Antihyperlipidemic drugs-In vitro effect on the function and structure of rat liver mitochondria

Three antihyperlipidemic drugs, clofibrate, p-chlorophenoxyisobutyrate (acid form of clofibrate), and tibric acid, were examined for their effect on the function and structure of freshly isolated rat liver mitochondria. All three compounds uncouple respiration from phosphorylation in analogy to the known uncoupling agent, 2,4-dinitrophenol. The efficacy of uncoupling is in the order: 2,4-dinitrophenol >clofibrate>tibric acid>p-chlorophenoxyisobutyrate. Tibric acid and p-chlorophenoxyisobutyrate result in immediate uncoupling when added to respiring mitochondria whereas uncoupling with clofibrate requires a short lag period. Clofibrate and tibric acid, when added at concentrations much higher than those necessary to elicit uncoupling, also inhibit respiration of intact mitochondria. The three antihyperlipidemic agents, also similar to 2,4-dinitrophenol, induce a stimulation of mitochondrial ATPase activity, but to a much lesser extent (<40% the value obtained with 2,4-dinitrophenol). Finally, antihyperlipidemic agents induce morphological changes in mitochondria, i.e. pronounced swelling characterized by enlarged mitochondria with diluted (lightly staining) matrices. In purified inner membrane particles antihyperlipidemic agents, unlike 2,4-dinitrophenol, inhibit respiration. Tibric acid inhibits respiration supported by β-hydroxybutyrate, succinate, and ascorbate+TMPD (N,N,N′,N′-tetramethyl-p-phenylenediamine). Clofibrate inhibits respiration supported by β-hydroxybutyrate and succinate, whereas p-chlorophenoxyisobutyrate inhibits only respiration supported by β-hydroxybutyrate. Respiration inhibited by antihyperlipidemic drugs cannot be restored by addition of 2,4-dinitrophenol. To explain these findings two models are presented, one which depicts the uncoupling properties of antihyperlipidemic drugs in intact respiring mitochondria as resulting from a dissipation of the proton gradient across the inner membrane, and a second which summarizes possible inhibitory sites of action of the drugs at the level of the electron transport chain in inner membrane particles.