A comparison of the effects of sarin and succinylcholine on respiratory parameters in anesthetized domestic swine

Differences in the "respiratory paralysis" caused by sarin (GB) and succinylcholine (SDC) were observed in a domestic swine model using a bedside pulmonary dynamics monitor. GB was administered intravenously (9 micrograms/kg/30 min) and compared with SDC administered intravenously (20 mg/30 min). All animals developed respiratory insufficiency indicated by decreased respiratory frequency. Minute ventilation was relatively maintained in animals that received GB by increasing tidal volume, whereas both of these parameters decreased in animals that received SDC. GB animals showed an increase in airway resistance and work of breathing. The former was unchanged and the latter was decreased in animals that received SDC. Mouth occlusion pressure at 100 milliseconds and tidal volume were relatively maintained in GB animals but decreased in SDC animals, suggesting a central mechanism for respiratory paralysis with GB and a peripheral mechanism for respiratory paralysis with SDC.

Effects of bis(2-chloroethyl)sulfide on ATP receptor-mediated responses of the rat vas deferens: possible relationship to cytotoxicity

Extracellular ATP is a broad-spectrum cytotoxic agent that produces effects via cell surface P2 purinoceptors. The ligand-gated P2X purinoceptor subtype has very high sequence homology with the RP-2 gene, which encodes for apoptosis. The P2X RNA found in rat vas deferens is expressed preferentially by apoptotic thymocytes. P2X purinoceptor-mediated phasic (twitch) motor responses of the isolated rat vas deferens to neurogenic or exogenous ATP were rapidly, specifically and irreversibly potentiated by bis(2-chloroethyl)sulfide (HD 10-100 microM). Both untreated and HD-potentiated neurogenic responses were Ca++ dependent, blocked in the absence of Ca++ plus 0.1 mM EGTA, by the neuronal Ca++ channel blocker omega-conotoxin-MVIIC (3 microM), by the P2 purinoceptor antagonist suramin (100 microM) and by tetrodotoxin (100 nM). HD also potentiated the effects of ATP on isolated guinea pig taenia caecum, where the nucleotide acts at G protein-coupled P2Y purinoceptor subtypes to cause relaxation. HD failed to inhibit the metabolism of ATP by ecto-ATPase in vas deferens or to cause the release of endogenous ATP. Potentiation of the twitch response to electric field stimulation by HD was attenuated or eliminated in tissues excised from rats previously challenged with topically applied HD, suggesting that HD absorbed into the systemic circulation had already effected maximal potentiation of ATP responses before in vitro testing. The physiological consequences of HD-induced potentiation of the extracellular actions of ATP are discussed in relation to apoptosis and necrosis.

Protective effect of an inhibitor of nitric oxide synthase on sulphur mustard toxicity in vitro

Although sulphur mustard is one of the oldest chemical warfare agents, its mechanism of toxic action is still not understood and as a consequence, no antidotes exist that are effective against this agent. Pretreatment of chick embryo neuron cultures with the well-known nitric oxide synthase (NOS) inhibitor L-nitroarginine methyl ester (L-NAME) was found to confer significant protection against sulphur mustard-induced cell death. However, these protective effects were not mediated through the inhibition of NOS. These findings may provide clues to the eventual understanding to sulphur mustard toxicity, and also suggest that L-NAME has significant novel pharmacological effects other than the inhibition of NOS.

A role for Q type Ca2+ channels in neurotransmission in the rat urinary bladder

1. In isolated bladder strips of the rat, a substantial component (46%) of the Ca(2+)-dependent contractile response to electrical field stimulation (5 Hz) was resistant to combined block of both N and P type Ca2+ channels by omega-conotoxin-GVIA (300 nM) and omega-agatoxin-IVA (100 nM) respectively. 2. The resistant portion (non-N, non-P) was sensitive to omega-conotoxin-MVIIC (3 microM), which in addition to N and P also blocks Q type channels at this concentration. omega-Conotoxin-MVIIC administered alone, inhibited the neurogenic response to the same degree as that observed in the combined presence of omega-agatoxin-IVA, omega-conotoxin-GVIA and omega-conotoxin-MVIIC. 3. omega-Agatoxin-IVA (100 nM), a concentration that fully inhibits P type channels, had a negligible effect on the neurogenic response. Following blockade of N type Ca2+ channels with omega-conotoxin-GVIA (300 nM), omega-agatoxin-IVA (3 microM) (a concentration well above that used to block P channels, inhibits Q type channels, but spares N type channels), inhibited the residual response to the same degree as omega-conotoxin-MVIIC alone. 4. Results suggest that neurotransmission in rat urinary bladder is supported by both N and Q type Ca2+ channels.

