Figure 1. Schematic diagram of dual microperfusion: the computational model.

Sarcolemmal and intracellular fluxes of solutes, as well as reaction events, are illustrated during partial exposure of a myocyte to small doses of NH₄Cl (3–5 mm). The result is an end-to-end pH_i gradient, as intracellular protonation of ammonia raises pH_i locally (by 0.1–0.6 pH units). Note that, because of buffering, the spatial diffusion of free H⁺ is insignificant, all proton movement being via buffers (e.g. Vaughan-Jones et al. 2006). Sarcolemmal acid/base transporters are not included in the model, but it appears their effect on the pH_i gradient is negligible (Swietach et al. 2005a). The computational model is used to generate look-up-tables for apparent intracellular proton diffusion coefficient (D^app_H) as a function of the amplitude of the pH_i gradient for a given mean pH_i (see Methods).