Anion channels in Chara corallina tonoplast membrane: Calcium dependence and rectification

Tonoplast K+ channels of Chara corallina are well characterized but only a few reports mention anion channels, which are likely to play an important r ole in the tonoplast action potential and osmoregulation of this plant. For experiments internodal cells were isolated. Cytoplasmic droplets were form ed in an iso-osmotic bath solution according to a modified procedure. Ion c hannels with conductances of 48 pS and 170 pS were detected by the patch-cl amp technique. In the absence of K+ in the bath solution the 170 pS channel was not observed at negative pipette potential values. When Cl- on either the vacuolar side or the cytoplasmic side was partly replaced with F-, the reversal potential of the 48 pS channel shifted conform to the Cl- equilibr ium potential with similar behavior in droplet-attached and excised patch m ode. These results showed that the 48 pS channel was a Cl- channel. In drop let-attached mode the channel rectified outward current flow, and the slope conductance was smaller. When Chara droplets were formed in a bath solutio n containing low (10(-8) M) Ca2+, then no Cl- channels could be detected ei ther in droplet-attached or in inside-out patch mode. Channel activity was restored if Ca2+ was applied to the cytoplasmic side of inside-out patches. Rectification properties in the inside-out patch configuration could be co ntrolled by the holding pipette potential. Holding potential values negativ e or positive to the calculated reversal potential for Cl- ions induced opp osite rectification properties. Our results show Ca2+-activated Cl- channel s in the tonoplast of Chara with holding potential dependent rectification.