Neuromuscular effects of candoxin, a novel toxin from the venom of the Malayan krait (Bungarus candidus).

1 Candoxin (MW 7334.6), a novel toxin isolated from the venom of the Malayan krait Bungarus candidus, belongs to the poorly characterized subfamily of nonconventional three-finger toxins present in Elapid venoms. The current study details the pharmacological effects of candoxin at the neuromuscular junction. 2 Candoxin produces a novel pattern of neuromuscular blockade in isolated nerve-muscle preparations and the tibialis anterior muscle of anaesthetized rats. In contrast to the virtually irreversible postsynaptic neuromuscular blockade produced by curaremimetic alpha-neurotoxins, the neuromuscular blockade produced by candoxin was rapidly and completely reversed by washing or by the addition of the anticholinesterase neostigmine. 3 Candoxin also produced significant train-of-four fade during the onset of and recovery from neuromuscular blockade, both, in vitro and in vivo. The fade phenomenon has been attributed to a blockade of putative presynaptic nicotinic acetylcholine receptors (nAChRs) that mediate a positive feedback mechanism and maintain adequate transmitter release during rapid repetitive stimulation. In this respect, candoxin closely resembles the neuromuscular blocking effects of d-tubocurarine, and differs markedly from curaremimetic alpha-neurotoxins that produce little or no fade. 4 Electrophysiological experiments confirmed that candoxin produced a readily reversible blockade (IC(50) approximately 10 nM) of oocyte-expressed muscle (alphabetagammadelta) nAChRs. Like alpha-conotoxin MI, well known for its preferential binding to the alpha/delta interface of the muscle (alphabetagammadelta) nAChR, candoxin also demonstrated a biphasic concentration-response inhibition curve with a high- (IC(50) approximately 2.2 nM) and a low- (IC(50) approximately 98 nM) affinity component, suggesting that it may exhibit differential affinities for the two binding sites on the muscle (alphabetagammadelta) receptor. In contrast, curaremimetic alpha-neurotoxins have been reported to antagonize both binding sites with equal affinity.

Evidence for the existence of a constitutive nitric oxide synthase in vascular smooth muscle.

1. We have identified a neuronal nitric oxide synthase (NOS)-like constitutive form of NOS in vascular smooth muscle (VSM) using a functional contractility approach as well as immunohistochemical methods. 2. N(G)-Nitro-L-arginine methyl ester, N(G)-monomethyl-L- arginine and N(G)-nitro-L-arginine (L-NOARG), the competitive inhibitors of NOS, inhibited Mg(2+)-induced relaxation of de-endothelialized rat aorta precontracted with phenylephrine (PE). This Mg(2+) relaxation of VSM was not affected by inhibitors of inducible NOS. 3. Electrical field stimulation (EFS; 30-70 Hz) caused relaxation of rat aorta in the presence of tetrodotoxin (therefore not a neurogenic effect) and this EFS relaxation was effectively inhibited by L-NOARG, oxyhemoglobin and methylene blue. 4. Immunohistochemical studies of dog saphenous vein using antibodies raised against neuronal NOS indicated prominent staining along the plasmalemma in a punctate pattern similar to the distribution of antibodies against caveolin-1, a major constituent of the plasmalemmal caveolae. 5. We propose that a constitutive NOS of non-endothelial, non-neuronal origin is present in a special caveolae domain of VSM cell membranes and could be activated by an ionic mechanism yet to be characterized.

Pharmacological characterization of mikatoxin, an alpha-neurotoxin isolated from the venom of the New-Guinean small-eyed snake Micropechis ikaheka.

