Figure 9.

Schematic representation of proposed model. NBC: intracellular CA activity delivers H⁺ ions to a diffusionally restricted domain at the cytoplasmic surface of NBC protein, the H⁺ ions being necessary for the uncatalysed titration of its HCO₃⁻ anion cargo. Reaction releases CO₂, which diffuses freely across the myocyte and its surface membrane. The relevant CA_i is itself supplied with H⁺ ions derived from the bulk cytoplasmic H⁺ ion shuttle (composed mainly of mobile histidyl dipeptides). NHE: this transporter is not located in a diffusionally restricted space and can thus be supplied directly by H⁺ ions derived from the bulk cytoplasmic H⁺ ion shuttle.