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Electrical properties of the plasma membrane of microplasmodia ofPhysarum polycephalum

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Summary

Microplasmodia ofPhysarum polycephalum have been investigated by conventional electrophysiological techniques. In standard medium (30mm K+, 4mm Ca++, 3mm Mg++, 18mm citrate buffer, pH 4.7, 22°C), the transmembrane potential differenceV m is around −100 mV and the membrane resistance about 0.25 Ω m2.V m is insensitive to light and changes of the Na+/K+ ratio in the medium. Without bivalent cations in the medium and/or in presence of metabolic inhibitors (CCCP, CN−, N −3 ),V m drops to about 0 mV. Under normal conditions,V m is very sensitive to external pH (pH o ), displaying an almost Nernstian slope at pH o =3. However, when measured during metabolic inhibition,V m shows no sensitivity to pH o over the range 3 to 6, only rising (about 50 mV/pH) at pH o =6. Addition of glucose or sucrose (but not mannitol or sorbitol) causes rapid depolarization, which partially recovers over the next few minutes. Half-maximal peak depolarization (25 mV with glucose) was achieved with 1mm of the sugar. Sugar-induced depolarization was insensitive to pH o . The results are discussed on the basis of Class-I models of charge transport across biomembranes (Hansen, Gradmann, Sanders and Slayman, 1981,J. Membrane Biol. 63:165–190). Three transport systems are characterized: 1) An electrogenic H+ extrusion pump with a stoichiometry of 2 H+ per metabolic energy equivalent. The deprotonated form of the pump seems to be negatively charged. 2) In addition to the passive K+ pathways, there is a passive H+ transport system; here the protonated form seems to be positively charged. 3) A tentative H+-sugar cotransport system operates far from thermodynamic equilibrium, carrying negative charge in its deprotonated states.

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  1. Abteilung Membranbiochemie, Max-Planck-Institut für Biochemie, D-8033, Martinsried, West Germany

    J. Fingerle & D. Gradmann

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Fingerle, J., Gradmann, D. Electrical properties of the plasma membrane of microplasmodia ofPhysarum polycephalum . J. Membrain Biol. 68, 67–77 (1982). https://doi.org/10.1007/BF01872255

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