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