Mitochondrial respiration under conditions of varying osmolarity

Abstract

Inhibition of mitochondrial respiration with NAD-linked substrates and with succinate occurs in hyperosmolar media containing sucrose, mannitol, NaCl or KCl as the principal osmotic agent. Mitochondrial NADH levels have been found to be lower in hyperosmolar than in normoosmolar media and uncoupling agents do not release the respiratory inhibition. Therefore, a block in energy transfer does not appear to be the main cause of the respiratory inhibition. On the other hand, under appropriate conditions the reduction of NAD + is not inhibited by hyperosmolarity as shown in studies of reversal of electron flow utilizing tetramethyl-p- phenylenediamine-ascorbate as substrate. The respiratory block in hyperosmolar media appears therefore to be situated before NAD with NAD-linked substrates and before cytochrome b with succinate as substrate. The elimination of this inhibition after destroying the integrity of the mitochondrial membranes by freezing and thawing suggests that there may be an impediment to entry of substrates or a decreased accessibility of substrate to appropriate dehydrogenases under conditions of hyperosmolarity and that this impediment is the principal cause of the respiratory inhibition.