Paralysis of autonomic ganglia by methonium salts

Sympathetic Pathophysiology in Hypertension Origins: The Path to Renal Denervation

The importance of the sympathetic nervous system in essential hypertension has been recognized in 2 eras. The first was in early decades of the 20th century, through to the 1960s. Here, the sympathetic nervous system was identified as a target for the treatment of hypertension, and an extensive range of antiadrenergic therapies were developed. Then, after a period of lapsed interest, in a second era from 1985 on, the development of precise measures of human sympathetic nerve firing and transmitter release allowed demonstration of the importance of neural mechanisms in the initiation and maintenance of the arterial blood pressure elevation in hypertension. This led to the development of a device treatment of hypertension, catheter-based renal denervation, which we will discuss.

The sympathetic nervous system alterations in human hypertension

Several articles have dealt with the importance and mechanisms of the sympathetic nervous system alterations in experimental animal models of hypertension. This review addresses the role of the sympathetic nervous system in the pathophysiology and therapy of human hypertension. We first discuss the strengths and limitations of various techniques for assessing the sympathetic nervous system in humans, with a focus on heart rate, plasma norepinephrine, microneurographic recording of sympathetic nerve traffic, and measurements of radiolabeled norepinephrine spillover. We then examine the evidence supporting the importance of neuroadrenergic factors as promoters and amplifiers of human hypertension. We expand on the role of the sympathetic nervous system in 2 increasingly common forms of secondary hypertension, namely hypertension associated with obesity and with renal disease. With this background, we examine interventions of sympathetic deactivation as a mode of antihypertensive treatment. Particular emphasis is given to the background and results of recent therapeutic approaches based on carotid baroreceptor stimulation and radiofrequency ablation of the renal nerves.

A historical perspective on peripheral reflex cardiovascular control from animals to man

Although drug treatment of human hypertension has greatly improved, there is renewed interest in non-drug methods of blood pressure reduction. Animal experiments have now shown that arterial baroreflexes do control long-term blood pressure levels, particularly by nervously mediated renal excretion of sodium and water. This Paton Lecture provides a review of the historical development of knowledge of peripheral circulatory control in order to supplement prior Paton Lectures concerned with cerebral cortical and other areas of influence. I also discuss how improved understanding of nervous control of the circulation has led to current methods of non-drug blood pressure control in man by implanted carotid baroreceptor pacemakers or by renal denervation. Finally, the role of other therapy, particularly listening to music, is reviewed.

Rat whisker movement after facial nerve lesion: evidence for autonomic contraction of skeletal muscle

Vibrissal whisking is often employed to track facial nerve regeneration in rats; however, we have observed similar degrees of whisking recovery after facial nerve transection with or without repair. We hypothesized that the source of non-facial nerve-mediated whisker movement after chronic denervation was from autonomic, cholinergic axons traveling within the infraorbital branch of the trigeminal nerve (ION). Rats underwent unilateral facial nerve transection with repair (N=7) or resection without repair (N=11). Post-operative whisking amplitude was measured weekly across 10weeks, and during intraoperative stimulation of the ION and facial nerves at ⩾18weeks. Whisking was also measured after subsequent ION transection (N=6) or pharmacologic blocking of the autonomic ganglia using hexamethonium (N=3), and after snout cooling intended to elicit a vasodilation reflex (N=3). Whisking recovered more quickly and with greater amplitude in rats that underwent facial nerve repair compared to resection (P<0.05), but individual rats overlapped in whisking amplitude across both groups. In the resected rats, non-facial-nerve-mediated whisking was elicited by electrical stimulation of the ION, temporarily diminished following hexamethonium injection, abolished by transection of the ION, and rapidly and significantly (P<0.05) increased by snout cooling. Moreover, fibrillation-related whisker movements decreased in all rats during the initial recovery period (indicative of reinnervation), but re-appeared in the resected rats after undergoing ION transection (indicative of motor denervation). Cholinergic, parasympathetic axons traveling within the ION innervate whisker pad vasculature, and immunohistochemistry for vasoactive intestinal peptide revealed these axons branching extensively over whisker pad muscles and contacting neuromuscular junctions after facial nerve resection. This study provides the first behavioral and anatomical evidence of spontaneous autonomic innervation of skeletal muscle after motor nerve lesion, which not only has implications for interpreting facial nerve reinnervation results, but also calls into question whether autonomic-mediated innervation of striated muscle occurs naturally in other forms of neuropathy.

