Mechanism of action of HI-6 on soman inhibition of acetylcholinesterase in preparations of rat and human skeletal muscle; comparison to SAD-128 and PAM-2

HI-6 is presently considered the most potent oxime antidote against soman poisoning in mice, rats, dogs and monkeys. However, it is still an open question whether efficiency of HI-6, observed in experimental animals, can be extrapolated to soman intoxicated humans. In this paper efficiency of HI-6 and possible mechanisms of action were compared in rat and human fresh muscle preparations. In rat muscle, about 50% of control AChE activity could be recovered by both therapeutic (5 min after soman) and prophylactic (5 min before soman) application of HI-6. On the other hand, in human muscle therapeutic treatment restored only 5%, while prophylactic application of HI-6 again resulted in about 50% recovery of control AChE activity. As revealed by comparison of the prophylactic effects of HI-6 and the non-oxime bispyridinium compound SAD-128, competitive inhibition of AChE plays a minor role as a protective mechanism. Immediate reactivation of rapidly aging human AChE must therefore be instituted for successful protective treatment by HI-6. Retardation of aging, a direct effect of SAD-128, was roughly estimated to improve reactivation by HI-6 for about 10% of control AChE activity of the human muscle. PAM-2 proved completely inefficient as a therapeutic and as a prophylactic agent on both rat and human muscle preparations.     

The mechanism underlying the vasodilator action of bunitrolol: contribution of alpha 1-adrenoceptor blocking action

The mechanism of the vasodilator action of bunitrolol was investigated in pentobarbital-anesthetized dogs. When injected intraarterially, bunitrolol increased blood flow through the femoral arterial bed more effectively than that through the vascular bed of the left anterior descending coronary artery (LAD). The former is rich in alpha-adrenoceptors and tonically controlled by the sympathetic nerves, whereas the latter is not. Intraarterial prazosin increased femoral flow but not LAD flow, whereas intraarterial nitrendipine increased equieffectively both femoral and LAD flows. In the saphenous arterial bed of dogs that also underwent spinal anesthesia and received atropine and nadolol, intravenous bunitrolol suppressed more effectively vasoconstrictor responses to saphenous nerve stimulation than those to intraarterial norepinephrine. These effects of bunitrolol were similar to those of prazosin and dissimilar to those of yohimbine. In similarly treated dogs, bunitrolol suppressed more effectively increases in mean systemic arterial pressure in response to methoxamine than those to B-HT 920. From these results, it was concluded that an alpha 1-adrenoceptor blocking action is mainly involved in the acute vasodilator effect of bunitrolol. This action may also contribute to the decrease in total peripheral resistance seen in hypertensive patients treated chronically with bunitrolol.     

Compulsive gnawing in rats after implantation of drugs into the ventral thalamus: A contribution to the mechanism of morphine action

1 Implantation of morphine into various parts of the corpus striatum of rats evokes only weak gnawing responses.2 Deposition of apomorphine, morphine or methadone in the region of the nucleus ventralis thalami produces a biphasic response, i.e. general excitation, followed by a period of intense gnawing.3 The effect of both apomorphine and morphine is blocked by chlorpromazine, haloperidol and pimozide. However, pretreatment with alpha-methyltyrosine methyl ester or alpha-methyldopa prevents only the gnawing response to morphine, but not to apomorphine.4 Systemic nalorphine, morphine or pethidine suppress the gnawing response, evoked by thalamic implants of apomorphine or morphine.5 Systemic amphetamine potentiates the effect of thalamic deposits of morphine.6 Compulsive gnawing, following implantation of morphine into the ventral region of the thalamus, probably results from enhanced production and release of catecholamines.     

Molecular orbital calculations as a contribution to the elucidation of the mechanism of action of penicillin (author's transl)]

Molecular Orbital Calculations as a Contribution to the Elucidation of the Mechanism of Action of Penicillin. The most stable conformations of methylpenicillin, N-acetyl-D -alanyl-D -alanine and N-acetyl muramic acid are calculated by means of the Extended Hückel Method. A structural similarity between methylpenicillin and N-acetyl-D -alanyl-D -alanine which was already assumed from molecular models is confirmed.     

Central and peripheral contribution to the antihypertensive action of indoramin.