Effect of ruthenium red on voltage-sensitive Ca++ channels

The organometallic dye, ruthenium red (RR) inhibited Ca++ influx, omega-conotoxin GVIA-sensitive Ca++ binding and Ca(++)-dependent neurotransmitter release in a qualitatively similar manner in rat and chicken synaptosomes and in mammalian neuromuscular preparations, but had no effect on Ca(++)-dependent processes mediated by the dihydropyridine-sensitive Ca++ channel. These effects are specific for the RR complex, as RuCl3 affected neither Ca++ influx, omega-conotoxin GVIA binding nor neurotransmitter release, but did, however, in contrast to RR, displace [3H]nitrendipine from synaptosomes. RR, in a manner similar to omega-conotoxin MVIIC and omega-agatoxin IVA (AgaIVA), also effectively inhibited the response of the rat diaphragm to nerve stimulation and blocked AgaIVA-sensitive Ca++ influx in the rat brain, suggesting a significant interaction at the P-type voltage-sensitive Ca++ channel. These effects of RR suggest that this amino complex affects both the N and the P domain of the Ca++ channel in the chicken brain and both the N- and P-type Ca++ channel which is intimately coupled to the Ca++ influx and neurotransmitter release in rat synaptosomes. Its ability to block all of the Ca++ influx in mammalian brain preparations and to inhibit completely the nerve-stimulated rat neuromuscular junction certainly indicates a significant action at the P-type voltage-sensitive Ca++ channel, similar to omega-conotoxin MVIIC or AgaIVA. RR should prove to be a valuable synthetic, inexpensive tool with which to probe the neuropharmacology of the mammalian neurotransmitter-linked voltage-sensitive Ca++ channels.

Effect of omega-agatoxin-IVA on autonomic neurotransmission

omega-Agatoxin-IVA, a peptide from the venom of the funnel-web spider Agelenopsis aperta and a P type Ca2+ channel inhibitor, was examined for effects on responses to nerve stimulation in isolated autonomic neuroeffector preparations from the rabbit, guinea-pig and rat. Ca(2+)-dependent, tetrodotoxin sensitive, noradrenergic excitatory responses of rabbit pulmonary artery, rat vas deferens, and anococcygeus muscles, and cholinergic guinea-pig myenteric plexus preparations (all highly sensitive to the N type Ca2+ channel inhibitor omega-conotoxin-GVIA) were unaffected by omega-agatoxin-IVA (100 nM). Similarly, the neurogenic response of rat bladder, which has cholinergic, and non-adrenergic non-cholinergic (NANC) excitatory components, and the NANC inhibitory response of rat jejunum (atropine 0.5 microM- and guanethidine 5.0 microM-treated), which are partially sensitive and insensitive to omega-conotoxin-GVIA, respectively, were unaffected by omega-agatoxin-IVA (100 nM). Neurogenic NANC inhibitory responses of the guinea-pig taenia caecum, and rat anococcygeus muscles (atropine- and guanethidine-treated, and tone raised with prostaglandin F2 alpha), were also insensitive to omega-agatoxin-IVA. These results suggest that P type Ca2+ channels, if present, play an insignificant role in supplying the Ca2+ necessary for neurotransmitter release in the peripheral autonomic nervous system.

Pharmacological identification of a novel Ca2+ channel in chicken brain synaptosomes

Ca2+ influx was measured in rat and chicken brain synaptosomes in the presence of a number of pharmacological tools which have recently been used to define voltage-sensitive Ca(2+)-channel (VSCC) types. In chicken brain synaptosomes. VSCCs which, because of their sensitivity to inhibition by omega-conotoxin (omega-CgTx), are thought to be exclusively N-type, the P-type VSCC polyamine inhibitor FTX (from Agelenopsis aperta venom; 1 microliters/ml), its synthetic analogue, sFTX (1-5 mM) and the polypeptides AgaIVA (IC50 0.29 microM) and omega-CgTx MVIIC (IC50 0.0022 microM) inhibited 70-100% of the measurable K+ stimulated Ca2+ influx. The prototypical N-channel VSCC inhibitor omega-CgTx GVIA (IC50 0.014 microM), Cd2+ (50 microM) and diluted venom from Hololena curta (1:2,500) also caused complete or almost complete, inhibition of Ca2+ influx. In comparable studies using rat brain synaptosomes, sFTX (1-10 mM) caused a dose-dependent reduction of Ca2+ influx, while FTX (1 microliters/ml) and AgaIVA (IC50 0.02 microM) completely inhibited Ca2+ influx. Similar to the findings in chicken synaptosomes, Cd2+ (50 microM) and H. curta (1:2,500 dilution) both inhibited K+ stimulated influx by > 80% whereas omega-CgTx (1 microM) only caused a maximum 25% inhibition. Both sFTX and its congener spermine, inhibited [125I]omega-CgTx binding to rat and chicken synaptosomal membranes. These results strongly implicate P-type channels as the major VSCC in rat brain. The results also clearly demonstrate a heretofore unrecognized, novel, FTX/AgaIVA/omega-CgTx GVIA/omega-CgTx MVIIC-sensitive VSCC in chicken brain.

