The behaviour of mammalian striated muscle in the presence of 2:4-dinitrophenol

Salicylic acid: old and new implications for the treatment of type 2 diabetes?

Efficacy of salicylic acid as a treatment for diabetes was first established well over a century ago. Antihyperglycaemic effects are thought to include improved peripheral insulin sensitivity and suppression of hepatic glucose production. For most of this period, the molecular mechanisms underlying these effects have been poorly understood and these are still a focus of considerable research, which is reviewed here. Antihyperglycaemic effects are observed only at much higher concentrations than analgesic, antipyretic and antithrombotic properties, suggesting that different targets underlie the antidiabetic aspects of salicylate pharmacology. In the 1950s, antihyperglycaemic responses were linked to mitochondrial uncoupling effects of the drug. Then at the beginning of this century, antihyperglycaemic effects were linked to anti-inflammatory effects of the drug on NF-κB signalling. More recently, new work suggests that direct activation of AMPK may contribute to antihyperglycaemic/antihyperlipidemic actions of salicylates. Better understanding of the mechanism of salicylate’s anthyperglycaemic effects may ultimately accelerate the development of new drugs for human use.

INHIBITION OF CONTRACTION IN A MOLLUSCAN SMOOTH MUSCLE BY THIOUREA, AN INHIBITOR OF THE ACTOMYOSIN CONTRACTILE MECHANISM

The action of thiourea on a molluscan 'catch’ muscle was investigated, to determine whether the actomyosin contractile system can be inactivated directly in the surviving muscle by a poison known to inactivate actomyosin in vitro . Thiourea in a concentration of 0.5 to 0.6 M abolishes reversibly all contractile responses after stimulation with acetylcholine, cathodal direct current, isotonic potassium chloride, caffeine, and chloroform. The inhibiting effect is not caused by blocking the excitation mechanism at the membrane, since although thiourea inhibits the acetylcholine—and KCl contraction, it does not inhibit depolarization by acetylcholine and KCl. It also inhibits caffeine contracture although the latter is independent of the polarization of the membrane. The action of thiourea is not due to an osmotic effect. Thiourea diffuses into the muscle cell: the inside concentration reaches half the outside concentration in about 6 min. The rate of outflux is similar. Since the degree of inhibition depends on the thiourea concentration inside rather than outside it may be concluded that the site of action lies inside. Unlike DNP in certain muscles, thiourea does not reduce the ATP -store in concentrations in which it produces full inhibition of contractile responses. Again, quite unlike DNP , it inhibits the ATP contraction of the isolated contractile apparatus and both the ATP -ase of fibrils and the tension development of actomyosin threads. The dependence of the inhibition on the inhibitor concentration is similar in isolated systems and in the surviving ABRM after equilibration with thiourea. Urea, which produced little inhibition of the ATP contraction in isolated systems, had very little effect on the contrac­tility of living muscles, in concentrations of 0.6 M. The simpliest and probably the most plausible hypothesis to account for all these findings is that thiourea inhibits contractility in the living muscle by inhibiting the contractile and enzymic activity of the actomyosin contractile system. Such a direct action may or may not be implemented by an additional effect on the coupling between excitation and contraction.

Prolonged aerobic exercise: physiological studies in rat gastrocnemius with additional observations on the effects of acute mitochondrial blockade

Protracted low frequency (1 and 5 Hz) stimulation of rat gastrocnemius in vivo leads to fatigue and fall out of glycolytic fibres with the development of a stable twitch response with continuation of the stimulus train. This model was used to examine the physiological effects of acute mitochondrial blockade on oxidative fibre function with the injection of a mitochondrial uncoupler (dinitrophenol) and a site I inhibitor (diphenyleneiodonium) intraarterially after a stable twitch response had developed. Dinitrophenol leads to progressive failure of contractility, closely followed by action potential failure and electrically silent contracture: external work accelerated this sequence but it also developed in resting muscle suggesting that DNP lead to active ATP hydrolysis and a more severe energy depletion than that encountered in human disease states. Diphenyleneiodonium also leads to progressive twitch tension and action potential failure but contracture was late and inconstant, considerable recovery in twitch parameters was seen with rest and restimulation lead to pathological fatiguability of twitch tension. This model has some similarity to human mitochondriopathies with pathological fatiguability. This acute model should allow ready testing of any therapeutic approaches which bypass respiratory chain blocks.

Experimentally induced defects of mitochondrial metabolism in rat skeletal muscle. Biological effects of the mitochondrial uncoupling agent 2,4-dinitrophenol

Infusion of dinitrophenol intra-arterially into rat hind limb caused an irreversible failure of isometric twitch tension and the induction of a severe progressive contracture. Metabolite analysis of muscle in which the twitch response had grossly fatigued revealed low levels of ATP and phosphocreatine together with lactate accumulation. Studies using 31P-n.m.r. confirmed the decrease in ATP and creatine phosphate concentrations and indicated a fall in intracellular pH. It is concluded that dinitrophenol-induced myopathy does not represent a good model for the human mitochondrial myopathic condition as has been previously suggested.

