Temporal behaviour of cellular H+ buffering

The [H+] is a potent modulator of several physiological processes. Thus, the regulation of cytosolic [H+] is of central importance for the cell and the pH has to be maintained in a narrow range. One fundamental mechanism to maintain pH, besides the transport of acid/base equivalents across the cell membrane, is the buffering of cytosolic H+. The degree to which protons are buffered is quantified by the buffer capacity (or buffer strength or power). M. Koppel & K. Spiro, (Biochem. Zeitschr. 65, 1914) and D.D. van Slyke, (J. Biol. Chem. 52, 1922) independently defined the buffer capacity v as the additive inverse of the differential quotient given by the infinitesimal amount of acid S divided by the actual infinitesimal change in pH:v=-dS/dpH. Until now, the definition was only used for steady state buffering (instantaneous buffering assumed for the intrinsic buffering). In the case that the buffering is not instantaneous (for the CO2/HCO3- buffer system), two different buffer capacities can be defined: vdyn for the dynamic reaction system and veq for the corresponding equilibrated system. While the last value corresponds to the classical buffer capacity, the last one is by itself less informative. More interesting is its rate of change with respect to time, given by the time derivative v'dyn=dvdyn/dt , which can be seen a quantitative measure how fast the buffering occurs under non-steady-state conditions. We will show in Xenopus laevis oocytes that v'dyn can be modulated without effecting veq. Such a modulator is e.g. carbonic anhydrase II (CAII) in the presence of the CO2/HCO3- buffer system.