Identification of noradrenaline in venom from the funnel-web spider Hololena curta

Venom from the funnel-web spider Hololena curta was added to the Krebs-Henseleit solution bathing isolated ring preparations of rat thoracic aorta, suspended in water-jacketed organ baths, for the purpose of tension recordings. Hololena curta venom at dilutions of 1:100,000 to 1:1000 caused a marked vasoconstriction, which was completely inhibited by the alpha 1 adrenoceptor antagonist prazosin (1 microm). The vasoconstriction appears to be due to the direct effects on alpha 1 adrenoceptors of a venom constituent, which we have identified using HPLC/ECD as the catecholamine, noradrenaline.

Evidence of mammalian Ca2+ channel inhibitors in venom of the spider Plectreurys tristis

Plectreurys tristis venom inhibited K(+)-stimulated Ca2+ influx in a concentration-dependent manner in rat (0.5-4.0 micrograms venom protein/ml) and chicken (1.0-64.0 micrograms venom protein/ml) brain synaptosomes. In contrast to Hololena curta venom or omega conotoxin GVlA which both show selectivity for avian synaptosomes, inhibition of Ca2+ influx by the venom appeared to be relatively selective for rat synaptosomes. Plectreurys tristis venom also inhibited K(+)-evoked release of [3H](-)-noradrenaline from labeled rat cortical synaptosomes. Responses to electric field stimulation of the sympathetically innervated rat vas deferens in vitro were inhibited by Plectreurys tristis venom at dilutions similar to those which inhibited Ca2+ influx in synaptosomes. Inhibition persisted following washout of the venom. K(+)-evoked contractions of rat aortic rings were relaxed by the dihydropyridine antagonist (-)-202-791, but not by Plectreurys tristis venom, thus precluding an effect on K(+)-depolarized smooth muscle L-type channels. Contractions to exogenous (-)-noradrenaline in rat aorta were not inhibited by Plectreurys tristis venom, ruling out an effect on alpha 1-adrenergic receptors, and further suggesting a prejunctional site of action. The results suggest that this venom inhibits N-type Ca2+ channels, as well as unclassified Ca2+ channels, which are neither N- nor L-type.

Pharmacological characterization of voltage-sensitive Ca2+ channels in autonomic nerves

Hololena curta venom a potent inhibitor of voltage sensitive Ca2+ channels and neurotransmitter release in mammalian brain, and synthetic funnel web spider toxin an inhibitor of P channels, were examined for their activity on autonomic nerves. Hololena curta (0.5 to 5.0 micrograms venom protein/ml) potently inhibited motor responses of the cholinergic guinea pig ileum myenteric plexus and the adrenergic rat anococcygeus muscle. Synthetic funnel web spider toxin was inactive at concentrations up to 100 microM. Hololena curta inhibited K+, and electrically evoked release of tritium from labeled superfused tissues. Furthermore, K(+)-contracted rat aorta was not relaxed by Hololena curta thereby precluding effects of Hololena curta on postjunctional L type smooth muscle Ca2+ channels. The pattern of effects of Hololena curta on peripheral autonomic nerves was similar to the N channel inhibitor omega-conotoxin GVIA. These results suggest that Hololena curta venom constituents block Ca2+ channels in peripheral autonomic nerves. The study failed to establish the presence of functional P type Ca2+ channels on these peripheral autonomic nerves and further suggests that N type channels may be exclusively responsible for supplying the Ca2+ necessary for neurotransmitter release in these nerves.

Inhibition of a dihydropyridine, omega-conotoxin insensitive Ca2+ channel in rat synaptosomes by venom of the spider Hololena curta

Inhibition of the N and L type Ca2+ channels with omega conotoxin GVIA (omega-CgTx) together with the dihydropyridine (-)-202-791 produces slight reduction (congruent to 25%) of K(+)-evoked Ca2+ influx in mammalian synaptosomes. These results and others suggest the existence of a third high threshold voltage sensitive calcium channel (VSCC) responsible for the majority of influx. Venom from the funnel web spider Hololena curta potently and persistently inhibited Ca2+ influx in rat cortical synaptosomes (IC50 1:10,000 or 4.21 micrograms/venom protein/ml of synaptosomes). Also Ca2+ influx in cerebellar synaptosomes was inhibited in a similar manner. K(+)-evoked tritium release from synaptosomes labeled with [3H]noradrenaline was inhibited by Hololena venom (congruent to 60% reduction at 10 micrograms/venom protein). Inhibition of Ca2+ influx by venom was unaffected by combined omega-CgTx and (-)-202-791 pretreatment (both 1 microM). Hololena venom and its active constituent should provide useful tools to investigate the role of this novel Ca2+ channel in neuronal function.

