Cl $^-$ Channels in Chara [and Discussion]

The action potential in the cells of the freshwater alga Chara corallina is slower than that in the nerve by about 1000-fold. The depolarization phase is brought on by the outflow of the Cl $^-$ ions. Voltage-clamp studies show that this Cl $^-$ current can be described by the Hodgkin-Huxley equations for the Na $^+$ transient in the squid axon. The only change necessary to the form of the Hodgkin-Huxley equations is an introduction of a time delay between the stimulus and the onset of excitation. This mathematical model of the Chara action potential facilitates a quantitative description of the effects of pH and temperature. While a pH shift alters various Hodgkin-Huxley parameters, temperature change influences mainly the activation and inactivation time constants but leaves the voltage-dependence of these parameters unaffected. The delays in excitation are both temperature and potential dependent. In future some corrections to the Hodgkin-Huxley picture of the Chara action potential may be necessary, as recent impedance measurements suggest a change in the membrane capacitance at the time of excitation.