Rodent cardiovascular responses to baroreceptor unloading: effect of plane of anaesthesia

The objective of this study was to determine whether a plane of urethane-α-chloralose anaesthesia that suppresses motor reflexes would affect baroreflex cardiovascular control relative to a plane of anaesthesia that leaves motor reflexes intact. Adult male Sprague-Dawley rats were anaesthetized to either a light (motor reflexes intact) or deep (motor reflexes suppressed) plane of anaesthesia. Animals were exposed to graded (-2 to -10 mm Hg) lower body negative pressure while heart rate, vascular resistance, and mean arterial pressure were assessed. No between-group differences were observed in baseline hemodynamics. Graded lower body negative pressure progressively increased heart rate (p < 0.01) and vascular resistance (p < 0.001) and reduced mean arterial pressure (p < 0.001) similarly in light and deep planes of anaesthesia. Therefore, the deep plane of anaesthesia was not associated with a degradation of the autonomic response to baroreceptor unloading beyond that observed at the light plane. These data support the use of urethane-α-chloralose anaesthesia in studies examining reflex cardiovascular control concomitant with some degree of noxious stimulation.

Pharmacological properties of pempidine (1:2:2:6:6-pentamethylpiperidine), a new ganglion-blocking compound

Pempidine (1:2:2:6:6-pentamethylpiperidine) is a long-acting ganglion-blocking compound which is effective by mouth. By intravenous injection it has a similar potency to hexamethonium on the preganglionically stimulated nictitating membrane of the cat. The compound blocks the effects of intravenous nicotine and of peripheral vagal stimulation on the blood pressure; it also causes dilatation of the pupil after removal of the sympathetic innervation. On the guinea-pig ileum, the predominant effect of the compound is to inhibit nicotine contractions. Pempidine is well absorbed from the gastro-intestinal tract as judged by (a) the low ratio (6.9) of oral to intravenous toxicities, (b) the rapid development of mydriasis in mice after oral administration of small doses, and (c) the rapid onset of hypotension when the compound is injected directly into the duodenum of anaesthetized cats. Other actions include neuromuscular paralysis of curare-like type when large doses of the compound are injected intravenously and central effects such as tremors which occur with near toxic doses. In cats with a low blood pressure, large intravenous doses have a slight pressor action.

Transmission failure in sympathetic nerves produced by hemicholinium

It has been shown by others that hemicholinium (alpha,alpha'-dimethylethanolamino-4,4'-biacetophenone) inhibits the synthesis of acetylcholine, an effect which is reversed by choline. Hemicholinium produces a failure of response to nerve stimulation in the following sympathetically innervated preparations: guinea-pig isolated vas deferens, rabbit isolated uterus, rabbit isolated colon, perfused rabbit ear, cat isolated atria and the piloerector muscles in the cat's tail. The blocking action of hemicholinium on the responses to postganglionic sympathetic stimulation resembles its blocking action against cholinergic nerve stimulation observed on rabbit isolated atria with vagus nerves, rabbit isolated vagina with pelvic nerves, and guinea-pig isolated diaphragm with phrenic nerve. The failure of transmission produced by hemicholinium in sympathetic nerves and in cholinergic nerves can be reversed by choline. It is suggested that if there were a cholinergic junction at sympathetic nerve endings the mechanism of the blocking action of hemicholinium at these endings could be explained by inhibition of acetylcholine synthesis.

