Skeletal muscle stimulants. Substituted benzoic acids

An Iron-Sulfur Cluster with a Highly Pyramidalized Three-Coordinate Iron Center and a Negligible Affinity for Dinitrogen

Attempts to generate open coordination sites for N2 binding at synthetic Fe-S clusters often instead result in cluster oligomerization. Recently, it was shown for Mo-Fe-S clusters that such oligomerization reactions can be prevented through the use of sterically protective supporting ligands, thereby enabling N2 complex formation. Here, this strategy is extended to Fe-only Fe-S clusters. One-electron reduction of (IMes)3Fe4S4Cl (IMes = 1,3-dimesitylimidazol-2-ylidene) forms the transiently stable edge-bridged double cubane (IMes)6Fe8S8, which loses two IMes ligands to form the face-bridged double-cubane, (IMes)4Fe8S8. The finding that the three supporting IMes ligands do not confer sufficient protection to curtail cluster oligomerization prompted the design of a new N-heterocyclic carbene, SIArMe,iPr (1,3-bis(3,5-diisopropyl-2,6-dimethylphenyl)-2-imidazolidinylidene; abbreviated as SIAr), that features bulky groups strategically placed in remote positions. When the reduction of (SIAr)3Fe4S4Cl or [(SIAr)3Fe4S4(THF)]+ is conducted in the presence of SIAr, the formation of (SIAr)4Fe8S8 is indeed suppressed, permitting characterization of the reduced [Fe4S4]0 product. Surprisingly, rather than being an N2 complex, the product is simply (SIAr)3Fe4S4: a cluster with a three-coordinate Fe site that adopts an unusually pyramidalized geometry. Although (SIAr)3Fe4S4 does not coordinate N2 to any appreciable extent under the surveyed conditions, it does bind CO to form (SIAr)3Fe4S4(CO). This finding demonstates that the binding pocket at the unique Fe is not too small for N2; instead, the exceptionally weak affinity for N2 can be attributed to weak Fe-N2 bonding. The differences in the N2 coordination chemistry between sterically protected Mo-Fe-S clusters and Fe-only Fe-S clusters are discussed.

The chloride-channel blocker 9-anthracenecarboxylic acid reduces the nonlinear capacitance of prestin-associated charge movement

The basis of the extraordinary sensitivity and frequency selectivity of the cochlea is a chloride-sensitive protein called prestin which can produce an electromechanical response and which resides in the basolateral plasma membrane of outer hair cells (OHCs). The compound 9-anthracenecarboxylic acid (9-AC), an inhibitor of chloride channels, has been found to reduce the electromechanical response of the cochlea and the OHC mechanical impedance. To elucidate these 9-AC effects, the functional electromechanical status of prestin was assayed by measuring the nonlinear capacitance of OHCs from the guinea-pig cochlea and of prestin-transfected human embryonic kidney 293 (HEK 293) cells. Extracellular application of 9-AC caused reversible, dose-dependent and chloride-sensitive reduction in OHC nonlinear charge transfer, Qmax . Prestin-transfected cells also showed reversible reduction in Qmax . For OHCs, intracellular 9-AC application as well as reduced intracellular pH had no detectable effect on the reduction in Qmax by extracellularly applied 9-AC. In the prestin-transfected cells, cytosolic application of 9-AC approximately halved the blocking efficacy of extracellularly applied 9-AC. OHC inside-out patches presented the whole-cell blocking characteristics. Disruption of the cytoskeleton by preventing actin polymerization with latrunculin A or by decoupling of spectrin from actin with diamide did not affect the 9-AC-evoked reduction in Qmax . We conclude that 9-AC acts on the electromechanical transducer principally by interaction with prestin rather than acting via the cytoskeleton, chloride channels or pH. The 9-AC block presents characteristics in common with salicylate, but is almost an order of magnitude faster. 9-AC provides a new tool for elucidating the molecular dynamics of prestin function.

