Fig. 1. Time-resolved determination of the proton secretion rate from human myotubes. (A ) Principle of pH recording. A charge movement, induced by pulsed illumination of the sensor chip is modulated by the voltage V (left ). The current–voltage relation is displaced along the voltage axis (right ) when protons that are secreted into the bath solution compensate negative surface charges of the chip. The charged groups originate from a thin insulating layer of silicon oxide and nitride. Ref = reference electrode. (B ) Four consecutive pump cycles of the perfusion system and corresponding pH-dependent alterations in the inflection points of the current–voltage relation resulting from the combined action of proton extrusion from the myotubes in the microchamber and of the flow rate of the perfusion system. During the intervals labeled “pump-on,” the chamber was perfused with the experimental solution. The acidification rate was determined from the pH decrease in the “pump-off” intervals. (C ) Acidification rates as determined in B plotted versus time. The numbers 1–4 indicate the four perfusion cycles shown in B . 4-CmC = 4-chloro-m-cresol.