The effect of Na+, H+ and Mg2+ ions on net calcium exchange induced in digitonin-treated myocytes has been investigated. Raising the [Na] from 1.4 to 31.4 mM revealed a sodium-sensitive fraction of net calcium exchange with a K1/2 for Na+ ions of 12 mM, alongside the respiration-dependent accumulation of calcium. An acidosis, but not an alkalosis, was found to depress both of these processes. Mg2+ ions exerted an effect solely on the respiration-dependent calcium sequestration. A simple semi-empirical model based on the experimental data was formulated to assess the effects that altering sarcoplasmic [Na+] and [H+] would have on the calcium-handling properties of cardiac mitochondria. It is concluded that part of the inotropic effects of these ions could be mediated via this organelle.
Animals Biological Transport