Indoramin has been reported to reduce blood pressure in experimental animals and humans. The complex pharmacological profile of the compound suggests that it may also exert central actions. Effects on spontaneous sympathetic outflow and on sympathetic nerve and blood pressure responses to hypothalamic stimulation were examined in anesthetized cats in the present study. At a low dose (1 mg/kg) indoramin reduced blood pressure but did not significantly influence the level of efferent sympathetic nerve activity. A higher dose (5 mg/kg) lowered pressure further and inhibited activity in the splanchnic, cardiac and renal nerves indicating that reduced sympathetic outflow is a contributing factor to the hypotensive response at this dose. However, in contrast to other centrally acting antihypertensive agents, e.g., clonidine, indoramin did not inhibit the increase in sympathetic nerve activity produced by hypothalamic stimulation.     

A classification of drug substances according to their mechanism of action

Different classification systems for therapeutic agents exist. The most commonly used one is the ATC Code (ATC: Anatomy, Therapeutic properties, Chemical, pharmacological properties). Here, an alternative classification system (TCAT: Target-Chemistry-Anatomy-Therapy) is proposed which refers to the molecular mechanism of action or rather, target. The main subgroups of targets are: enzymes; substrates, metabolies, proteins; receptors; ion channels; transporter molecules and systems; nucleic acids, ribosomes; physicochemical mechanisms; antigen-antibody reactions; unknown targets. This target-oriented approach may be particularly useful in teaching advanced medicinal chemistry.     

A possible mechanism of the antiarrhythmic action of glucagon

The effects of glucagon on ouabain-induced ventricular arrhythmias were studied in dogs with normal cardiac conduction systems and in dogs with surgically induced complete heart block. Glucagon, 20 μg/kg, effected conversion to a sinus rhythm in eight of nine animals with normal conduction systems, and in each instance the conversion occurred with a sinus tachycardia whose rate exceeded the rate of pre-existing ventricular arrhythmia. The one animal which failed to convert to a sinus rythm did not develop a sinus nodal rate faster than the rate of the ventricular arrhythmia. In 10 animals with ouabain-induced ventricular arrhythmias atrial pacing was employed to overdrive the arrhythmia, and in 6 cases this was successful. In all 6 experiments subsequent administration of glucagon after pacing resulted in conversion to a sinus rhythm at a rate greater than the minimum atrial pacing rate required to effect capture of the ventricules. In 4 of the 10 animals neither atrial overdrive nor glucagon were successful in effecting sinus conversion, but ventricular rate was slowed by lidocaine. In addition, in 5 animals with complete heart block and ouabain-induced ventricular tachycardia which were given glucagon, 20 μg/kg, there was no change in the tachycardia and no change in the ECG pattern associated with the arrhythmia. It was concluded that the antiarrhytmic action of glucagon in this arrhythmia is mediated through a supraventricular action to elevate sinus rate above that of the dominant ventricular focus and thus allow a return to dominance of the sinus node.     

The contribution of cyclooxygenase-2 to endocannabinoid metabolism and action

The development of sensitive analytical methods for measurement of endocannabinoids, their metabolites, and related lipids, has underlined the complexity of the endocannabinoid system. A case can be made for an 'endocannabinoid soup' (akin to the inflammatory soup) whereby the net effect of a pathological state and/or a pharmacological intervention on this system is the result not only of changes in endocannabinoid levels but also of their metabolites and related compounds that affect their function. With respect to the metabolism of anandamide and 2-arachidonoylglycerol, the main hydrolytic enzymes involved are fatty acid amide hydrolase and monoacylglycerol lipase. However, other pathways can come into play when these are blocked. Cyclooxygenase-2 derived metabolites of anandamide and 2-arachidonoylglycerol have a number of properties, including effects upon cell viability, contraction of the cat iris sphincter (an effect mediated by a novel receptor), mobilization of calcium and modulation of synaptic transmission. Nonsteroidal anti-inflammatory agents, whose primary mode of action is the inhibition of cyclooxygenase, can also interact with the endocannabinoid system both in vitro and in vivo. Other enzymes, such as the lipoxygenase and cytochrome P450 oxidative enzymes, can also metabolize endocannabinoids and produce biologically active compounds. It is concluded that sensitive analytical methods, which allow for measurement of endocannabinoids and related lipids, should provide vital information as to the importance of these alternative metabolic pathways when the primary hydrolytic endocannabinoid metabolizing enzymes are inhibited.     

Scavenging effect of 5-aminosalicylic acid on neutrophil-derived oxidants. Possible contribution to the mechanism of action in inflammatory bowel disease.