Effects of 2,4-dinitrophenol amylobarbitone and certain other drugs on the rate of oxygen consumption and force of contraction of isolated curarized diaphragm muscle of the rat

1 A technique has been developed for studying over periods of 10 min or longer the effects of drugs on both the force of electrically-induced contractions and the oxygen consumption of an isolated, curarized, mammalian, skeletal muscle preparation.2 The resting oxygen consumption of the muscle was increased substantially by 2,4-dinitrophenol in concentrations (0.02 mM and higher) that eventually produced contracture. Two other uncoupling agents, 4,6-dinitro-o-cresol and carbonylcyanide-p-trifluoromethoxyphenylhydrazone, behaved similarly.3 The oxygen consumption over 10 min of the stimulated muscle was also increased by 2,4-dinitrophenol (0.05 mM), although the strength of the ;maximal' contractions was lessened.4 Amylobarbitone increased the strength of contraction at a concentration (0.2 mM) that did not affect oxygen consumption significantly. Amylobarbitone and pentobarbitone also increased it at a concentration (1 mM) that depressed oxygen consumption. They decreased both strength of contraction and oxygen consumption at a concentration of 5 mM. Phenobarbitone had a weaker action.5 S-n-decyl-thiouronium increased oxygen consumption when given at a concentration (1 mM) that diminished strength of contraction and eventually produced contracture of the muscle.6 Both S-methyl-thiouronium (1 mM) and 4-aminopyridine (0.1 mM and 0.5 mM) increased strength of contraction without increasing oxygen consumption. Neither strength of contraction nor oxygen uptake was affected by ouabain (up to 0.01 mM) or by phenformin (0.1 mM).7 It is concluded that the response to 2,4-dinitrophenol is due mainly, if not wholly, to its known ability to uncouple oxidative phosphorylation; that the response to the barbiturates is due to a combination of a known metabolic action (viz., blocking of the respiratory chain) and a stimulant action on muscle; and the response to S-n-decyl-thiouronium to a disruptive action on cell membranes. The disproportionate actions of 4-aminopyridine and S-methyl-thiouronium on strength of contraction relative to oxygen consumption are also attributed to a non-metabolic action.

Uncoupling activity of the anthelmintic oxyclozanide in rodents

The uncoupling activity of oxyclozanide in warm blooded animals has been studied in whole animals, isolated tissue in vitro and on mitochondrial preparations. The onset of post mortem rigidity in mice and rats is accelerated and a contracture of striated muscle is produced. Oxyclozanide (1 muM) stimulated rat liver mitochondrial respiration and stimulated an ATP-ase activity.

Exchange of structurally bound phosphate in muscular activity

1. When rectus abdominis and sartorius muscles of the frog are incubated with (32)P-orthophosphate and subsequently minced, extracted with carbonate-bicarbonate buffer and water, and converted to acetone powders, considerable radioactivity remains associated with the solid material.2. Muscles labelled with (32)P and subjected to contracture at room temperature with KCl or ACh yield acetone powders which, in comparison with those of uncontracted control muscles, show a significantly decreased radioactivity. There is no evidence for a corresponding decrease in the total phosphate content.3. Relaxation of a contracted muscle restores the (32)P content of its acetone powder at least to the level for a resting, control muscle.4. When two recti are incubated with 5 x 10(-4)M-DNP after labelling with (32)P, subsequent depolarization of one muscle with KCl usually results in an increase in the activity of the bound phosphate fraction, with little or no contracture in addition to that already caused by the DNP.5. It is concluded that the bound phosphate fraction undergoes exchange during activation, shortening and relaxation of muscle, and may represent a source of energy for muscular activity.

The biological action of chlorophenols

The biological action of a series of chlorinated phenols has been investigated. With increasing chlorination there is an increase in toxicity in which the convulsant action of phenol is replaced by the signs characteristic of poisoning by dinitrophenol; the higher chlorinated phenols produce a contracture of the isolated rat phrenic nerve diaphragm and a stimulation of in vitro oxygen uptake of rat brain homogenate. These actions of the chlorinated phenols have been correlated with their dissociation constants; it is suggested that the higher chlorinated phenols interfere with oxidative phosphorylation, and that this property may be attributed to the chlorophenate ion. The convulsant action of the lower chlorinated phenols is probably associated with the undissociated molecule.

The effect of metabolic inhibitors on the electrical and mechanical activity of the smooth muscle of the guinea-pig's taenia coli

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Relation between work and labile phosphate content in the isolated dog heart

Starling heart-lung preparations of the dog (with only minor differences in heart rate) were subjected to changes in work load to study their effect on the amount of labile phosphate in the left ventricle. While increasing the work load three to fourfold by increasing output caused no change, doubling the work load by increasing the resistance was accompanied by a marked reduction in labile phosphate. The data suggest that the heart expends phosphate bond energy chiefly in overcoming resistance rather than in systolic emptying.