Pharmacology and surface electrostatics of the K channel outer pore vestibule

Inhibition of Kv2.1 Potassium Channels by MiDCA1, A Pre-Synaptically Active PLA2-Type Toxin from Micrurus dumerilii carinicauda Coral Snake Venom

MiDCA1, a phospholipase A₂ (PLA₂) neurotoxin isolated from Micrurus dumerilii carinicauda coral snake venom, inhibited a major component of voltage-activated potassium (Kv) currents (41 ± 3% inhibition with 1 μM toxin) in mouse cultured dorsal root ganglion (DRG) neurons. In addition, the selective Kv2.1 channel blocker guangxitoxin (GxTx-1E) and MiDCA1 competitively inhibited the outward potassium current in DRG neurons. MiDCA1 (1 µM) reversibly inhibited the Kv2.1 current by 55 ± 8.9% in a Xenopus oocyte heterologous system. The toxin showed selectivity for Kv2.1 channels over all the other Kv channels tested in this study. We propose that Kv2.1 channel blockade by MiDCA1 underlies the toxin's action on acetylcholine release at mammalian neuromuscular junctions.

Nonspecific block of voltage-gated potassium channels has greater effect on distal schaffer collaterals than proximal schaffer collaterals during periods of high activity

Previous studies established different responses between proximal and distal portions of Schaffer collateral axons during high‐frequency and burst stimulation, with distal axons demonstrating biphasic changes in excitability (hyperexcitability followed by depression), but proximal axons showing only monophasic depression. Voltage‐dependent potassium (K_V) channels are important determinants of axonal excitability, and block of K_V channels can promote axon hyperexcitability. We therefore hypothesized that block of K_V channels should lead to biphasic response changes in proximal Schaffer collaterals, like those seen in distal Schaffer collaterals. To test this hypothesis, we made extracellular recordings of distal Schaffer collateral responses in stratum radiatum of hippocampal area CA1 and proximal Schaffer collateral responses in stratum pyramidale of area CA3 during high‐frequency stimulation (HFS) at 100 Hz and burst stimulation at 200 msec intervals (5 Hz or theta frequency). We then applied a nonselective K_V channel blocker, tetraethlylammonium (TEA, 10 mmol/L) or 4‐aminopyridine (4‐AP, 100 μmol/L), and assessed effects on Schaffer collateral responses. Surprisingly, block of K_V channels had little or no effect on proximal Schaffer collateral responses during high‐frequency or burst stimulation. In contrast, K_V channel blockade caused more rapid depression of distal Schaffer collateral responses during both high‐frequency and burst stimulation. These findings indicate that K_V channels are important for maintaining distal, but not proximal, Schaffer collateral excitability during period of sustained high activity. Differential sensitivity of distal versus proximal Schaffer collaterals to K_V channel block may reflect differences in channel density, diversity, or subcellular localization.

Maitotoxin induces two dose-dependent conductances in Xenopus oocytes. Comparison with nystatin effects as a pore inductor

Maitotoxin (MTX)-induced conductances in Xenopus oocytes were thoroughly characterized using the two-electrode voltage clamp technique with a hyperpolarizing voltage protocol. MTX 5-100pM induced an inward current with maximal amplitude between 0.1 and 10microA. The kinetics of this current had rising and decaying phases, which were non-voltage dependent. Its reversal potential (Erev) was close to 0mV in high K+ or Na+ external solution, indicating the participation of non-selective cation channels (NSCC). A second conductance was developed at MTX doses higher than 200pM whose amplitude increased continuously. This current showed a large instantaneous component and a voltage-independent decay, as well as similar selectivity for Na+ and K+ ions (Erev approximately 0 mV). Moreover, the maximal current amplitude was about 34% bigger in high K+ than in high Na+. The MTX effect was reversible at all doses in pM range. All the properties found are similar to those of NSCC. The differences in the current kinetics suggest that the MTX-elicited currents reflect the activation of two sets of voltage-independent NSCC. As MTX has been proposed to act by forming pores directly into the plasma membrane, we compared its effects with those of nystatin, a well-known membrane pore inductor. We found strong differences between the effects of both substances suggesting different mechanisms for these drugs.