Blocking action of decamethonium at different sites in the autonomic nervous system of the cat

Doses of decamethonium sufficient to paralyse skeletal and respiratory muscles in the cat for 20 to 30 min can reversibly block transmission at several sites in the autonomic nervous system. The sympathetic vasodilator outflow to skeletal muscle was blocked at the post-ganglionic nerve endings, probably by preventing the release of acetylcholine. The effects of vagal stimulation on heart-rate and intestinal contraction were blocked in most experiments, possibly by an action on pre-ganglionic as well as post-ganglionic nerve endings. However, decamethonium did not block all cholinergic nerve endings-for example, it did not diminish either the effects of stimulation of the chorda tympani on the submandibular salivary gland or those of pelvic nerve stimulation on the bladder.

Open-channel blockers at the human alpha4beta2 neuronal nicotinic acetylcholine receptor

To extend our knowledge of the pharmacological profile of human alpha4beta2 neuronal nicotinic receptors, we investigated the action of hexamethonium on the major brain human nicotinic acetylcholine receptor (nAChR) stably expressed in human embryonic kidney 293 cells. This compound displays all of the characteristics of an open-channel blocker at the human alpha4beta2 nAChR: a voltage-dependent inhibition (more pronounced at hyperpolarized potentials), absence of competition, and use dependence. Moreover, we observed that classic N-methyl-D-aspartate open-channel blockers amantadine, 3,5-dimethyl-1-adamantanamine (memantine), and dizocilpine [(+)-MK-801] and the calcium channel antagonist 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate are powerful inhibitors of the human alpha4beta2 nAChR. Dose-inhibition curves yield, at -100 mV, IC50 values in the micromolar range for all of compounds and Hill coefficients below unity. Whole-cell current-voltage relationships display a strong rectification profile at hyperpolarized potentials, and current blockades are fitted adequately by a mathematical model that describes the mechanism of an ion channel block. We conclude that these molecules are powerful human alpha4beta2 open-channel blockers ranking in the following order of potency: amantadine > memantine = hexamethonium > 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate approximately (+)-MK-801.

The channel-blocking action of methonium compounds on rat submandibular ganglion cells. 1983

The effects of drugs of the polymethylene bis-trimethylammonium (methonium) series on the characteristics of the synaptic currents (e.s.cs) recorded from voltage-clamped rat submandibular ganglion cells have been studied. The drugs studied were from C4 to C10 (decamethonium).

All of the drugs except C4 shortened the initial decay phase of the e.s.c.; C9 and C10 produced an additional slowly decaying component. These effects were interpreted in terms of an open channel block mechanism, the calculated rate constants for association with the open channel at − 80 mV being fairly similar (5.9 × 10⁶ to 18.1 × 10⁶M⁻¹ s⁻¹) for all of the compounds except C4, which had no effect on the e.s.c. decay.

All of the compounds produced use-dependent block when tested with short trains of stimuli at 10 Hz, or with trains of ionophoretic pulses of acetylcholine, consistent with their channel blocking property. Tubocurarine had a similar effect, but not trimetaphan or mecamylamine.

Recovery from use-dependent block with short chain methonium compounds, up to C8, was very slow in the absence of agonist, being incomplete even after several minutes. With C9 or C10 or tubocurarine, recovery from use-dependent block was complete within a few seconds. With C6 recovery in the absence of agonist was unaffected by membrane potential, but could be accelerated by applying acetylcholine with the cell depolarized to − 40 mV. This persistent block was ascribed to the ability of the blocking molecule to become trapped by closure of the channel. With C9 and C10 it is assumed that their presence inhibits channel closure, so they can escape without the help of agonist.

When use-dependent block is avoided by leaving the ganglion unstimulated during equilibration with the blocking drug, the first e.s.c. elicited shows no appreciable reduction of amplitude, though with C6, C7 or C8 subsequent responses elicited at 0.1 Hz become progressively more blocked.

Even at 1 Mm, C6 does not prevent acetylcholine from opening ionic channels.

