Figure 4. Original mass spectrometric recordings of the decay of C¹⁸O¹⁶O concentration in suspensions of isolated proximal colonocytes.

The first phase (phase 1) reflects the spontaneous decay of labelled CO₂ in the absence of cells and of CA. After the additions of colonocytes, the second phase (phase 2, middle shaded area) exhibits a rapid fall in C¹⁸O¹⁶O, which is due to fast entry of C¹⁸O¹⁶O into the cells where it is rapidly converted into HC¹⁸O¹⁶O₂⁻ (this process requiring high membrane P_CO2 and high intracellular CA activity). The subsequent slower phase (phase 3), among other processes, represents influx of HC¹⁸O¹⁶O₂⁻ into the cells and is governed by the membrane permeability for HCO₃⁻. A, cells added at the arrow are isolated proximal colonocytes, no extracellular CA inhibitor is present. After addition of Triton X-100, all CA, extra- as well as intracellular, is directly accessible from the reaction solution and behaves therefore as CA in solution, giving a linear slope in the semi-logarithmic plot. The slope of this line over the slope of the initial phase in the figure represents total CA activity of the cells. B, proximal colonocytes were added at the arrow after having been pre-incubated in 10⁻⁵m STAPTPP, which continued to be present during the mass spectrometric experiment. Because A_ex is inhibited here, the exchange process is determined by A_in, P_CO2 and P_HCO3. At the end of both experiments, soluble CA is added in excess to rapidly establish isotopic equilibrium.