INFLUENCE OF SODIUM CONCENTRATION ON CHANGES OF MEMBRANE CAPACITANCE ASSOCIATED WITH THE ELECTROGENIC ION-TRANSPORT BY THE NA,K-ATPASE

Electrogenic ion transport by the Na,K-ATPase was investigated in a mo del system of protein-containing membrane fragments adsorbed to a lipi d bilayer. Transient Na+ currents were induced by photorelease of ATP from inactive caged ATP. This process was accompanied by a capacitance change of the membrane system. Two methods were applied to measure ca pacitances in the frequency range 1 to 6000 Hz. The frequency dependen t capacitance increment, Delta C, was of sigmoidal shape and decreased at high frequencies. The midpoint frequency, f(0), depended on the io nic strength of the buffer. At 150 mM NaCl f(0) was about 200 Hz and d ecreased to 12 Hz at high ionic strength (1 M). At low frequencies (f much less than f(0)) the capacitance increment became frequency indepe ndent. It was, however, dependent on Na+ concentration and on the memb rane potential which was generated by the charge transferred. A simple model is presented to analyze the experimental data quantitatively as a function of two parameters, the capacitance of the adsorbed membran e fragments, C-P, and the potential of maximum capacitance increment, psi(0). Below 5 mM Na+ a negative capacitance change was detected whic h may be assigned to electrogenic Na+ binding to cytoplasmic sites. It could be shown that the results obtained by experiments with the pres ented alternating current method contain the information which is dete rmined by current-relaxation experiments with cell membranes.