Ca2+-dependent K+ (KCa) channels are divided according to biophysical properties and gene homology into two main groups. KCa 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 BKCa or maxiK, encoded by the slo or KCNMA1 gene)1.
BKCa 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).
BKCa channels are widely distributed in brain, smooth muscle, cochlea and pancreatic islets1. Interestingly, putative BKCa channels were found using the Anti-KCNMA1 (KCa1.1) (1097-1196) Antibody (#APC-021) in the inner membrane of cardiomyocyte mitochondria3. BKCa channels belong to the KV 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 BKCa 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 BKCa channels α subunit, several structural and functional differences exist. Like the prototypic KV channels, BKCa 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 BKCachannels.
The first peptidyl toxin to be described was Charybdotoxin (#STC-325), which is a strong but not very specific blocker of BKCa channels (blocks also several types of KV1 channels).
Iberiotoxin (#STI-400), another scorpion toxin, is a very selective BKCa 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 BKCa channels, depending on the presence of β subunits7 and on the α splice variant8. In fact the Charybdotoxin and Iberiotoxin blockade of BKCa channels is also dependent on the presence of a β subunit and on the type of the β isoform9,10. 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 BKCa channels in these two tissues and in expression systems, when the α subunit is coexpressed with either β.
Figure 1. Sequence Aligment of Three Scorpion Toxins
Other Potent Blockers
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 BKCa 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 IK(Ca) in CA1 pyramidal cells15.
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 BKCa channels regardless of the expression of β1 subunit16. Several PiMA derivatives, including Isopimaric Acid (#I-370), had similar effects and potency in enhancing BKCa currents16. We are pleased to add both PiMA and Isopimaric Acid to our growing K+ channel modulators product line (see related products for details).
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5. Jiang, Y. et al. (2002) Nature 417, 515.
6. Piskorowski, R. and Aldrich, R.W. (2002) Nature 420, 499.
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15. Aoki, T. and Baraban, S.C. (2000) J. Neurophysiol. 83, 3453.
16. Imaizumi, Y. et al. (2002) Mol. Pharmacol. 62, 836.