Electrical noise measurements on red beet vacuoles : another way to detect the ATPase activity

The vacuolar potential (V(vac)) and its fluctuations were recorded in red beet vacuoles (Beta vulgaris L.). Measurements with vacuoles in their suspension medium gave V(vac) = 10 +/- 2 millivolts (referred to the external medium) when 3 molar KCl microelectrodes were used. Buffering the microelectrode filling solution at pH 7.7 reversed the sign of the potential: V(vac) = -7 +/- 2 millivolts. The magnitude of the potential fluctuations was lowered by dilution (5-1000 times) with the suspension medium containing components released by the cells during the mechanical preparation. Fluctuations were decreased by 50 millimolar KNO(3) while they were enhanced by 5 millimolar ATP-Mg. No noticeable change in membrane resistance was detected. The presence of an ATPase bound to the tonoplast may explain the recorded noise spectra. These spectra imply a close connection between the rate of ATPase functioning and the magnitude of ionic fluxes across the tonoplast. It is suggested that noise analysis could be used to detect ATPase (or related enzyme) activity in vacuoles. Possible use of H(+) diffusion through a buffered microelectrode, to modify intravacuolar pH, is also suggested.