Symptoms of envenomation by the New-Guinean small-eyed snake Micropechis ikaheka (Elapidae) include peripheral neurotoxicity and myotoxicity. We have now purified to homogeneity a long-chain neurotoxin, mikatoxin, from M. ikaheka venom by successive gel filtration and reverse-phase chromatography. Electrospray ionization mass spectrometry showed mikatoxin to be a homogenous peptide of MW 7775.6. Mikatoxin was devoid of any phospholipase A(2) activity associated with the crude venom and did not exhibit any intrinsic anticholinesterase activity. In the chick biventer cervicis muscle, it produced an irreversible, concentration-dependent block of responses to exogenously applied acetylcholine and carbachol as well as twitches evoked by nerve, but not by direct muscle stimulation. Moreover, mikatoxin, like alpha-bungarotoxin and erabutoxin-b, did not show significant fade response to train-of-four stimulation of the mouse phrenic nerve-hemi diaphragm muscle. It also failed to block ganglionic transmission in the guinea pig ileum and muscarinic responses in the rat anococcygeus muscle. Our study provides strong evidence for the presence of a neurotoxin (mikatoxin) in M. ikaheka venom that produces neuromuscular blockade in skeletal muscle attributable to selective and irreversible antagonism of postsynaptic nicotinic acetylcholine receptors of the neuromuscular junction and likely contributes to the peripheral neurotoxicity observed in M. ikaheka envenomation.

Involvement of the L-arginine-nitric oxide synthase pathway in the relaxant responses of the rat isolated anococcygeus muscle to a scorpion (Leiurus quinquestriatus quinquestriatus) venom.

Venom from several species of scorpions can cause generalized depolarization of peripheral nerves with enhancement of neurotransmitter release. The effects of the venom (LQV) from the scorpion Leiurus quinquestriatus quinquestriatus were investigated using the rat isolated carbachol (CCh) precontracted anococcygeus muscle (Acm) mounted in Krebs solution containing phentolamine (5 microM). LQV (0.2 microgram/ml and 1.5 micrograms/ml) markedly relaxed the tone of the CCh precontracted (non-stimulated) Acm by 31.5 +/- 5.1% and 35.5 +/- 4.7%, respectively; the onset was immediate after the high dose, followed by a slow and gradual return of the muscle tone to 88.7 +/- 1.8% of the initial peak tension in 50.2 +/- 4 min. Subsequent doses of LQV produced essentially no appreciable changes in muscle tone, but the addition of L-arginine (250 microM) or, especially sodium nitroprusside (SNP; 1 microM) produced marked and rapid relaxations of the Acm. Similar results were obtained with LQV during electrical field stimulation (EFS) of the precotracted Acm; however, during the gradual return of the muscle tone from the relaxed state induced by LQV, the EFS-induced relaxant (NANC) responses were also progressively inhibited by 84.9 +/- 3.4% and 64.3 +/- 4.5 by LQV 0.2 microgram/ml and 1.5 micrograms/ml, respectively. Tetrodotoxin (TTx; 2 microM) or NG-nitro-L-arginine methylester (L-NAME; 50 microM) markedly inhibited the relaxant responses of the Acm to EFS as well as to LQV but not the responses to SNP: L-arginine (250 microM) partially restored the relaxant (NANC) responses of the Acm to EFS. Thus, the L-arginie-nitric oxide synthase-nitric oxide pathway is involved in mediating the marked relaxant responses of the CCh precontracted Acm to LQO.

L-arginine-nitric oxide pathway involvement in the nerve-evoked relaxant responses of the 5-HT precontracted chick isolated upper oesophagus.

