Large Conductance Ca²⁺-Dependent K⁺ (BK_Ca) Channels

Introduction

Ca²⁺-dependent K⁺ (K_Ca) channels are divided according to biophysical properties and gene homology into two main groups. K_Ca were first divided according to their single channel conductance, which represents the speed by which the K⁺ passes via the open channel. The first group consists of small and intermediate conductance channels (SK and IK respectively, the KCNN gene family). The second group is comprised of large/big conductance channels (called BK_Ca or maxiK, encoded by the slo or KCNMA1 gene)¹.

BK_Ca channels are both voltage and [Ca²⁺]_in dependent and their response to both signals results in extensive K⁺ efflux (due to their large single channel conductance) and membrane hyperpolarization (see for example²).

BK_Ca channels are widely distributed in brain, smooth muscle, cochlea and pancreatic islets¹. Interestingly, putative BK_Ca channels were found using the Anti-KCNMA1 (K_Ca1.1) (1097-1196) Antibody (#APC-021) in the inner membrane of cardiomyocyte mitochondria³. BK_Ca channels belong to the K_V super family of ion channels in which a functional channel is formed by a tetramer of the principle pore forming α subunit that may also interact with an auxiliary β subunit⁴. Four β subunits of BK_Ca channels were identified to date (with their human genes called KCNMB1-4). These auxiliary subunits bind to and modulate the channel activity¹, including pharmacological profiles (see below).

However, for the BK_Ca channels α subunit, several structural and functional differences exist. Like the prototypic K_V channels, BK_Ca channels are tetramers in which each subunit possesses the six transmembrane (S1-S6) topology (including the voltage sensor in S4 and the pore region between S5 and S6) but has an additional (seventh, called S0) α-helix towards its N-terminal region, with the N-terminal loop being extracellular.⁴ In addition, a C-terminal, intracellular Ca²⁺ receptor exists in these channels⁵, (but see⁶).

Differential Sensitivity to Toxin Blockers

A battery of compounds including peptidyl scorpion toxins and small molecules were found to affect BK_Cachannels.

The first peptidyl toxin to be described was Charybdotoxin (#STC-325), which is a strong but not very specific blocker of BK_Ca channels (blocks also several types of K_V1 channels).

Iberiotoxin (#STI-400), another scorpion toxin, is a very selective BK_Ca channel inhibitor.

Recently another scorpion toxin was described, Slotoxin (#STS-410), that is not only a potent and selective blocker, but, in addition, differentially inhibits BK_Ca channels, depending on the presence of β subunits⁷ and on the α splice variant⁸. In fact the Charybdotoxin and Iberiotoxin blockade of BK_Ca channels is also dependent on the presence of a β subunit and on the type of the β isoform^9,10. It was also shown that the glycosylation of the β4 subunit is essential for the manifestation of this phenomenon¹¹. In general, the assembly with β subunits reduces the sensitivity of the α subunit to these toxins. The β1 is the main smooth muscle isoform and β4 is the main neuronal isoform. Indeed there are marked pharmacological differences between BK_Ca channels in these two tissues and in expression systems, when the α subunit is coexpressed with either β.

Other Potent Blockers

Several tremorgenic indole alkaloids (lipid soluble) molecules such as Paxilline (#P-450), Penitrem A (#P-650) and Verruculogen (#V-500) are also potent blockers of BK_Ca channels¹².

Paxilline was shown to inhibit the peptidyl toxin resistant combination of α+β4 channels heterologously expressed in CHO cells². However, it was as potent as Iberiotoxin in action potential broadening in hippocampus² and amygdala¹³.

Penitrem A blocked BK_Ca channels in smooth muscle cells, eliminated the transient repolarization and increased the plateau depolarization associated with action potentials in these cells¹⁴. Verruculogen also had similar effects to Iberiotoxin in inhibition of early I_K(Ca) in CA1 pyramidal cells¹⁵.

Novel Openers

Recently, a set of novel potent openers was described, which are derived from Pimaric Acid (PiMA #P-270).

PiMA was shown to increase both the voltage and Ca²⁺ sensitivity of single BK_Ca channels regardless of the expression of β1 subunit¹⁶. Several PiMA derivatives, including Isopimaric Acid (#I-370), had similar effects and potency in enhancing BK_Ca currents¹⁶. We are pleased to add both PiMA and Isopimaric Acid to our growing K⁺ channel modulators product line (see related products for details).

References

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