Some biological effects of experimentally induced inhibition of oxidative phosphorylation in rat skeletal muscle

I) Selective defects in the mitochondrial respiratory chain and in the coupling of respiration to phosphorylation underlie weakness and exercise intolerance in certain mliopatbies (see Carafoli and Roman 1980: Morgan-Hughes 1981). In this study specific defects in these pathways have been reproduced in the rat using the respiratory chain inhibitors diphenyleneiodonium and antimycin A and the uncoupling agent dinitrophenol. These disease models have been characterised using electrophysiological, biochemical and morphological techniques and the findings have furthered the understanding of the pathophysiology of human mitochondrial myopathies. Mitochondrial inhibitors were either injected intraarterially into anaesthetised animals or given by repeated daily subcutaneous injection. Isometric twitch tension and compound muscle action potentials evoked by nerve or by direct muscle stimulation were recorded from the gastrocnemius muscle in vivo during various stimulus patterns. The gastrocnemius muscle was either freeze clamped for metabolite assay or fixed for histological study at various times after injection. In a second series of experiments, the biochemical changes were followed using 31 phosphorous nuclear magnetic resonance. The effects of acute ischaemia and blocked glycolysis were also studied in vivo for comparative purposes. In vitro intracellular recordings were made from individual soleus muscle fibres in separate experiments. 2) Intra-arterial injection of dinitrophenol resulted in irreversible failure of twitch tension and loss of muscle action potential associated with the development of an electrically silent contracture. In vitro intracellular recordings from soleus muscle fibres demonstrated a progressive broadening of the action potential, with a fall in amplitude and eventual failure of propogation. This was associated with a small reduction in resting potential. Force failure was accompanied by a rapid depletion of phosphocreatine and adenosine triphospbate (ATP) accumulation of inosine monophosphate (IMP) and a fall in intramuscular pH with lactate accumulation. Enlarged mitochondria with whorled, fractured or lysed cristae, myelin figures, and intracristal linear inclusions were seen 60 minutes after infusion. 3) Intra-arterial or repeated subcutaneous injection of diphenyleneiodonium which is known to inhibit NADH ubiquinone reductase resulted in a progressive fall in twitch tension with 1 and 5c/sec stimulation patterns accompanied by a marked prolongation of relaxation time. Failure of muscle excitation paralleled force failure. Contracture as determined by a rise in resting tension ; developed inconsistently as a late finding only in animals given large doses of DPI. Considerable recovery of force generation occurred with rest. After a single injection a pattern of rapid fatiguability followed by a Partial recovery was established with repeated bursts of 5c/sec stimulation. With DPI, force failure was associated with depletion of PCr but ATP levels were similar to those in control muscle exposed ,1 to the same stimulus, pattern. A massive rise in muscle lactate concentrations with light work was seen in animals given subcutaneous DPI. With DPI mitochondrial changes were minor after intra-arterial infusion. In animals given repeated subcutaneous injections, the changes were more marked and resembled those'seen more acutely_ after dinitrophenol. 4) Abnormal force failure followed injection of antimycin A, but systemic toxicity limited the usefulness of this agent and only 5 experiments were undertaken. 5) Intra-arterial infusion of the glycolytic inhibitor iodoacetate resulted in a rapid failure of twitch tension and muscle exictability, associated with contracture. Severe ATP depletion and IMP accumulation were found in contractured_muscle.