Potassium Transport Across the Membranes ofChara

Abstract

Smith, J. R., Smith, F. A. and Walker, N. A. 1987. Potassium transport across the membrane of Chara. I. The relationship between radioactive tracer influx and electrical conductance.—J. exp. Bot. 38:731–751. The ⁴²K influx ($ϕKin$) and the electrical conductance (G_m) were measured simultaneously for the ‘membrane’ of internodal cells of Chara australis as a function of the external [KCl] (K°. In bathing solutions of pH = 5·0, $ϕKin$ progressively increased from 20±5 to 430±60 nmol m⁻² s⁻¹ and G_m increased from 0·36±0·02 to 3·8±0·8 S m⁻² when K° was increased from 0·1 to 10 mol m⁻³. The resting membrane potential difference (p.d.) was approximately -135 mV for low K° and approached the expected Nernst equilibrium p.d. for K⁺ ions when K° > 1·0 mol m⁻³. Measurements of ³⁶Cl influx suggested that the ⁴²K influx was predominantly electrogenic. The equivalent Goldman permeability to K⁺ ions (P_k) was approximately 20–30 nm s⁻¹ and did not vary significantly with increasing K°. The equivalent conductance $GϕV$ attributable to the electrogenic transport of K⁺ ions was calculated from $ϕKin$ assuming passive, independent diffusion of K⁺ ions and the ratio $GϕV/Gm$ was found to be typically close to one. It was also found that the magnitudes of $GϕV$ and G_m measured simultaneously for each individual cell were also well correlated for K° ≥ 1·0 mol m⁻³, and that the slope of the line of best fit was close to one. For each K° it was found that the conductance not attributable to K⁺ translocation and presumably associated primarily with the transport of protons or their equivalents was typically 0·2–0·5 Sm⁻². For K° > 1·0 mol m⁻³ the results indicated that the transport of K⁺ ions was essentially independent, i.e. there was no evidence for flux interactions. The results also indicated that the equivalent conductance derived from the measured ⁴²K influx could usefully indicate the fraction of the electrical conductance attributable to the translocation of K⁺ ions.