1. The effects of tetrodotoxin (TTx) and the selective nitric oxide synthase (NOS) inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) on relaxant responses of the 5-HT precontracted chick isolated upper oesophagus to electrical field stimulation (EFS: 25-30V, 5 Hz for 10 s, 1 ms pulse width every 100 s) were investigated; the oesophagus was mounted under 1 g tension in Krebs solution containing 1 microM atropine. Appropriate tissue sections (30 microM thickness) of the chick oesophagus were also processed for NADPH-diaphorase histochemistry. 2. TTx (2 microM) and L-NAME (100-200 microM) inhibited the relaxant responses of the 5-HT precontracted chick oesophagus to EFS in a concentration-dependent manner; L-arginine (0.5-1 mM), but not D-arginine (0.5-1 mM), reversed the inhibition by L-NAME. In the absence of atropine and muscle tone, EFS produced contractile responses of the chick oesophagus that were completely abolished by 1 microM atropine, which also blocked the contractile response to acetylcholine (50 microM). 3. Under light microscopy, NADPH-diaphorase histochemistry confirmed the presence of nitric oxide synthase (NOS)-containing neurones and nerve fibres in the chick oesophagus. 4. The relaxant responses of the 5-HT precontracted chick isolated upper oesophagus to EFS are, therefore, mediated via the stimulation of non-adrenergic non-cholinergic nerves. These are likely to correspond to the histochemically identified NOS-containing neurones involved, presumably, in the synthesis and release of nitric oxide as the relaxant (inhibitory) neurotransmitter in this avian smooth muscle.

Prejunctional and postjunctional inhibition of adrenergic transmission in the rat-isolated anococcygeus muscle by cimetidine.

1. Cimetidine and ranitidine can inhibit various cholinergic sites, which can account for some of their clinically documented adverse effects; ranitidine can also inhibit adrenergic transmission, closely resembling the action of guanethidine. The effects of cimetidine on adrenergic transmission in the rat isolated anococcygeus muscle (Acm) were therefore investigated. 2. The contractile (motor) responses of the Acm to electrical field stimulation (EFS; 20-30V, 10 s, 1 ms pulse width, every 2 min) at varying frequencies (Hz: 5, 10, 20) and to 3 microM noradrenaline (NA) were inhibited in a concentration-dependent manner by cimetidine (mM: 1, 2, 4, 8). Inhibition of the EFS-induced responses was inversely related to the stimulation frequency. 3. Cimetidine (nM: 2, 4, 8) produced a concentration-dependent and non-parallel shift of the NA cumulative log concentration-response curves (CRCs; curves 2, 3, 4) to the right of the control curve (curve 1); at the highest concentration (8 mM) used, cimetidine produced a 4.3-fold shift of curve 4 accompanied by a decline of 9.4 +/- 1.5% in the maximal response to NA (compared to essentially no change in maximal responses for the corresponding CRCs in the NA control series). Cimetidine therefore inhibited the postjunctional alpha-adrenoceptor sites. 4. The contractile responses of the Acm to EFS (i.e. prejunctionally mediated responses) were more sensitive to inhibition by cimetidine than the NA-induced (postjunctionally mediated) responses: 8 mM cimetidine inhibited the responses to EFS by about 97%, whereas the responses to NA were inhibited by only 41 +/- 5%.(ABSTRACT TRUNCATED AT 250 WORDS)

Prejunctional action of the venom from the Indian red scorpion Mesobuthus tamulus on adrenergic transmission in vitro.

Venom (RSV) from the Indian red scorpion, Mesobuthus tamulus (Buthus tamulus), can cause increased peripheral sympathetic activity with consequent enhancement of adrenergic responses. We have therefore investigated the effects of RSV on adrenergic transmission in the rat isolated anococcygeus muscle. The effects of phentolamine (5 microM), tetrodotoxin (2 microM), guanethidine (5 microM), desipramine (1 microM) and reserpine pretreatment in vivo (5 mg/kg s.c. x 24 hr and 5 mg/kg i.p. x 3 hr) on the contractile responses of the rat anococcygeus muscle to RSV (1.5 microgram/ml), field stimulation, noradrenaline (NA, 1 microM or 3 microM) and tyramine (15 microM) were compared. The contractile responses to RSV and to field stimulation were completely blocked by phentolamine, tetrodotoxin, guanethidine and reserpine pretreatment, but the responses were potentiated by desipramine. The contractile responses to tyramine were completely blocked by phentolamine, reserpine pretreatment as well as desipramine. The responses to NA were completely blocked by phentolamine, but were potentiated by guanethidine, desipramine and reserpine. Relatively low concentrations (0.1 microgram/ml x 4) of RSV which did not produce any observable increase in tone of the anococcygeus muscle, potentiated the contractile response of the anococcygeus muscle to field stimulation, but not the responses to exogenous NA; 4-aminopyridine (25 microM x 2) also potentiated the muscle responses to field stimulation. HPLC measurements revealed only very low concentrations (0.10 +/- 0.03 mumol/g venom) of NA in RSV. Thus, the adrenergic agonist action of RSV in the rat isolated anococcygeus muscle can be attributed to the involvement of some prejunctional mechanism(s) of action that stimulates the release of neurotransmitter which differs from the indirect action mediated by tyramine.

