RESPONSE OF ISOLATED RAT-LIVER MITOCHONDRIA TO VARIATION OF EXTERNAL OSMOLARITY IN KCL MEDIUM - REGULATION OF MATRIX VOLUME AND OXIDATIVE-PHOSPHORYLATION

When isolated rat liver mitochondria are incubated in KCI medium, matr ix volume, flux, and forces in both hypo-and hyperosmolarity are time- dependent. In hypoosmotic KCI medium, matrix volume is regulated via t he K+/H+ exchanger. In hyperosmotic medium, the volume is regulated in such a manner that at steady state, which is reached within 4 min, it is maintained whatever the hyperosmolarity. This regulation is Pi-and Delta<(mu)over tilde>H+-dependent, indicating Pi-K salt entry into th e matrix. Under steady state, hyperosmolarity has no effect on isolate d rat liver mitochondria energetic parameters such as respiratory rate , proton electrochemical potential difference, and oxidative phosphory lation yield. Hypoosmolarity decreases the NADH/NAD(+) ratio, state 3 respiratory rate, and Delta<(mu)over tilde>H+, while oxidative phospho rylation yield is not significantly modified. This indicates kinetic c ontrol upstream the respiratory chain. This study points out the key r ole of potassium on the regulation of matrix volume, flux, and forces. Indeed, while matrix volume is regulated in NaCl hyperosmotic medium, flux and force restoration in hyperosmotic medium occurs only in the presence of external potassium.