HCO3− Influx across the Plasmalemma of Chara corallina

The effect of 10 mm K(+) on the HCO(3) (-) influx in Chara corallina has been used to distinguish a Ca(2+)-dependent membrane integrity site from the HCO(3) (-) transport site which is also Ca(2+)-dependent (Lucas and Dainty, Plant Physiology 1977 60: 862-867).In the presence of 0.2 mm Ca(2+) at pH 9, 10 mm K(+) inhibits the HCO(3) (-) influx and depolarizes the membrane potential. Inhibition of the HCO(3) (-) influx may be prevented by raising the Ca(2+) concentration in the solution or by addition of Mg(2+), Sr(2+), or Mn(2+). Protection is also afforded by 20 mm Na(+) and Cs(+) but not by Rb(+) which acts as a weak analog of K(+) in producing inhibition of the HCO(3) (-) influx and depolarization of the membrane potential. With the exception of Na(+), ions which prevent inhibition of the influx also prevent depolarization of the membrane potential.Once inhibited by 10 mm K(+), HCO(3) (-) transport cannot be restored by addition of Ca(2+), Mg(2+), or Sr(2+) in less than 3 hours. Addition of Mn(2+), however, results in a progressive restoration of the proportion of cells with influxes equal to the control value. Mn(2+) also produces an increase in the membrane potential with a sharp hyperpolarization occurring at a threshold of about-180 mv. This sudden recovery of the HCO(3) (-) influx in individual cells contrasts with the gradual recovery observed when the K(+) concentration is reduced to 0.2 mm. Since Mn(2+) cannot substitute for Ca(2+) at the HCO(3) (-) transport site, restoration of HCO(3) (-) transport by Mn(2+) involving the membrane integrity site, as evidenced by the effect on the membrane potential and resistance, is clearly separate. The other divalent cations were able to restore HCO(3) (-) transport if applied at a concentration of 2 mm for 14 hours.The OH(-) efflux is also inhibited by 10 mm KCl. It is postulated that voltage dependency of the OH(-) or HCO(3) (-) transport systems may account for the observed effects of 10 mm K(+) on the HCO(3) (-) influx.