The black scorpion Heterometrus longimanus: pharmacological and biochemical investigation of the venom.

Documentation on the biological activity (including the lethality) of the venom (BSV) from the black scorpion Heterometrus longimanus is lacking. We have investigated the effects of BSV on adrenergic transmission using the rat isolated anococcygeus muscle (Acm), since the venom from several species of scorpions causes peripheral sympathetic nerve stimulation with enhanced adrenergic responses. The catecholamine content in BSV was also measured by HPLC. The effects of phentolamine (5 microM), guanethidine (5 microM), desipramine (1.5 microM), tetrodotoxin (2 microM) and reserpine pretreatment in vivo (5 mg/kg s.c. x 24 hr and 5 mg/kg i.p. x 3 hr) on contractile responses of the rat Acm to field stimulation, crude BSV (2-10 microliters in 6 ml bath), noradrenaline (3 microM), tyramine (10-15 microM), carbachol (2-3 microM) and potassium chloride (50-75 mM) were investigated. BSV mimicked the agonist actions of noradrenaline (NA) by acting directly on postjunctional alpha-adrenoceptors in the anococcygeus muscle. The LD50 of crude BSV injected i.v. into mice was 0.13 ml per kg mouse. Sequential ultrafiltration of the crude BSV revealed the presence of a substance of low mol. wt which mediates the postjunctional alpha-agonist actions of BSV. HPLC measurements confirmed the presence of noradrenaline (NA; mean concentration of 1.8 +/- 0.3 mM) in BSV; the dopamine concentration (mean of 31 +/- 4 microM) was 60-fold lower than that of NA, whereas adrenaline was not detected in all the 15 samples investigated. Thus, the presence of NA in BSV can account for the postjunctional alpha-agonist actions of the venom in the Acm.

Dual action of ranitidine at cholinergic sites in the rat isolated anococcygeus muscle.

1. The effects of ranitidine on contractile responses of the rat isolated anococcygeus muscle to acetylcholine (ACh) and carbachol (CCh) were investigated. 2. Four consecutive cumulative concentration-response curves (CRCs) for ACh or CCh were constructed in the absence or presence of ranitidine. The mean EC50 values were determined from the individual CRCs and used for the calculation of the corresponding EC50 ratios. 3. Ranitidine (0.01, 0.05 and 0.5 mM) produced a progressively leftward shift of the ACh curves; the corresponding EC50 ratios obtained were 3.2 +/- 0.4, 5.7 +/- 0.5 and 9.4 +/- 1.4. Neostigmine 0.5 microM alone or in combination with ranitidine 0.1 mM produced very marked leftward shifts of the ACh curves (EC50 ratios 68.2 +/- 14.3 and 56.3 +/- 9.6 respectively). In contrast, ranitidine (0.05, 0.5 and 2 mM) produced a rightward and parallel shift of the CCh curves (EC50 ratios 1.0 +/- 0.1, 3.4 +/- 0.32 and 12.5 +/- 1.7 respectively). 4. Ranitidine can therefore mediate a dual action at cholinergic sites in the rat isolated anococcygeus muscle, actions which can be attributed to its inherent anticholinesterase and antimuscarinic activity. The relatively marked anticholinesterase action of ranitidine was rather surprising in view of the limited cholinergic innervation present in the rat anococcygeus muscle.

