The slow and the quick anion conductance in whole guard cells: their voltage-dependent alternation, and the modulation of their activities by abscisic acid and CO2.

Abstract

We explored the functioning of the slowly activating anion conductance, S-type or SLAC, and of the quickly activating anion conductance, R-type or QUAC, in whole guard cells of Vicia faba L.; details of QUAC activity had not previously been demonstrated in guard cells possessing their walls. The discontinuous single-electrode voltage-clamp method was used to record current responses to voltage pulses and voltage ramps as well as the free-running membrane voltage. At all voltages tested between -200 and 60 mV, SLAC activated with two components, one had a time constant similar to 7 s, the other similar to 40 s. The current-voltage relationship resembled that obtained by patch-clamp experiments. In pulse experiments and 1-s ramps, QUAC activity appeared with half-maximum activation near -50 mV and full activation above -10 mV; it inactivated with a half-time of approximately 10 s. Inactivation of QUAC at -40 mV led to the appearance of SLAC. After deactivation of SLAC at -200 mV, QUAC could be activated again. We concluded that voltage-dependent interchanges between SLAC and QUAC had occurred. Frequently, SLAC and QUAC were active simultaneously in the same cell. Abscisic acid (ABA, 20 microM) activated SLAC as well as QUAC. External Ca2+ was not required, but enhanced the activation of QUAC. Rises in the partial pressure of CO2, in the range between 0 and 700 microbar, caused rapid and reversible increases in the activity of SLAC (and outward currents of K+). QUAC also responded to CO2, however in an unpredictable manner (either by increased or by decreased activity). Oscillations in the free-running membrane voltage arose either spontaneously or after changes in CO2. They were correlated with periodic activations and inactivations of QUAC and required the simultaneous activity of an electrogenic pump.