SK channels are small conductance Ca²⁺-activated K⁺ channels, responsible for the afterhyperpolarisation (AHP) of neurones following single or bursts of action potentials. Apamin, a bee venom toxin, potently inhibits rSK-2 and -3 channels, and in some, but not all neurones, inhibits the AHP. The sensitivity of SK-1 channels is less clear; apamin reportedly inhibits hSK-1 channels when transiently expressed in mammalian cells but not in oocytes (Kohler et al. 1996; Shah & Haylett, 2000). The present study investigates more fully the apamin-sensitivity of recombinant hSK-1 and -2 channels.

hSK-1 and hSK-2 stable cell lines in Chinese hamster ovary (CHO) cells were generated. Whole-cell currents were recorded using conventional patch-clamp techniques under symmetrical K⁺ conditions. Internal (pipette) free Ca²⁺ was buffered at 1 µM or 100 nM for inhibitor or activator studies, respectively. Voltage-ramps (1 s, -100 to +70 mV) were applied. Inward current that persisted when external K⁺ was substituted for Na⁺ was considered leak. In CHO hSK-1 cells, an inwardly rectifying K⁺ conductance was recorded in the presence, but not the absence, of [Ca²⁺]_i (mean at -100 mV = 257 pA pF^-1). No currents were observed in untransfected CHO cells. Apamin (0.1 nM-1 µM) only partially inhibited hSK-1 currents, even at saturating concentrations. The maximal block was 58 ± 9 % (mean ± S.E.M., n = 8) of the basal current with IC₅₀ and slope values of 8 nM [95 % CI 6-10] and 1.1 ± 0.1 (n = 6). The residual component was inwardly rectifying and could be blocked by the non-specific blocker, cyproheptadine (1 mM). In ion substitution experiments it was evident that the permeability of the apamin-sensitive and insensitive fractions were the same (K⁺ > Cs⁺ > Na⁺ = Li⁺). 1-Ethyl-2-benzimidazolinone (EBIO) activated hSK-1 channels; the concentration-response curve was monophasic with an IC₅₀ of 631 µM [352-938] and slope of 2.0 ± 0.2 (n = 4). Channel activation by 1 mM EBIO still occurred in the presence of 1 µM apamin. hSK-2 channels expressed in CHO cells were also only partially blocked by apamin (maximum block 47 ± 7 %, IC₅₀ 140 pM [60-330], slope 1.1 ± 0.2; n = 4-7).

Our data indicate that, at least in a recombinant expression system, SK channels can be partially apamin insensitive. The two components of hSK-1 do not differ in their biophysics, ionic permeability, activation by EBIO or sensitivity to cyproheptadine. The explanation for the apamin insensitivity of a part of the SK current is unclear, but these data suggest that apamin-insensitive AHPs may yet be mediated by channels of the SK family.

Kohler, M., Hirschberg, B., Bond, C.T., Kinzie, J.M., Marrion, N.V., Maylie, J. & Adelman, J.P. (1996). Science 273, 1709-1714.

Shah, M. & Haylett, D.G. (2000). Br. J. Pharmacol. 129, 627-630.