Ranitidine inhibits adrenergic but not nonadrenergic noncholinergic transmission in the rat isolated anococcygeus muscle.

The effects of ranitidine on adrenergic and nonadrenergic noncholinergic transmission in the rat isolated anococcygeus muscle and on the contractile responses of the muscle to noradrenaline, potassium chloride, 5-hydroxytryptamine and carbachol were investigated. Ranitidine (2-8 mM) produced a concentration-dependent and readily reversible inhibition of motor (excitatory) responses of the anococcygeus muscle evoked via field stimulation (20-30 V, 10 Hz x 10 sec, 1 msec pulse width, every 2 min), whereas the contractile response of the anococcygeus muscle to exogenous noradrenaline (3 microM) was potentiated by 57 +/- 3.7%; the contractile response to potassium chloride (50 mM) was only slightly decreased, whereas the responses to carbachol (3 microM) or 5-hydroxytryptamine (30 microM) were completely inhibited by ranitidine (8 mM). When the tone of the muscle was raised by noradrenaline (3 microM) after complete inhibition of the motor responses by ranitidine (8 mM), field stimulation produced relaxant (inhibitory) responses which were inhibited by tetrodotoxin (2 microM) or NG-nitro-L-arginine methylester (50 microM). Similar results were obtained with guanethidine (0.5-3 microM) on adrenergic and non-adrenergic noncholinergic transmission. The results clearly show that ranitidine can selectively inhibit adrenergic transmission in the anococcygeus muscle with consequent unmasking of nonadrenergic noncholinergic transmission; ranitidine can also produce a super-sensitivity-type response of the muscle to exogenous noradrenaline but not to potassium chloride, carbachol or 5-hydroxytryptamine. Thus, the inhibition of adrenergic transmission by ranitidine in the rat isolated anococcygeus muscle closely resembles the action of guanethidine on adrenergic transmission.

Ranitidine inhibits adrenergic transmission in the rat isolated anococcygeus muscle.

1. The effects of ranitidine on adrenergic transmission in the rat isolated anococcygeus muscle were investigated. 2. Cumulative doses (2-8 mmol/L) or ranitidine produced a concentration-dependent inhibition of motor responses of the rat isolated anococcygeus muscle evoked by field stimulation (20-25 V, 10 Hz for 10 s, 1 ms pulse width) every 2 min, but also potentiated the contractile response to exogenous noradrenaline (5 mumol/L). The inhibited motor responses recovered rapidly and completely after washing out ranitidine. 3. 4-Aminopyridine (100 mumol/L) effectively reversed the partially inhibited (55% or greater) motor responses. 4. The results strongly suggest that ranitidine can inhibit adrenergic transmission in the anococcygeus muscle by a prejunctional mechanism with, presumably, consequent development of supersensitivity of the effector cells to noradrenaline.

In vitro time course studies on train-of-four fade induced by hexamethonium, pancuronium and decamethonium in the rat hemidiaphragm.

1. In vitro time course studies on the effects of hexamethonium (7 mmol/L), pancuronium (5 mumol/L) and decamethonium (220 mumol/L) on nerve-evoked (2 Hz for 2 s every 20 s) maximal twitches (T1, T2, T3, T4) of the rat hemidiaphragm were conducted. All three drugs progressively depressed all four twitches in a given train but at different rates (T4 greater than T3 greater than T2 much greater than T1). 2. The response-time profiles for T1 and T4 varied widely for the three drugs such that, for the same degree of T1-block, each drug produced a different magnitude of T4-block during the onset of and recovery from neuromuscular blockade. 3. Analysis of the T1 versus T4/T1 plot showed that, at 50% T1-block, the corresponding T4/T1 (i.e. train-of-four ratios) during the onset (and recovery) phase were 0.16 (0.29), 0.46 (0.40) and 0.66 (0.53) for hexamethonium, pancuronium and decamethonium, respectively. Thus, for the same degree (i.e. 50%) of twitch (T1) tension depression, the three drugs differed widely in their ability (hexamethonium much greater than pancuronium greater than decamethonium) to produce fade as reflected in the respective train-of-four ratio. 4. Our results therefore show that the train-of-four ratio (T4/T1) at 50% T1-block obtained from such in vitro time course studies is a useful quantitative index of the potential of various drugs to cause train-of-four fade. Based on this index a classification of various compounds already studied is proposed as follows: hexamethonium much greater than pancuronium approximately (+)-tubocurarine greater than decamethonium approximately succinylcholine much greater than alpha-bungarotoxin.

