Conductance stability and Na+ interaction with Shab K+ channels under low K+ conditions.

K⁺ ions exert a structural effect that brings stability to K⁺ selective pores. Thus, upon bathing Shab channels in 0 K⁺ solutions the ion conductance, G_K, irreversibly collapses. Related to this, studies with isolated KcsA channels have suggested that there is a transition [K⁺] around which the pore takes one of two conformations, either the low (non-conducting) or high K⁺ (conducting) crystal structures. We examined this premise by looking at the K⁺-dependency of G_K stability of Shab channels within the cell membrane environment. We found that: K⁺ effect on G_K stability is highly asymmetrical, and that as internal K⁺ is replaced by Na⁺ G_K drops in a way that suggests a transition internal [K⁺]. Additionally, we found that external permeant ions inhibit G_K drop with a potency that differs from the global selectivity-sequence of K⁺ pores; the non-permeant TEA inhibited G_K drop in a K⁺-dependent manner. Upon lowering internal [K⁺] we observed an influx of Na⁺ at negative potentials. Na⁺ influx was halted by physiological external [K⁺], which also restored G_K stability. Hyperpolarized potentials afforded G_K stability but, as expected, do not restore G_K selectivity. For completeness, Na⁺ interaction with Shab was also assessed at depolarized potentials by looking at Na block followed by permeation (pore unblock) at positive potentials, in solutions approaching the 0 K⁺ limit. The stabilizing effect of negative potentials along with the non-parallel variation of Na⁺ permeability and conductance-stability herein reported, show that pore stability and selectivity, although related, are not strictly coupled.