$\mathrm{epsilon}=e^{\frac{-\mathrm{V\_m}F}{RT}}\mathrm{J\_HCO3}=\frac{\frac{\mathrm{P\_HCO3}\mathrm{V\_m}F}{RT}(\mathrm{HCO3\_o}-\mathrm{HCO3\_i}\mathrm{epsilon})}{1-\mathrm{epsilon}}\frac{d \mathrm{HCO3\_i}}{d t}=0.5$