Charybdotoxin, a protein inhibitor of single Ca2+-activated K+ channels from mammalian skeletal muscle

Identification of two toxins from scorpion (Leiurus quinquestriatus) venom which block distinct classes of calcium-activated potassium channel

Two polypeptide toxins from scorpion (Leiurus quinquestriatus) venom which block distinct classes of calcium-activated potassium channels have been identified and partially purified. One toxin, at 50-100 ng/ml, blocks apamin-sensitive potassium fluxes in hepatocytes and inhibits [125I]monoiodoapamin binding. The other, more basic, toxin blocks apamin-insensitive potassium fluxes in erythrocytes at 200 ng/ml and, to our knowledge, is the first toxin shown to block the erythrocyte calcium-activated potassium channel with high affinity. The possible co-identity of this latter toxin with charybdotoxin is discussed.

A Drosophila mutation that eliminates a calcium-dependent potassium current

A mutation of Drosophila, slowpoke (slo), specifically abolishes a Ca2+-dependent K+ current, IC, from dorsal longitudinal flight muscles of adult flies. Other K+ currents remain normal, providing evidence that IC is mediated by a molecularly distinguishable set of channels. The pharmacological properties of IC are similar to those of Ca2+-dependent currents in some vertebrate cells. The muscle action potential was significantly lengthened in slo flies, indicating that IC plays the major role in its repolarization.

Ionic channels: II. Voltage- and agonist-gated and agonist-modified channel properties and structure

This article reviews the different forms of ionic channels: voltage-gated, agonist-gated, and agonist- and second messenger-modified channels. The recent advances in our knowledge of the amino acid sequence of the sodium channel and the nicotinic acetylcholine receptor and the relationship of the primary structure to the channels' quarternary structure and function are discussed.

Leiurus quinquestriatus venom inhibits different kinds of Ca2+-dependent K+ channels

A minor protein component of Leiurus quinquestriatus venom has been reported to inhibit selectively the apamin-insensitive Ca2+-dependent K+ channels of mammalian skeletal muscle (Miller, C., Moczydlowski, E., Latorre, R. and Phillips, M. (1985) Nature 313, 316-318). We report the effect of the venom on both the apamin-insensitive channels of the human erythrocyte, the Ehrlich cell and the rat thymocyte and the apamin-sensitive channel of the guinea pig hepatocyte. The venom inhibited Ca2+-dependent K+ transport in all the cases with a Ki value within the range of 1 to 10 micrograms/ml, similar to that reported previously in muscle. Valinomycin-induced K+ transport was also antagonized by the venom but its sensitivity was about 1/10 as much as that of the Ca2+-dependent K+ channel.

Blocking mechanisms of batrachotoxin-activated Na channels in artificial bilayers

The effects of various pharmacological agents that block single batrachotoxin-activated Na channels from rat muscle can be described in terms of three modes of action that correspond to at least three different binding sites. Guanidinium toxins such as tetrodotoxin, saxitoxin, and a novel polypeptide, mu-conotoxin GIIIA, act only from the extra-cellular side and induce discrete blocked states that correspond to residence times of individual toxin molecules. Such toxins apparently do not deeply penetrate the channel pore since the voltage dependence of block is insensitive to toxin charge and block is not relieved by internal Na+. Many nonspecific organic cations, including charged anesthetics, exhibit a voltage-dependent block that is enhanced by depolarization when present on the inside of the channel. This site is probably within the pore, but binding to this site is weak, as indicated by fast blockade that often appears as lowered channel conductance. A separate class of neutral and tertiary amine anesthetics such as benzocaine and procaine induce discrete closed states when added to either side of the membrane. This blocking effect can be explained by preferential binding to closed states of the channel and appears to be due to a modulation of channel gating.