Effects of organophosphorus agents on sarcoplasmic reticulum in skinned skeletal muscle fibers

These experiments were designed to determine whether skinned skeletal muscle fibers could be useful in screening new antidotes to organophosphorus poisons. Isometric force and fiber diameter were measured in mechanically skinned fibers from mice and frogs. Fibers were depleted of calcium and placed in a calcium loading solution that contained 0.5 mM EGTA with pCa 6.25. The elapsed time (zero time) before a contracture began and the maximum rate of force development (slope) were measured and divided by the square of the diameter (normalized zero time, normalized slope). The zero time was assumed to be the time required for the sarcoplasmic reticulum to attain a threshold concentration for calcium-induced calcium release, and the slope was assumed to indicate primarily the rapidity of the release of calcium from the sarcoplasmic reticulum. Organophosphorus agents, sarin, soman, tabun, and VX were also placed in the loading solutions. Only sarin failed to shorten the normalized zero times of mouse fibers compared to controls, and all agents decreased the normalized slopes. The normalized zero times of frog fibers were not altered by the agents, but the normalized slopes were altered by some agents. Pralidoxime chloride (PAM) and 3-Cl-2,5,6-trimethylbenzoic acid (TBA) were also added to the loading solution for mouse fibers; PAM was marginally effective in moderating some actions of the organophosphates. Because the effects of the agents on the fibers were so definite, we concluded that the skinned muscle fiber might indeed be useful as a screening tool for developing and testing new antidotes to organophosphorus poisons.

Comparison of UV and fluorescence spectrophotometry for the quantification of a potent myotonia inducer: anthracene-9-carboxylic acid, in plasma, urine, and saline perfusion fluids

UV and fluorescence spectrophotometry were used to establish the analytical profile of a potent myotonia inducer, anthracene-9-carboxylic acid (I). UV spectrophotometry is useful for the determination of I when it is dissolved in physiological solutions (Ringer's, Tyrode's, etc). In these fluids there is a linear relationship between UV absorption and I concentration between 500 and 2000 ng/ml (2.25-9.0 X 10(-6)M). However, in biological fluids there are interferences in the UV absorption due to organic substances. On the other hand, fluorescence spectrophotometry is more sensitive than UV for determinations in plasma and urine. Within the range of 200-1000 ng/ml (0.9-4.5 X 10(-6) M) fluorescence intensity increases linearly with concentration. Furthermore, when both emission and excitation spectra are combined there are no interferences due to organic substances normally present in those fluids. An extraction procedure of I from plasma and urine is also described, and the importance of I determinations in relation to the problem of this myotonia-inducing aromatic monocarboxylic acid is discussed.

Myotonia as a side effect of diuretic action

1. Commonly used loop diuretics produce side effects in man which are similar to chemically induced myotonia. These diuretics have structural affinity with known myotonic agents. 2. We have observed EMG myotonia in vivo in leg muscles of rats treated with intravenous frusemide. 3. In the presence of several different diuretics, rat isolated diaphragm, soleus and extensor digitorum longus muscles as well as frog sartorius muscles produce typically myotonic contractions with relaxation times up to several seconds. 4. Intracellular recording of action potentials from diuretic-treated muscles reveals long lasting after-discharges following a brief electrical stimulus, again typical of chemically induced myotonia. 5. Having demonstrated a myotonic action of several diuretics we suggest a need for caution in using these drugs in persons with hereditary myotonia and a need to be aware of possible provocation of myotonia in subclinical cases. Myopathies and neuropathies which are known to result from chronic exposure to myotonic agents also need to be considered. 6. In our study, the diuretic, acetazolamide, unmasked subthreshold myotonia. This seems to be at variance with reports of its usefulness in the treatment of myotonia. 7. Diuretics should probably not be employed in the treatment of herbicide intoxication where their myotonic activity would be expected to add to the known myotonic activity of the herbicide.

Subacute skeletal myopathy induced by 2,4-dichlorophenoxyacetate in rats and guinea pigs

Rats and guinea pigs were injected with 2,4-dichlorophenoxyacetate (2,4-D) in amounts sufficient to keep them myotonic for over 12 hr daily. The biceps brachii muscles were studied by histochemistry, epoxy resin histology, and electron microscopy after one to five days. Changes observed include proliferation of the longitudinal component of the sarcoplasmic reticulum, production of new myofilaments on the periphery of cells and in nuclei, activation of satellite cells, accumulation of lipid droplets, myofibrillar damage with Z-disc streaming, necrosis, and regeneration. The action mehanisms of 2,4-D are not well understood. Some of the phenomena observed could be classified as proliferative and may be analogous to the auxin action of 2,4-D in plants. The production of intranuclear filaments is probably related to the diffusion of newly formed cytoplasmic proteins into nuclei.

