Interaction of barium ions with potassium channels in squid giant axons

Abstract

Blocking of potassium channels by internally and externally applied barium ions has been studied in squid giant axons. Internal Ba (3-5 mM) causes rapid decay or "inactivation" of potassium current (IK). The kinetics and degree of block are strongly voltage-dependent. Large positive voltages speed blocking and make it more profound. Raising the external potassium concentration (Ko) from 0 to 250 mM has the opposite effect: block is made slower and less severe. In contrast, for positive voltages block by the tetraethylammonium derivative 3-phenylpropyltriethylammonium ion is almost independent of Ko and voltage. Recovery from block by internal Ba has a rapid phase lasting a few milliseconds and a slow phase lasting approximately 5 min. Internal Ba causes a "hook" in the IK tails recorded on repolarizing the fiber in high potassium external medium. External Ba, on the other hand, blocks without much altering IK time-course. KD (the dissociation constant) for block by external Ba is a few millimolar, and depends on the internal potassium concentration, the holding potential, and other factors. A reaction scheme for Ba and K channels is presented, postulating that internal and external Ba reach the same point in the channel. Once there, Ba blocks and also stabilizes the closed conformation of the channel. The extreme stability of the Ba channel complex implies the existence of negative charge within the channel.

Characterization of a voltage-gated K+ channel that accelerates the rod response to dim light

“…The effect of external Ba 2+ on I Kx is interesting for two reasons: Ba 2+ does not have a similar effect on the activation curve of either the M-current (this study) or the delayed rectifier (Armstrong and Taylor, 1980). In addition, Ba 2+ is known to prolong the light response in rods (Brown and Flaming, 1978;Fain et al, 1980) and to delay dark adaptation (Walter et al, 1982).…”

Molecular determinants of external barium block in Shaker potassium channels

“…In fact, it hate [Ba-(MES)2] replaced Na-MES in the following solution: 117 has been proposed that Ba 2÷ blocks these channels by binding mM Na-MES, 2 mM K-MES, and 10 mM HEPES. Ba 2+ solutions at a K + site within the permeation pathway [1][2][3][4]. Consistent for measurement of gating currents had a constant divalent concenwith this idea, K + can act as a competitive inhibitor of Ba 2+ tration of 2 mM with Ba-(MES)2 replacing Ca-(MES)2, e.g.…”

“…Introduction made by mixing stock isotonic solutions (240 mOsm) of the main cation buffered with 10 mM N- [2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid] (HEPES) at pH 7.0; methanesulfonate (MES) Barium ions block a wide variety of functionally and strutwas used as the main anion. The external solution in control experiturally distinct K + selective ion channels [1][2][3][4][5][6][7][8][9][10]. In many cases ments was 115 mM sodium methanesulfonate (Na-MES), 2 mM calBa 2+ can block these channels from either side of the memcium methanesulfonate [Ca-(MES)2], 2 mM potassium methanesulfobrane suggesting that Ba 2+ can interact with a structural feahate (K-MES), 0.1 mM EGTA and 10 mM HEPES.…”

“…When added to the internal side of the squid tion containing 0.1 mM Ba 2+ was as follows: 0.1 mM Ba-(MES)2, 1.9 mM Ca-(MES)2, 2 mM K-MES, 117 mM Na-MES. The solution in axon delayed rectifer, Ba 2+ blocks at a site that can only be contact with the intracellular compartment was 120 mM potassium accessed when the channel pore is open, deep within the mereglutamate buffered with 10 mM HEPES, The intracellular microelecbrane field [2,3]. Molecular biological approaches have given trode was filled with 2.7 M Na-MES, l0 mM NaCI, 10 mM EGTA, some insight into the structural domains influencing internal and 10 mM HEPES.…”

K+ transport in Malpighian tubules of Tenebrio molitor L.: is a KATP channel involved?

“…In our study, a low bath [K + ] (5 mmol l -1 ), and therefore a high [Na + ] (141 mmol l -1 ), could be responsible for activating the Na + /K + -ATPase, thereby increasing the open probability of the KATP channels. The initial large depolarization seen when changing the bath [K + ] from a low to a high value is most probably due to the following: (1) Ba 2+ , being a competitive inhibitor of K + channels, is 'knocked-off' by the inward flux of K + ions at high bath [K + ] (Eaton and Brodwick, 1980;Armstrong and Taylor, 1980) and (2) the initial intracellular [ATP] is relatively low, which means the open probability of the KATP channels is high, allowing an initial influx of K + ions. However, at a bath concentration containing no NaCl (140 mmol l -1 KCl), the Na + /K + pump stops functioning, resulting in a time-dependent increase of intracellular [ATP] and therefore the closing of KATP channels.…”