Comparison of several oximes against poisoning by soman, tabun and GF

Three oximes currently being evaluated for adoption as replacement nerve agent therapy by various countries were compared for therapeutic efficacy against the toxic organophosphate inhibitors soman and tabun under a standard set of conditions. These oximes together with PAM-Cl and toxogonin, were also compared for efficacy against GF, an agent weaponized by Iraq. The order of effectiveness against soman was HI-6 greater than HLö-7 greater than pyrimidoxime. HLö-7 was very effective against tabun poisoning while HI-6 and pyrimidoxime were of moderate value. Against GF, HI-6 and HLö-7 were extremely effective, toxogonin was moderately effective, and PAM-Cl and pyrimidoxime were the least effective. HI-6 provided a high level of protection against all of the agents tested as did HLö-7 to a slightly lesser degree. The other oximes suffered from their lack of effects against one or more of the organophosphates.

Pharmacological evidence for an omega-conotoxin, dihydropyridine-insensitive neuronal Ca2+ channel

Inactivation of N-type voltage-sensitive Ca2+ channels (VSCC) with omega-conotoxin (omega-CgTx) in tissue obtained from chicken brain produces a concentration dependent (0.01-0.1 microM) inhibition of K(+)-stimulated Ca2+ influx (delta K+), the rise in [Ca2+]i and acetylcholine (ACh) release. In identical preparations from rat brain, Ca2+ influx and the rise in [Ca2+]i were only marginally affected by much higher (1-10 microM) concentrations of omega-CgTx. The release of ACh, however, was inhibited to the same degree with similar amounts of omega-CgTx as those used in chicken brain. An L-type VSCC inhibitor failed to affect any of these parameters alone, or to augment the effect of omega-CgTx. The results suggest that almost all the VSCC in chicken brain are of the N type and that these channels regulate neurotransmitter release. In rat brain, on the other hand, Ca2+ channels resistant to N- or L-type blockers account for almost 75% of the measurable Ca2+ influx and rise in [Ca2+]i. The conspicuous dissociation between the regulation of Ca2+ influx and ACh release demonstrated in rat brain by using omega-CgTx, suggest that neurotransmitter release is governed by only a small proportion of strategically located N-type, omega-CgTx sensitive, VSCC in the presynaptic terminal.

The effect of neuropeptide Y on voltage-sensitive Ca2+ channels

Neuropeptide Y (NPY) (50-1000 nM) failed to modify basal or K(+)-stimulated Ca2+ influx in cortical or hippocampal synaptosomes from rat brain, whereas the voltage-sensitive Ca2+ channel (VSCC) blocker Cd2+ (50 microM) caused major inhibition. In cortical synaptosomes from chicken brain NPY (1.0 microM) failed to modify, whereas omega-conotoxin GV1A (0.1 microM) markedly inhibited Ca2+ influx. NPY does not appear to modify synaptosomal Ca2+ influx, however it may still affect VSCCs spatially distinct or 'upstream' from the nerve terminals.

Distribution of the bispyridinium oxime [14C] HI-6 in male and female rats

Female rats poisoned with multiple LD50s of soman or tabun have been shown previously to respond to the protective effects of HI-6 more positively than male rats. This present study was designed first to determine the distribution pattern and concentration of [14C] HI-6 in rats, and secondly, to determine the possibility that HI-6 might be located in high concentrations in critical tissues in the female as opposed to the male. To these ends, [14C] HI-6 was administered to groups of male and female rats and its radiolabelled distribution determined by whole body autoradiography and/or by measurement of its actual concentration, by scintillation spectrometry. The experiments were repeated in the presence of 2 x LD50 soman and supporting therapy with atropine. In both sexes, HI-6 levels were highest in the kidney, followed in order by cartilage greater than plasma greater than liver greater than heart greater than or equal to lung greater than or equal to diaphragm greater than brain and spinal cord. The relative distribution in the two sexes was confirmed by both methods and was not significantly altered in the presence of soman and atropine. The lack of a measurable difference in tissue distribution of [14C] HI-6 derived radioactivity between males and females suggested that the hormone-dependent difference in the protective effects previously observed was not due to selective accumulation of [14C] HI-6 in organs believed to be important in its therapeutic activity, such as brain or diaphragm.

