Interaction of the Depolarization-Activated K⁺ Channel of Samanea saman with Inorganic Ions: A Patch-Clamp Study

Abstract

A depolarization-activated K+ channel capable of carrying the large K+ currents that flow from shrinking cells during movements of Samanea saman leaflets has been described in the plasmalemma of Samanea motor cell protoplasts (N Moran et al [1988] Plant Physiol 88:643-648). We now characterize this channel in greater detail. It is selective for K+ over other monovalent ions, with the following order of relative permeability: K+ > Rb+ > Na+ = Cs+ Li+. It is blocked by Cs+ and by Ba2+ in a voltage dependent manner, exhibiting a 'long-pore' behavior, similarly to various types of K+ channels in animal systems. Cadmium, known for its blockage of Ca2+ channels in animal systems, and Gd3+, closely related to La3+, which also blocks Ca2+ channels in animal cells, both block K+ currents in Samanea in a voltage-independent manner, and without interfering with the kinetics of the currents. The suggested mechanism of block is either (a) by a direct interaction with the K+ channel, but external to its lumen, or, alternatively, (b) by blocking putative Ca2+ channels, and preventing the influx of Ca2 , on which the activation of the K+ channels may be dependent.that this channel plays a role in the passive efflux of K+ during cell shrinkage. The channel has the following characteristics: (a) it is voltage gated, opening upon membrane depolarization with a time constant of 1 to 2 s; (b) at moderate depolarizations, it does not inactivate for many minutes; (c) its conductance ranges from 15 to 40 pS at external K+ concentrations between 5 and 125 mm, respectively; (d) it can be blocked by TEA3 and quinine, compounds that block K'+ channels in membranes of other organisms; and (e) the channel is similar in both extensor and flexor cells.TEA and quinine also inhibited the movement of excised leaflets, thereby supporting the view that the flow of K+ current through these channels is necessary for leaflet movement. To characterize this channel in greater detail, we now extend our description of the channel's 'signature' to include its interaction with various inorganic ions: Li+, Na+, Rb+, Cs+, Ba2+, Cd2+, and Ga3+. Preliminary results appeared in abstract form ( 18).

External pH effects on the depolarization-activated K channels in guard cell protoplasts of Vicia faba.

“…The patch-clamp technique is described in detail by Hamill, Marty, Neher, Sakmann, and Sigworth (1981). The application to plant cell protoplasts is described in detail by Moran et al (1988), and by Moran, Fox, and Satter (1990). Briefly, a drop of the protoplast suspension was added to ~ 200 p.1 of the recording solution, the protoplasts were allowed to settle and stick to the glass bottom of the recording chamber and then washed with ~ 2-3 ml of the recording solution.…”

A transient outward-rectifying K + channel current down-regulated by cytosolic Ca 2+ in Arabidopsis thaliana guard cells

“…1 and 2), and depolarization-activated outward-rectifying K ϩ channels (K ϩ out ), which mediate K ϩ efflux. K ϩ out currents have been identified in a variety of cell types and plant species, such as guard cells of Vicia faba, maize, tobacco, and Arabidopsis (3-7); trap-lobe cells of Dionaea muscipula (8); Samanea and Mimosa pulvinar motor cells (9,10); mesophyll cells of Arabidopsis and tobacco (11,12), and root cells of various species (13)(14)(15). The most common property of these plant K ϩ out currents is time-and voltage-dependent activation and lack of inactivation, with currents remaining stable for many minutes during continuous depolarization (16).…”

The combined effect of Cd2+ and ACh on action potentials of Nitellopsis obtusa cells

“…Decrement of AP peak found in our experiments after Cd 2+ treatment could be explained by an impact of Cd 2+ on the intracellular calcium level. Cd 2+ toxic effects on membrane permeability can be attributed to the influence on the transport systems, involved in plant membrane electrogenesis [14,32,33]. However, it is thought that there may be mechanisms in the plant systems that can resist the toxic effect of Cd 2+ up to certain concentrations [34].…”