Train-of-four fade during neuromuscular blockade induced by tubocurarine, succinylcholine or alpha-bungarotoxin in the rat isolated hemidiaphragm.

1. Nerve-evoked maximal twitches (T1, T2, T3, T4) of the rat isolated hemidiaphragm to train-of-four (TOF) stimulation (2 Hz X 2 s) were recorded continuously in the absence or presence of tubocurarine (1.5 mumol/l), succinylcholine (40 mumol/l) or alpha-bungarotoxin (1 mumol/l). The T1 and T4 response-time profiles for the three drugs were analysed with respect to amplitude depression and the TOF ratio (T4/T1) during the development of and recovery from neuromuscular blockade. 2. Tubocurarine produced T1 block accompanied by intense TOF fade; for the same degree of T1 block, the TOF ratio was lower during the recovery from blockade after washing out tubocurarine from the bath than during the onset of blockade. There was also a correspondingly slower recovery of the TOF ratio from 90% T1 block to control levels when compared with the time for complete T1 recovery. Fade and twitch tension depression were shown clearly to be separate responses, each with its own response-time profile. Fade is therefore not simply a consequence of postjunctional cholinoceptor blockade. 3. Succinylcholine produced T1 block with only moderate TOF fade; similar recovery rates from 90% T1 block to control levels were obtained for T1 and the TOF ratio. 4. alpha-Bungarotoxin produced irreversible and complete neuromuscular blockade during which TOF fade was virtually absent. 5. The results obtained in this study closely resemble those from other similar studies in animals and in humans and clearly demonstrate that the rat isolated hemidiaphragm is a suitable in vitro model for time course studies on TOF fade.

Cimetidine and ranitidine: a study of competitive and ion-channel blockade in the toad isolated rectus abdominis muscle.

Responses of the toad isolated rectus abdominis muscle to cumulative doses of carbachol were recorded in the absence or presence of varying concentrations of cimetidine or ranitidine. The corresponding cumulative log concentration-response curves for carbachol were then plotted for each antagonist studied. Cimetidine (1 mmol/l) produced a straight and parallel 1.8-fold shift of the carbachol curve to the right of the corresponding control curve with no significant change in the maximal response. Cimetidine, at doses of 2.5 mmol/l and 5 mmol/l, and ranitidine, at doses of 0.1 mmol/l, 0.25 mmol/l and 0.5 mmol/l, produced a concentration-dependent and non-parallel shift of the carbachol curve to the right of the corresponding control curve accompanied by a marked decline of maximal responses. The results provide further evidence that ion-channel block may be involved in the neuromuscular blockade produced by cimetidine or ranitidine, the latter being more potent in this respect. Competitive antagonism may also be partly responsible for cimetidine-induced neuromuscular blockade, especially at lower drug concentrations.

Actions of cimetidine and ranitidine at some cholinergic sites: implications in toxicology and anesthesia.

Cimetidine and ranitidine are specific and potent H2-receptor antagonists widely used in the effective therapy of peptic ulcer disease. The drugs also possess other pharmacological properties unrelated to H2-receptor antagonism. More recently large experimental doses of cimetidine or ranitidine were found to have anticholinesterase, ganglion blocking and neuromuscular blocking activities. Actions of the drugs at such cholinergic sites may account for some of their clinically documented adverse effects. The toxicological implications of these findings including the potential for drug interactions to occur, especially during some anesthetic procedures, are discussed.

