The accumulation ratio of K+, Na+, Ca2+ and tetrapropylammonium in steady-state Mitochondria.

The accumulation ratio of a permeant cation under steady-state conditions after active uptake, is defined as: ($\text{{cat}{}_{\mathrm{i}}\text{}}\text{{cat}{}_0\text{}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>z</mtext>}}$, where Z is the valence of the cation, and {cat_i} and {cat₀} are the internal and external cation activities, respectively. The electrogenic proton pump predicts that the accumulation ratio should be independent of (i) the chemical structure of the cation and (ii) the degree of permeability of the membrane to cations. Furthermore the accumulation ratio should be the same for all permeant cation species simultaneously present. In the present study it is found that under steady-state conditions: (i) the accumulation ratio is not the same for potassium in the presence of valinomycin, for tetrapropylammonium in the presence of tetraphenylboron, and for calcium in the presence of acetate; (ii) the accumulation ratio is not identical for two cations such as potassium and sodium present simultaneously in the presence of gramicidin; (iii) the accumulation ratio is dependent on the external carrier concentration, such as valinomycin or tetraphenylboron. It is concluded that the cation distribution ratios under steady-state conditions are not compatible with the predictions of the electrogenic proton pump model.