Effect of endothelin on Ca2+ influx, intracellular free Ca2+ levels and ligand binding to N and L type Ca2+ channels in rat brain

The actions of endothelin, an endogenous vasoconstrictor compound with potent effects on various parameters of Ca2+ metabolism in peripheral tissue, were studied in several neuronal preparations. Endothelin, by itself, did not alter resting intracellular free Ca2+ levels or Ca2+ influx in either rat or chicken brain preparations; nor did it affect depolarization (K+) induced changes in these parameters. Endothelin also had no effect on the binding of [3H]-nitrendipine or [125I]-omega-conotoxin to "L " or "N" type channels respectively nor did it induce the release of endogenous acetylcholine from brain slices. The results show that, despite the proposed role of endothelin on voltage sensitive Ca2+ channels in peripheral tissue and despite the existence of endothelin binding sites on both smooth muscle and neurons, endothelin has no measurable effects on Ca2+ metabolism in neural tissue of central origin.

HI-6 therapy of soman and tabun poisoning in primates and rodents

The bis-pyridinium oxime HI-6, in conjunction with atropine, was found to offer significant protection against multiple LD50 challenges with the organophosphorus compounds soman and tabun. In adult rhesus macaques, the therapeutic administration of HI-6 with atropine and diazepam protected three of four animals from the lethal effects of 5 x LD50 of soman and three of three animals from 5 x LD50 of tabun. However, when toxogonin was substituted for HI-6 in the therapeutic mixture, all three animals poisoned with 5 x LD50 of soman died. In rats, the 24 h protective ratios against tabun and soman with HI-6 were 2 and 3.5, respectively, whereas in guinea pigs these values were between 4 and 6 for both agents. No evidence was obtained for acetylcholinesterase (AChE) reactivation by HI-6 in tissue from tabun-poisoned rodents or following soman or tabun in primate plasma. The results underscore the significant therapeutic benefit of HI-6 in primates, a species specific efficacy against tabun, and argue for some mechanism of action of HI-6 at least partly unrelated to AChE reactivation.

Hormone- and dose schedule-dependent protection by HI-6 against soman and tabun poisoning

The protective ratio produced by HI-6 (with atropine) against soman and tabun poisoning in rats and guinea pigs was determined. The amount of protection afforded by HI-6 decreased with time following poisoning, prompting us to examine the effects of repeated doses of HI-6 (four additional) given over a 5-hr period. In addition, it was determined that HI-6 produced much better protection in female rats than male rats, which led to a study of the hormone dependence of this activity. When in addition to the first dose of HI-6 four additional doses were given over a 5-hr period, the protective ratio, defined as LD50 in treated animals/LD50 in untreated animals, in males against soman, increased from 4.2 to 7.8, and against tabun, from 2.5 to 6.6. A single dose of HI-6 produced a protective ratio in females of 10.5 against soman and 4.3 against tabun, whereas multiple doses increased these values to greater than 27 and 22, respectively. A regimen consisting of gonad removal and long-term treatment with a sex hormone of the opposite gender reversed the sex-related differences in response to the protective effects of HI-6. In addition a single injection of HI-6 (plus atropine), 1 min following either soman or tabun, produced good protective ratios in guinea pigs against both soman (between 4 and 5) and tabun (5.1); however, there was no apparent hormone-dependent effect similar to that obtained in rats. The results demonstrate unequivocal protective effects of HI-6 against tabun and a very dramatic hormone-dependent factor in its activity against either organophosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence of omega-conotoxin GV1A-sensitive Ca2+ channels in mammalian peripheral nerve terminals

The existence of omega-conotoxin GV1A (omega-CgTx)-sensitive, voltage-sensitive Ca2+ channels (VSCCs) in mammalian peripheral nerves was investigated in the guinea pig ileum myenteric plexus longitudinal smooth muscle preparation (GPI). omega-CgTx (0.01-1.0 microM) reduced the electrically stimulated GPI twitch height, failed to modify exogenously applied acetylcholine (ACh) contractions, and inhibited Ca2+-dependent KCl-stimulated ACh release as measured by chemiluminescence. The 1,4-dihydropyridine VSCC antagonist (-) 202-791 (0.1-1.0 microM) inhibited the GPI twitch height, reduced contractions to exogenous ACh, but failed to affect ACh release. In the rat aorta, a nerve free preparation, omega-CgTx failed to affect contractions to KCl which were inhibited by (-) 202-791 and potentiated by the VSCC agonist (+) 202-791. The results provide evidence of neuronal N type VSCCs in mammalian peripheral cholinergic nerve terminals.

