Pore Properties of Lymnaea Stagnalis Neuron Stretch-Activated K⁺ Channels Available to Purchase

In many neurons, variations in membrane excitability are determined by a resting K⁺ conductance whose magnitude is modulated via neurotransmitters. The S-channel in Aplysia californica mechanosensory neurons is such a conductance, but it has also been shown to be a stretch-activated K⁺ channel. In this, it resembles stretch-activated K⁺ channels common to all molluscan neurons. Comparable channels are widespread, having been reported in molluscan and insect muscle and various vertebrate cells. The pore properties of the S-channel and similar stretch-activated K⁺ channels have received only sporadic attention. Here we examine, at the single-channel level, the permeation characteristics of a stretch-activated K⁺ channel from neurons of the mollusc Lymnaea stagnalis.

Michaelis–Menten constants (K_m) for the conductance, obtained separately for inward (28 mmol l⁻¹) and outward (91 mmol l⁻¹) K⁺ currents, suggest that the channel presents to the external medium, where [K⁺] is lower, a higher-affinity site than it presents to the cytoplasmic medium. This may help to ensure that influx is not diffusion-limited at potentials near the resting potential, i.e. near the K⁺ equilibrium constant. Anomalous mole fraction behavior, observed when the ratio of permeant ion (K⁺ and Rb⁺) was varied, indicated that the stretch-activated K⁺ channel is a multi-ion pore. The ion selectivity sequence determined using reversal potentials under bi-ionic conditions was Cs⁺>K⁺>Rb⁺>NH₄⁺>Na⁺>Li⁺, and using relative conductance in symmetrical solutions, the sequence was Tl⁺=K⁺>Rb⁺>NH₄^+⪢Na⁺=Li⁺=Cs⁺. Extreme variations in extracellular pH from 4.7 to 11.4 had no effect on stretch-activated K⁺ channel conductance, whereas normal concentrations of extracellular Mg²⁺ reduced inward K⁺ current. Intracellular, but not extracellular, Ba²⁺ produced a slow, open channel block with an IC₅₀ of 140±80 μmol l⁻¹.

These pore properties are compared with those of other stretch-activated K⁺ channels and of K⁺ channels in general. In spite of a greater than half order of magnitude difference in the cytoplasmic [K⁺] in marine (Aplysia californica) and freshwater (Lymnaea stagnalis) molluscs, the conductances of stretch-activated K⁺ channels from the two groups are very similar.