DEPENDENCE OF FLUX SIZE AND EFFICIENCY OF OXIDATIVE-PHOSPHORYLATION ON EXTERNAL OSMOLARITY IN ISOLATED RAT-LIVER MITOCHONDRIA - ROLE OF ADENINE-NUCLEOTIDE CARRIER

The aim of this work was a thermodynamic and kinetic study of the infl uence of varying external osmolarity on overall oxidative phosphorylat ions in isolated rat liver mitochondria. When external osmolarity is i ncreased from 100 to 400 mosM by using a non-penetrant sugar: (i) matr ix volume diminishes, (ii) state 3 respiratory rate decreases when sta te 4 slightly varies, (iii) states 3 and 4 protonmotive force and NAD( P)H level increase, whereas oxidative phosphorylation efficiency (ATP/ O) decreases. Indeed, respiratory flux versus protonmotive force relat ionships depend on the osmolarity considered: the lower the external o smolarity, the higher the span of overall driving force necessary for the same respiratory rate. To further investigate the mechanism of the decrease in respiratory and ATP synthesis flux leading to a lowering in oxidative phosphorylation efficiency, we determined the adenine nuc leotide carrier control coefficient on respiratory and ATP synthesis r ates respectively. The main result is that the adenine nucleotide carr ier control coefficient on respiratory rate decreases, and conversely that adenine nucleotide carrier control on ATP synthesis rate increase s, from iso- to hyperosmolarity. Furthermore, whatever the osmolarity, when state 3 respiratory rate is titrated with carboxyatractyloside, the same relationship is observed between ATP/O ratio and respiratory flux. From many previous studies, it has been shown that an increase i n external osmolarity and a consequent decrease in matrix volume inhib its almost all mitochondrial proton pumps (coupling site 1 and 2 of re spiratory chain, ATPase) in different ways. In this work, we show that in phosphorylating mitochondria, the adenine nucleotide carrier plays a key role: its inhibition as the external osmolarity increases lower s the state 3 respiration close to state 4 level and consequently lead s to a decrease in oxidative phosphorylation efficiency.