Reversible closing of water channels inCharainternodes provides evidence for a composite transport model of the plasma membrane

Abstract

Treatment of internodal cells of Chara corallina with the water channel blocker HgCl 2 caused a decrease of the hydraulic conductivity of the membrane (Lp) by a factor of three to four. In the presence of (practically) non-permeating solutes such as sugars or salts, the osmotic responses were similar to those found in controls, i.e. reflection coefficients ( s ) were close to unity. However, when treating the internodes with osmotic solutions of rapidly permeating lipophilic substances such as low molecular weight alcohols or acetone, the pirture of biphasic pressure relaxations due to the exchange of both water and solutes changed considerably. In the presence of HgCl 2 , reflection coefficients were substantially reduced and even became negative for some solutes (anomalous osmosis). Different from reflection coefficients, permeability coefficients (P s ) remained constant upon treatment. When using heavy water (HDO) as a small hydrophilic solute which should cross the membrane largely via water channels, results were different: the reflection coefficient of HDO increased and permeability decreased. Treating the cells with 5 mM 2-mercaptoethanol to remove the mercury from transport proteins (water channels) reversed changes in Lp and s . The results disagree with conventional membrane models, which imply a homogeneous membrane. They are readily explained by a composite model of the membrane in which proteinaceous arrays with specific water channels are arranged in parallel with lipid arrays. The latter account for most of the permeability of the small organic test solutes used and are not affected by the channel blocker. From the data, detailed information is obtained about transport properties of the arrays (water channels).