Inhibition of acetylcholinesterase by histamine agonists and antagonists.

The histamine H1 and/or H2-receptor agonists showed weak acetylcholinesterase inhibitory activity. Their dissociation constants (Kis and/or Kii) were 2-methyl histamine (Kis = 1751 mumol/l, s.e. = 163) less than 4-methyl histamine (Kis = 3551 mumol/l, s.e. = 414) less than dimaprit (Kis = 2931 mumol/l, s.e. = 605; Kii = 3668 mumol/l, s.e. = 901) less than histamine (Kis = 6480 mumol/l, s.e. = 1360). Both histamine H1 and H2-receptor antagonists showed stronger acetylcholinesterase inhibitory activity. Their dissociation constants were ranitidine (Kii = 1.56 mumol/l, s.e. = 0.14) less than oxmetidine (Kis = 14.7 mumol/l, s.e. = 1) less than mepyramine (Kis = 178 mumol/l, s.e. = 23; Kii = 440 mumol/l, s.e. = 98) less than cimetidine (Kis = 199 mumol/l, s.e. = 12; Kii = 827 mumol/l, s.e. = 126). Using the ROSFIT programs for fitting data to common enzyme kinetic models, the inhibition caused by histamine, 4-methyl histamine, 2-methyl histamine, oxmetidine and neostigmine was of the competitive type. Mepyramine, cimetidine and dimaprit appeared to exhibit the modern non-competitive type of inhibition with their primary action on the enzyme rather than on the enzyme-substrate complex. Ranitidine seemed to act on the enzyme-substrate complex rather than the enzyme, conforming to the uncompetitive inhibition model. The clinical implications of acetylcholinesterase inhibition by cimetidine and ranitidine are discussed.

Ganglion blocking activity of cimetidine in the anaesthetized cat.

Cimetidine produced a dose-dependent and reversible inhibition of contractions of the nictitating membrane elicited through stimulation of the preganglionic nerve supplying the superior cervical ganglion in the anaesthetized cat. Postganglionic nerve stimulation resulted in only a slight and variable inhibition of the contractions. Both hexamethonium and cimetidine also produced a dose-dependent and reversible fall in arterial blood pressure. The ganglion blocking activity of cimetidine was much weaker than that of hexamethonium; the ED50 ratio (cimetidine:hexamethonium) calculated from the cumulative log dose-response curves for the two drugs was 64. The possible mechanism(s) of ganglion blockade produced by cimetidine is discussed including a possible action at nicotinic receptors, either directly or indirectly (via its anticholinesterase activity), and ion channel blockade. The clinical implications of cimetidine-induced ganglionic blockade are also discussed, especially with respect to sexual impotence associated with the use of cimetidine.

Anticholinesterase activity of and possible ion-channel block by cimetidine, ranitidine and oxmetidine in the toad isolated rectus abdominis muscle.

Responses of the toad isolated rectus abdominis muscle to cumulative doses of acetylcholine were recorded in the absence or presence of varying concentrations of cimetidine, ranitidine or oxmetidine. The corresponding cumulative log concentration-response curves for acetylcholine were then plotted for each antagonist studied. Cimetidine (5 mmol/l), ranitidine (1 mmol/l) and oxmetidine (0.02 mmol/l) potentiated the effect of acetylcholine by 4-fold, 2.6-fold and 1.3-fold, respectively. At higher concentrations all three histamine H2-receptor antagonists produced a concentration-dependent and non-parallel shift of the acetylcholine curve to the right of the corresponding control curve accompanied by a depression of the maximal response. The results provide further evidence that the H2-antagonists studied possess anticholinesterase activity and also suggest that the H2-antagonists may produce neuromuscular blockade by ion-channel block. The clinical implications of the results obtained are also discussed.