Arylazido aminopropionyl ATP (ANAPP3): interaction with adenosine receptors in longitudinal smooth muscle of the guinea-pig ileum

Arylazido aminopropionyl ATP (ANAPP3), an ATP-receptor antagonist containing a photosensitive arylazido moiety coupled to the 3' hydroxyl of ATP, inhibited the twitch response of electrically stimulated ileal longitudinal muscle strips in a dose-dependent manner. These agonist responses to ANAPP3 were attenuated by the enzyme adenosine deaminase and antagonized by the adenosine receptor antagonist 8-phenyltheophylline. Schild analysis yielded similar pA2 values for ANAPP3 and adenosine suggesting a common receptor site. Several 3'-ribose-modified adenosine analogs were tested for agonist activity and found to be inactive. Results suggest that ANAPP3 interacts at the presynaptic adenosine receptor of the ileum following its metabolism to adenosine, which explains the lack of antagonism at adenosine receptors of ileal smooth muscle following photolysis of ANAPP3.

Differential effects of ketamine isomers on neuronal and extraneuronal catecholamine uptake mechanisms

Contractile responses of isolated rabbit aortic strips to epinephrine and norepinephrine were potentiated in a dose-related manner by (+) ketamine but not by (-) ketamine (1.1 X 10(-5) M - 3.7 X 10(-4) M). Potentiation was blocked completely by pretreatment with the extraneuronal uptake inhibitor cortisol (83-138 microM) but was unaffected by the neuronal uptake inhibitor cocaine (29 microM). Responses of the rat anococcygeus muscle to these catecholamines were potentiated by both isomers, with (+) ketamine being more potent than its optical antipode. These effects were blocked completely in tissues from 6-hydroxydopamine sympathectomized animals. Results suggest that inhibition of extraneuronal uptake of catecholamines by racemic ketamine is due solely to an action of the (+) isomer, whereas both isomers appear capable of inhibiting neuronal uptake.

Ketamine: evidence of tissue specific inhibition of neuronal and extraneuronal catecholamine uptake processes

Ketamine (1.1 X 10(-5) to 3.7 X 10(-4) M) potentiated catecholamine responses of rat anococcygeus muscle and rabbit aorta in vitro. In the anococcygeus, potentiation was abolished by cocaine (2.9 X 10(-5) M) pretreatment or by chemical sympathectomy using 6-hydroxydopamine (6-OHDA), but was unaffected by pretreatment with the extraneuronal uptake inhibitor cortisol (8.3 X 10(-5) M), or the catechol-O-methyltransferase inhibitor tropolone (2.4 X 10(-4) M). The action of ketamine mimicked the potentiating effect of cocaine on tyramine responses. In contrast, the potentiation by ketamine in rabbit aorta was unaffected by cocaine or 6-OHDA but was abolished by cortisol or tropolone; and ketamine potentiated tyramine responses, whereas cocaine inhibited them. Thus the mechanism of action by which ketamine produces potentiation of catecholamines in these two tissues is completely different. These results suggest that ketamine has the unusual ability to block neuronal and extraneuronal uptake and that the predominating mechanism will depend on the type of tissue examined and the morphology of its adrenergic innervation.

Evidence of reduced uptake of convulsant in brain following prostaglandin E2

The toxic and convulsant effects of the acetylcholinesterase (AChE) inhibitor Soman, were examined in mice pretreated with various doses of prostaglandin E2 (PGE2), administered by either intraperitoneal injection (i.p.) or by intracerebroventricular (i.c.v.) infusion. PGE2 (i.p.) reduced the lethal effects of Soman slightly. PGE2 (i.p. and i.c.v.) delayed the onset and reduced the severity of cholinergically-induced convulsions, resulting from Soman. Whole brain AChE was measured at various times after Soman or Soman preceded by PGE2. PGE2 (i.p. or i.c.v.) reduced the rate at which Soman inhibited brain AChE, which appeared to be related to the increased time to onset of convulsive activity. Repeated injections of PGE2 did not delay convulsions indefinitely nor were convulsions terminated once they had started. The results suggest that the anticonvulsant properties of PGE2 may have been due, in part, to decreased cerebral circulation with subsequent reduction in the access of the convulsant to the brain and in part to direct neuronal effects.

Examination of the role of central cholinergic mechanisms in the therapeutic effects of HI-6 in organophosphate poisoning

In atropine-pretreated rats, HI-6 (125 mg/kg i.p.) raised the LD50 of Soman (subcutaneous) 5.7 times. Addition of HI-6 (25 micrograms i.c.v.) failed to enhance this protection further. HI-6 (intraperitoneal) also protected animals from intracerebroventricular Soman. HI-6, administered intracerebroventricularly either alone or in combination with intraperitoneal HI-6, failed to increase protection, nor did it reactivate Soman-inhibited acetylcholinesterase (AChE) in several brain areas. HI-6 (125 or 62.5 mg/kg i.p.) protected rats from Sarin lethality, but only the higher dose significantly altered the brain AChE activity. Furthermore, HI-6 (intraperitoneal) failed to block the Soman-induced increase in acetylcholine (ACh) or choline (Ch) levels in any of the brain areas examined. These data indicate that HI-6 is a very beneficial therapy against Soman, but that no definitive central anticholinergic activity of the compound could be found to explain its protective effects. It is possible that HI-6 acts by noncholinergic central mechanisms, or that it produces its beneficial effects outside the CNS. Furthermore, brain AChE activity does not appear to be indicative of protective effects of this oxime. ACh or Ch levels in this study were not good parameters to predict the outcome of Soman poisoning.

