Na+ transport in Acetabularia bypasses conductance of plasmalemma

Na(+)-selective microelectrodes with the sensor ETH 227 have been used to measure the cytoplasmic Na+ concentration, [Na+]c, in Acetabularia. In the steady-state, [Na+]c is about 60 mM (external 460 mM). Steps in external Na+ concentration, [Na+]o, cause biexponential relaxations of [Na+]c which have formally been described by a serial three-compartment model (outside<==>compartment 1<==>compartment 2). From the initial slopes (some mMsec-1) net uptake and release of about 3 mumolm-2sec-1 Na+ are determined. Surprisingly, but consistent with previous tracer flux measurements (Mummert, H., Gradmann, D. 1991. J. Membrane Biol, 124:255-263), these Na+ fluxes are not accompanied by corresponding changes of the transplasmalemma voltage. [Na+]c is neither affected by the membrane voltage, nor by electrochemical gradients of H+ or Cl- across the plasmalemma, nor by cytoplasmic ATP. The results suggest a powerful vesicular transport system for ions which bypasses the conductance of the plasmalemma. In addition, transient increases of [Na+]c have been observed to take place facultatively during action potentials. The exponential distribution of the amplitudes of these transients (many small and few large peaks) points to local events in the more ore less close vicinity of the Na+ recording electrode. These events are suggested to consist of disruption of endoplasmic vesicles due to a loss of pressure in the cytoplasm.