The proton pump,high pH channels,and excitation: voltage clamp studies of intact and perfused cells ofNitellopsis obtusa

Summary The current-voltage (I/V) and conductance-voltage (G/ V) characteristics were recorded for intact and perfused (tonoplast-free) cells ofNitellopsis obtusa. In the pH₀ range 5 to 8, the I/V profile was sigmoidal and the G/V profile exhibited a maximum — these characteristics are attributed to the proton pump at the plasmalemma. The pH₀ dependence in this range was very similar to that found inChara corallina. At very alkaline pH₀ (11.0) the high conductance due to H⁺/OH^– channels was observed in intact cells but not in perfused cells. Young plants ofNitellopsis did not display bands of calcification, but did exhibit pH banding patterns in petri dishes. The pH bands were less than 5mm wide. The excitation transients in intact cells featured two peaks near the excitation threshold, but more peaks could be observed in the p.d. (potential difference) range –90 to –60 mV. The amplitude of the transients was strongly inhibited at pH₀ 11.0. In the perfused cells the currents lacked complete inhibition at some p.d. levels, but still exhibited one or two peaks. At high pH₀ this lack of inactivation was accentuated. The addition of 5 mM TEA to the outside medium abolished the excitation transients in perfused cells.Abbreviations EGTA ethylene glycol-bis (-amino-ethylether)-N,N-tetraacetic acid - PIPES piperazine-N,N-bis-(2-ethane sulfonic acid) - TEA tetraethyl ammonium