Evidence against ATP being the nonadrenergic, noncholinergic inhibitory transmitter in guinea pig stomach

THe adenosine-sensitive P1 purinoceptor antagonists 8-phenyl theophylline and 8-(p-bromophenyl)theophylline (4 microM) antagonized ATP-induced relaxation of spontaneous tone in guinea pig stomach, but caused no significant modification of relaxation to electrical field stimulation (0.2 - 10 Hz, 1 msec for 30 sec, atropine 1.5 microM, and guanethidine 4 microM treated). These results suggest that in fundic muscle ATP acts via hydrolysis to adenosine, with subsequent activation of P1 purinoceptors, and that ATP is not the nonadrenergic, noncholinergic inhibitory transmitter in stomach fundus.

Effect of arylazido aminopropionyl ATP (ANAPP3), on ATP responses of isolated guinea pig smooth muscle

Arylazido aminopropionyl ATP (ANAPP3), a photoaffinity analogue of adenosine 5'-triphosphate, photoactivated with visible light (+hv), specifically and irreversibly antagonized ATP contractions of the guinea pig vas deferens. ANAPP3 (30 microM) antagonized responses to exogenously added ATP in untreated, and in tissues pretreated with indomethacin (2.9 microM) and 6-(2-hydroxy-5-nitrobenzyl)-thio guanosine (10 microM). It was of interest to see if this pharmacological antagonist of ATP could be used to assess the validity of the purinergic nerve hypothesis by allowing a differentiation between, or proof of the identity of, responses to ATP and the non-adrenergic inhibitory transmitter in guinea pig stomach fundus. After photoactivation (+hv) in the organ bath and subsequent washout, ANAPP3 (30 and 100 microM) failed to antagonize relaxant responses to ATP (1.0 - 1000 microM) in fundic strips. In addition ANAPP3 failed to antagonize ATP-induced inhibition of the twitch response in electrically stimulated guinea pig ileum longitudinal muscle strips. We conclude that ANAPP3 does not antagonize all actions of ATP, which may limit its usefulness in assessing the above hypothesis. Results with this compound suggest that ATP excitatory receptors may differ from those mediating relaxation and other ATP actions.

Ketamine potentiates catecholamine responses of vascular smooth muscle by inhibition of extraneuronal uptake

Effects of ketamine on responses to sympathomimetic amines were studied using isolated aortic and pulmonary artery strips from the rabbit. Ketamine (1.1 x 10(-5) to 3.7 x 10(-4) M) potentiated adrenaline-contracted strips. Potentiation was not impaired in tissues from animals pretreated with reserpine, with 6-hydroxydopamine, or its tissues pretreated with cocaine. Pretreatment of the strips with the catechol O-methyltransferase (COMT) inhibitors tropolone or pyrogallol or the inhibitor of extraneuronal uptake 17 beta-estradiol blocked the potentiation by ketamine; in addition, potentiation by the COMT and extraneuronal uptake inhibitors was abolished or greatly reduced by ketamine. In rabbit aorta, ketamine potentiated responses to the catecholamines (adrenaline greater than nordefrine greater than noradrenaline) but not to the noncatecholamines phenylephrine, methoxamine, and synephrine; instead a slight relaxant effect was observed. Ketamine potentiated, whereas cocaine inhibited, responses to tyramine Experiments using the technique of oil immersion demonstrated that ketamine reduced the rate at which aortic strips inactivate adrenaline even when monoamine oxidase (MAO) and neuronal uptake processes were fully inhibited. Uptake studies showed that ketamine and 17 beta-estradiol reduced extraneuronal accumulation of [3H]adrenaline in aortic strips. We conclude that ketamine is an inhibitor of extraneuronal uptake in the vascular smooth muscles studied and the importance of this mechanism in producing its known cardiovascular effect is discussed.