On the inhibition of muscle membrane chloride conductance by aromatic carboxylic acids

25 aromatic carboxylic acids which are analogs of benzoic acid were tested in the rat diaphragm preparation for effects on chloride conductance (G(Cl)). Of the 25, 19 were shown to reduce membrane G(Cl) with little effect on other membrane parameters, although their apparent K(i) varied widely. This inhibition was reversible if exposure times were not prolonged. The most effective analog studied was anthracene-9-COOH (9-AC; K(i) = 1.1 x 10(-5) M). Active analogs produced concentration-dependent inhibition of a type consistent with interaction at a single site or group of sites having similar binding affinities, although a correlation could also be shown between lipophilicity and K(i). Structure-activity analysis indicated that hydrophobic ring substitution usually increased inhibitory activity while para polar substitutions reduced effectiveness. These compounds do not appear to inhibit G(Cl) by altering membrane surface charge and the inhibition produced is not voltage dependent. Qualitative characteristics of the I-V relationship for Cl(-) current are not altered. Conductance to all anions is not uniformly altered by these acids as would be expected from steric occlusion of a common channel. Concentrations of 9-AC reducing G(Cl) by more than 90 percent resulted in slight augmentation of G(I). The complete conductance sequence obtained at high levels of 9-AC was the reverse of that obtained under control conditions. Permeability sequences underwent progressive changes with increasing 9-AC concentration and ultimately inverted at high levels of the analog. Aromatic carboxylic acids appear to inhibit G(Cl) by binding to a specific intramembrane site and altering the selectivity sequence of the membrane anion channel.

Further studies on the therapy of organophosphorous anti-cholinesterase intoxication with veratrinic compounds; the role of calcium

Experiments were carried out to investigate the role of calcium in the therapy of soman intoxication with 9-anthroic acid (ANCA), a compound with veratrine-like pharmacological properties. The effects of ANCA on the respiratory paralysis and on the calcium content of the blood and that of the hindleg muscles were determined in anaesthetized, atropinized rats injected with 4 times LD50 soman. The respiratory paralysis which in control animals occurs within a few min after the injection of soman can be delayed about 2.5 hr by treatment with ANCA. It was found that ANCA causes a small decrease of the blood calcium content, an effect which is potentiated by soman. A comparison was made between the calcium accumulation in the indirectly stimulated gastrocnemius-soleus muscles in these animals with that in the non-stimulated muscles on the other side. Whereas the injection of soman or ANCA alone caused no change, the combination of the two drugs induced a two-fold increase in the accumulation of calcium in the stimulated muscles. The non-stimulated muscles remained unaffected. The accumulation of calcium in the stimulated muscles induced by soman and ANCA could be partly antagonized by lowering the free calcium concentration of the blood by EDTA. Moreover, treatment with EDTA improved the therapeutic effects of ANCA. It is concluded that the therapy of soman poisoning with ANCA falls short in completely preventing respiratory failure since ANCA causes an accumulation of calcium in the stimulated muscles of soman-poisoned animals.

Chloride conductance in normal and myotonic muscle fibres and the action of monocarboxylic aromatic acids

1. Cable parameters, component conductances, excitability and membrane potentials in isolated external intercostal fibre bundles at 38 degrees C from normal and myotonic goats were measured in normal and low-chloride Ringer, and in the presence of monocarboxylic aromatic acids that produce myotonic responses in mammalian muscle.2. The mean resting chloride conductance in mumho/cm(2) of myotonic fibres (range 0-147) was significantly less than that of normal fibres (range 376-951). The mean resting potassium conductance was higher in myotonic fibres (range 123-285) than in normal fibres (range 44-132). Potassium conductance increased about 10 mumho/cm(2) per mV increase in absolute resting potential.3. In normal fibres in normal Ringer 3-chloro-2,5,6-trimethylbenzoic acid; 5,6-dihydro-5,5-dimethyl-7-carboxybenz[c]acridine; phenanthrene-9-carboxylic acid; and anthracene-9-carboxylic acid at 10(-5)-10(-4)M decreased membrane conductance without consistently changing diameter or capacitance. In low-chloride Ringer 3-chloro-2,5,6-trimethylbenzoic acid (5 x 10(-5)M) increased potassium conductance in myotonic and normal fibres. It is concluded that these compounds block chloride conductance.4. The carboxylic acids produced myotonia in normal fibres similar to that in untreated myotonic fibres.5. Anthracene-9-carboxylic acid intravenously (8 mg/kg) in normal goats produced acutely a condition resembling myotonia congenita. The carboxylic acids produced no myotonic effects in frog muscle.