Cytoplasmic calcium affects the gating of potassium channels in the plasma membrane of Chara corallina: a whole-cell study using calcium-channel effectors

The action of a wide range of drugs effective on Ca²⁺ channels in animal tissues has been measured on Ca²⁺ channels open during the action potential of the giant-celled green alga, Chara corallina. Of the organic effectors used, only the 1,4-dihydropyridines were found to inhibit reversibly Ca²⁺ influx, including, unexpectedly, Bay K 8644 and both isomers of 202-791. Methoxyverapamil (D-600), diltiazem, and the diphenylbutylpiperidines, fluspirilene and pimozide were found not to affect the Ca²⁺ influx. Conversely, bepridil greatly and irreversibly stimulated Ca²⁺ influx, and with time, stopped cytoplasmic streaming (which is sensitive to increases in cytoplasmic Ca²⁺). By apparently altering the cytoplasmic Ca²⁺ levels with various drugs, it was found that (with the exception of the inorganic cation, La³⁺) treatments likely to lead to an increase in cytoplasmic Ca²⁺ levels caused an increase in the rate of closure of the K⁺ channels. Similarly, treatments likely to lead to a decrease in cytoplasmic Ca²⁺ decreased the rate of K⁺ channel closure. The main effect of bepridil on the K⁺ channels was to increase the rate of voltage-dependent channel closure. The same effect was obtained upon increasing the external concentration of Ca²⁺, but it is likely that this was due to effects on the external face of the K⁺ channel. Addition of any of the 1,4-dihydropyridines had the opposite effect on the K⁺ channels, slowing the rate of channel closure. They sometimes also reduced K⁺ conductance, but this could well be a direct effect on the K⁺ channel; high concentrations (50 to 100 μM) of bepridil also reduced K⁺ conductance. No effect of photon irradiance or of abscisic acid could be consistently shown on the K⁺ channels. These results indicate a control of the gating of K⁺ channels by cytoplasmic Ca²⁺, with increased free Ca²⁺ levels leading to an increased rate of K⁺-channel closure. As well as inhibiting Ca²⁺ channels, it is suggested that La³⁺ acts on a Ca²⁺-binding site of the K⁺ channel, mimicking the effect of Ca²⁺ and increasing the rate of channel closure.