membrane, delivery of standard activating pulses collapses the Shaker B K+ conductance. Prolonged depolarizations restore the ability to conduct K+. It has been proposed that the collapse of the conductance results from the dwelling of the channels in a stable closed (noninactivated) state (Gómez-Lagunas, 1997, J. Physiol. (Lond.). 499:3–15). Here it is shown that 1) Ba2+ impedes the collapse of the K+ conductance, protecting it from both sides of the membrane; 2) external Ba2+ protection (Kd=63μM at −80mV) decreases slightly as the holding potential (HP) is made more negative; 3) external Ba2+ cannot restore the previously collapsed conductance; on the other hand, 4) internal Ba2+ (and K+) protection markedly decreases with hyperpolarized HPs (−80 to −120mV), and it is not dependent on the pulse potential (0 to +60mV). Ba2+ is an effective K+ substitute, inhibiting the passage of the channels into the stable nonconducting (noninactivated) mode of gating