Ganglion blocking properties of some bispyridinium soman antagonists

Various doses of several bispyridinium compounds (HS-6, HI-6, HGG-12, HGG-42, and SAD-128) known to protect animals against the irreversible cholinesterase inhibitor soman were examined to determine their effects on the cardiovascular and respiratory system of cats. Although the potency varied considerably all of the compounds tested lowered the blood pressure, which appeared to be the result of ganglion blocking properties as determined by their reduction of the pressor response to dimethylphenylpiperazinium and the blockage of the contraction of the preganglionically stimulated cat nictitating membrane. Some of the compounds caused cessation of respiration at much lower doses than others but did so at doses greater than those causing ganglion blockage.

The ganglionic blocking properties of the cholinesterase reactivator, HS-6

Following intravenous administration of the cholinesterase reactivator HS-6 (30 mg/kg), blood pressure fell (up to 50 mmHg) and maximal blood levels of HS-6 reached 242 microgram/ml. HS-6 attenuated the pressor response resulting from carotid occlusion and the depressor effect of vagal stimulation. Doses of HS-6 below those used to protect against soman in different animal species (10--30 mumol/kg) progressively blocked the ganglion-stimulating effects of nicotine and dimethylphenylpiperazinium but not the pressor effect following adrenaline, a pattern similar to that produced by hexamethonium but only 1/84 as potent. HS-6, like hexamethonium and mecamylamine, progressively blocked the contraction of the nictitating membrane of the cat resulting from preganglionic stimulation. The results indicate that HS-6 possesses ganglion-blocking properties at doses likely to be used in the protection against soman poisoning. The ganglion-blocking properties of the drug may be a factor in the beneficial effects of HS-6.

Mechanism of the cardiovascular activity of dibenzoxazepine in cats

Small i.v. doses of dibenzoxazepine (DBO) (50--400 microgram/kg) given to anesthetized cats resulted in dose related increases in heart rate (up to 70 beats/min) and blood pressure (up to 80 mm Hg). The pressor response was blocked by pretreatment of the animals with phentolamine; pretreatment for 3 days with 6-hydroxdopamine; with mecamylamine and spinal transection between C1 and C2 but not by propranolol or adrenalectomy. The increase in heart rate was blocked by pretreatment with propranolol, 6-hydroxydopamine, mecamylamine and spinal transection whereas adrenalectomy only affected the response slightly. DBO produced only negative effects on the isolated rabbit heart. Bioassay of arterial blood showed an increased level of circulating catecholamines corresponding to the cardiovascular stimulation. DBO had no tyramine-like activity on the isolated rabbit aortic strip but slightly potentiated the contraction induced by noradrenaline. These findings strongly suggest that the cardiovascular effects resulted from central stimulation of the sympathetic nervous system. A minor part of the observed sympathomimetic effects may also be the result of the ability of DBO to potentiate the effects of noradrenaline perhaps by blocking catecholamine uptake.

The actions of ketamine on vascular smooth muscle

The responses of rabbit aortic strips superfused with noradrenaline, adrenaline and 5-HT were studied alone and in combination with ketamine (50 mug/ml). Ketamine caused a slight depression of the isolated aorta but potentiated responses to adrenaline but not to noradrenaline or 5-hydroxytryptamine. Ketamine did not potentiate aortic strips contracted to a stable level by pyrogallol and adrenaline. Experiments carried out with COMT from homogenates of rat liver showed that, in contrast to pyrogallol (10(-5) M), ketamine (10(-3) M) did not inhibit the enzyme. Other experiments with rabbits given 6-hydroxydopamine showed that aortas of these rabbits responded in a similar manner to controls when treated with ketamine and catecholamines. Results obtained with aortas contracted by adrenaline and noradrenaline with ketamine present, followed by oil immersion, showed that ketamine prolonged greatly the relaxation induced by adrenaline and to a lesser extent the relaxation induced by noradrenaline. The results of these studies indicate that ketamine prevented catecholamines from reaching the intracellular site of COMT. In this respect, ketamine can be termed an inhibitor of uptake site 2. If this hypothesis is valid then the action of ketamine on vascular tissue might explain the cardiovascular effects of the drug in man and experimental animals.

Further experimental evidence for a smooth muscle depressant effect of ketamine

The effect of ketamine on the rat uterus in oestrus and the guinea pig ileum was investigated to determine whether the drug has smooth muscle relaxant properties dependent upon beta adrenergic stimulation or an anticholinergic effect. The results with the rat uterus clearly showed that relaxation was not due to action at the beta adrenergic receptors. On this muscle preparation ketamine depressed carbachol-induced contractions. This anticholinergic action of ketamine was investigated on the guinea pig ileum. Dose response curves were obtained for acetylcholine, histamine and barium chloride in the presence of ketamine. The results showed that ketamine effectively reduced contractions elicited by the three agonists. It is concluded that ketamine is not a beta receptor agonist and it is unlikely that the drug is a specific cholinergic antagonist.