Inflammatory phagocytic leukocytes produce superoxide and hydrogen peroxide and secrete myeloperoxidase (MPO) into the extracellular medium. MPO catalyzes the oxidation of Cl- by H2O2 to yield chlorinated oxidants (e.g. HOCl and NH2Cl), which have been shown to induce pathologic changes in mucosal function. We examined the ability of 5-aminosalicylic acid (5-ASA), a drug used to treat inflammatory bowel disease (IBD), to inhibit oxidation of L-cysteine by NH2Cl, HOCl and H2O2. NH2Cl and HOCl were especially strong oxidants against L-cysteine. 5-ASA prevented L-cysteine oxidation by NH2Cl and HOCl; an interaction associated with the formation of characteristic absorption spectra due to the oxidation of 5-ASA was observed. NH2Cl and HOCl evoked characteristic increases in short-circuit current (Isc), indicative of net electrolyte transport, when added to the serosal side of stripped rat colon mounted in Ussing chambers. Premixing of NH2Cl with 5-ASA 10 min before addition to the tissue markedly reduced the secretory response to NH2Cl. In contrast, 5-ASA immediately reduced the response to HOCl. The reduction in the functional response to NH2Cl and HOCl by 5-ASA may contribute to its mechanism of action in the treatment of the symptoms of IBD.     

The contribution of intrinsic activity to the action of opioids in vitro.

The effects of opioids were compared in five field-stimulated isolated tissue models, the guinea-pig ileum and vasa deferentia from rat, rabbit and mice of the Alderley Park and C57BL/6 strains. Although the mu-receptor agonist [D-Ala2, MePhe4, Gly-ol5] enkephalin appeared to act at similar receptors in the guinea-pig ileum, rat vas deferens, mouse vas deferens and C57BL/6 mouse vas deferens preparations, its potency varied considerably between these preparations. Similar potency differences were also observed with the kappa-agonist, ethylketocyclazocine. It is proposed that these variations in potency reflect differences in the number of spare receptors present in each model. The finding that some drugs which have agonist activity in the more sensitive preparations behave as antagonists in the less sensitive tissues supports this proposal and highlights the importance of intrinsic activity in determining the action of opioids. Many of the prototypic opioid agonists were found to be either partial agonists (eg. morphine and bremazocine) or to possess affinity for more than one receptor type (eg. ethylketocyclazocine, Mr 2034).     

On the metabolism of methylated disulphonamides with a diuretic action—a contribution to the mechanism of action of chlorothiazide and similar compounds

The metabolism of two substances possessing a diuretic action, 4-amino-6-chlorobenzene-1,3-disulphonmethylamide (III) and 6-chloro-3,4-dihydro-2-methyl-7-methylsulphamoyl-2H-1,2,4-benzo[e]thiadiazine-1,1-dioxide (IV), has been studied on rats and human subjects. It has been shown that these compounds are not demethylated in the -SO₂.NHCH₃ group. They therefore do not act through liberation of substances with unsubstituted sulphonamido-groups which might inhibit carbonic anhydrase, and their mechanism of diuretic action cannot be considered due to inhibition of this enzyme.     

A pharmacological analysis of the mechanism of action of ethosuximide

Ethosuximide is one of the means of treatment of minor epilepsy but hardly any data on its mechanism of action are available in the literature. Anticonvulsant agents are known to bring about changes in the functions and in the interaction between some of the mediator systems within the central nervous system. An assessment of the status of neuromediator systems can be made on the basis of the response of isolated smooth muscle strips to the action of agonists and antagonists of various receptors. It was found by the pharmacological analysis of isolated strips from the rat stomach (antrum and corpus strips), the seminal duct and the cervical vein that ethosuximide induces a reduction in the physical contractile activity and the tone of smooth muscle preparations. Smooth muscle relaxation caused by ethosuximide is not blocked by different receptor inhibitors such as dihydroergotamine, propranolol, atropine, chlorpromazine, haloperidod and indomethacin. Ethosuximide causes a significant reduction in the physical contraction of smooth muscles produced by potassium chloride depolarization, with a stronger impact on the subsequent tonic contraction caused by calcium ions. A reduction in the potassium content of the solution has no effect on the nature of the action of ethosutimide. It is thus assumed that the probable mechanism of action of ethosuximide consists in lowering calcium transport since the inhibitors of calcium transport sodium nitroprusside and verapamil intensify the blocking effect of ethosuximide on smooth muscle contractile activity.     

Effects of 1,1'-oxydimethylene bis-(4-tert-butylpyridinium chloride) (SAD-128) and decamethonium on reactivation of soman- and sarin-inhibited cholinesterase by oximes

The effects of 1,1′-oxydimethylene bis-(4-tert-butylpyridinium chloride) (SAD-128) and decamethonium on reactivation by oxime of Soman- and Sarin-inhibited erythrocyte acetylcholinesterase (AChE; EC 3.1.1.7) are reported. The inclusion of SAD-128 or decamethonium (10^?3 M) together with either TMB-4 or Toxogonin markedly augmented the reactivation of Soman- or Sarin-inhibited AChE; the recovery of enzyme activity was more than doubled in each instance when compared to the recovery in the presence of oxime alone. No reactivation was observed with SAD-128 or decamethonium in the absence of oxime. The i.v. ld₅₀ of SAD-128 in rabbits is 11.9 mg/kg. When 5 mg/kg of SAD-128 was given i.v. to rabbits 2 min before poisoning with Soman. 15–20 per cent of the blood cholinesterase was protected from inhibition by Soman. The aging rate in vitro of Soman-inhibited erythrocyte AChE was decreased by a factor of 1.9 with 10^?3 M SAD-128. A concentration of 1.4 × 10^?4M SAD-128 reduced the rate of acetylcholine hydrolysis by erythrocyte AChE in vitro to 50 per cent of control. We suggest that SAD-128 protects against inhibition by Soman due to its ability to function as a reversible inhibitor and that SAD-128 and decamethonium might be allosteric modifiers of the inhibited enzyme, rendering it more susceptible to reactivation by TMB-4 or Toxogonin.     

A microdialysis study on the mechanism of action of gabapentin

To gain insight into the mechanism of action of the anti-epileptic, gabapentin, the effects of gabapentin on the in vivo extracellular gamma-aminobutyric acid (GABA) levels in the rat substantia nigra reticulata were studied using microdialysis. In order to investigate possible interference with different GABA-ergic compartments in the substantia nigra reticulata, we studied the effects of gabapentin under basal, K(+)-, nipecotic acid- and glutamate-stimulated conditions. Intraperitoneally (i.p.) administered gabapentin, at a dose of 100 mg/kg, did not significantly affect extracellular GABA levels under any condition. Thus, our data do not support the involvement of nigral GABA release in the mechanism of action of the anti-epileptic gabapentin.     

A mechanism of action of phenylephrine on heart

Phenylephrine exerted a positive chronotropic and inotropic effect on isolated, spontaneously beating, atria of reserpinised rabbits. Addition of phenoxybenzamine and phentolamine resulted in a depression of control contractile amplitude. Practolol, however, was devoid of this effect. The positive inotropic response to phenylephrine was significantly antagonised by all the three blockers used, while positive chronotropic response was annulled by phentolamine and practolol, but not with phenoxybenzamine. It is, therefore, suggested that phenylephrine exerts its cardiostimulant effects through mediation of both alpha and beta-1 adrenoceptors. A probable mechanism of action could be, that phenylephrine acts on some specific chemical group, shared by alpha and beta1 receptors. This specific group is probably blocked by both alpha and betaceptor antagonists separately, so phenylephrine becomes ineffective in presence of these antagonists.     

A new concept regarding the mechanism of action of omeprazole.

OBJECTIVES: In this paper we investigated in humans and in animals the in vitro and in vivo effect of omeprazole upon purified and erythrocyte carbonic anhydrase (CA) I and II isozymes, as well as on gastric mucosa CA IV. METHOD: In vitro, we observed the effect of omeprazole at concentrations between 10(-8)-10(-4) M on purified CA I and CA II, and also on isolated gastric mucosa CA IV, renal and pulmonary CA IV activity, using the dose-response relationship. In vivo, we studied the effect of omeprazole (Losec) on gastric CA I, II and IV, as well as on erythrocyte CA I and CA II, in humans and in animals. RESULTS: In vitro omeprazole inhibits pH-dependent purified CA I and CA II and gastric mucosa CA IV according to dose-response relationship. In vivo, the i.v. administration of omeprazole in rabbits and in humans shows a decrease of erythrocyte CA I and CA II activity as well as of gastric mucosa CA I, II and IV. CONCLUSIONS: Omeprazole in its active form (sulfenamide) selectively inhibits gastric mucosa CA IV and does not modify the activity of the same isozyme from the kidney and lung proving that the enzyme has an organ specificity. Our results lead to the conclusion that omeprazole possesses a dual mechanism of action: both H+K+ATPase and CA inhibition--enzymes that could be in a functional coupling. This dual mechanism of action might explain the higher effectiveness of treatment using substituted benzimidazole inhibitors compared to other therapies.