It is concluded that all of the effects on e.s.c. amplitude can be interpreted in terms of channel block, there being no evidence of any receptor blocking action.

EFFECTS OF CENTRAL DEPRESSANT DRUGS UPON ACETYLCHOLINE RELEASE

Several central depressant and other drugs have been examined for their effects upon acetylcholine release from the stimulated, perfused cat superior cervical ganglion and rat isolated phrenic nerve-diaphragm preparations. The acetylcholine released was assayed biologically. Amylobarbitone sodium, chloral hydrate, trichloroethanol, methylpentynol, methylpentynol carbamate, paraldehyde, procaine hydrochloride and troxidone reduced the presynaptic release of acetylcholine from the ganglion. They also exhibited a postsynaptic blocking action, this component of depressant activity being particularly prominent with paraldehyde and troxidone. Closely analogous findings were obtained at the neuromuscular junction with methylpentynol and its carbamate, paraldehyde, procaine hydrochloride, trichloroethanol and troxidone. At both sites the drug-induced depression, both of transmission and of acetylcholine output, was reversible. Whereas hexamethonium regularly blocked ganglionic transmission with no effect upon acetylcholine release, tetraethylammonium not only completely blocked ganglionic transmission but concomitantly augmented acetylcholine output. These results are discussed in relation to the electrophysiological and metabolic events associated with neuro-effector transmission.

THE SELECTIVITY OF DRUGS BLOCKING GANGLIONIC TRANSMISSION IN THE RAT

By comparing the effects on ganglionic transmission and on the pre- and post-ganglionic nerves in the isolated superior cervical ganglion preparation of the rat, the selectivity of several drugs was assessed quantitatively. Hexamethonium, tetraethylammonium, nicotine and tubocurarine blocked transmission in concentrations which did not affect nervous conduction and were considered to be highly selective in action. Atropine, amylobarbitone and paraldehyde depressed nervous conduction appreciably in ganglion-blocking doses, but not enough to account wholly for the block in transmission and they were therefore considered as being moderately selective. The ganglion blocking actions of mephenesin, procaine, methylpentynol, methylpentynol carbamate and benactyzine were nonspecific, showing general depression of neuronal activity. Ganglion block with bretylium was nonselective in its site of depression of the postganglionic neurone in concentrations which only partly depressed the preganglionic nerve.

A study of the bovine adrenal chromaffin nicotinic receptor using patch clamp and concentration-jump techniques

1. Voltage clamp records have been obtained from bovine adrenal chromaffin cells in the outside-out and whole-cell configurations, in response to step changes of acetylcholine (ACh) concentration. The concentrations used ranged from 50 nM to 20 mM. 2. At high acetylcholine concentrations, the activation and desensitization kinetics of the nicotinic receptor, as observed in outside-out patches, may be described by a model incorporating a single, fast agonist binding step, and relatively slow isomerization to the open state. The affinity of the closed receptor for ACh is 310 microM, the channel opening rate constant is 460 s-1, and the closing rate constant is 29 s-1. 3. Single channel events, observed when nanomolar ACh concentrations are applied to whole cells, have two distinct channel lifetimes: 0.6 ms and 11-15 ms. The variation of the frequencies of the events with ACh concentration, suggests that the short lifetimes are openings of a singly liganded receptor and the longer lifetimes are openings of a doubly liganded receptor. 4. Only a single exponential associated with receptor desensitization is seen with outside-out patches, but two are seen with whole cells. It is postulated that there are two nicotinic receptor types present on adrenal chromaffin cells. 5. The rate of desensitization (9 s-1 and 26 s-1, whole cells; 24 s-1, patches), is fast enough to be significant in determining the open channel lifetime. 6. A sudden increase in current (rebound) is observed when a high concentration of ACh is abruptly removed from outside-out patches. This is evidence for a blocked state. The affinity of the blocking site for ACh is 1400 microM (outside-out patches). 7. The total number of activatable nicotinic channels per whole cell is estimated to be 2600.

Conductance and kinetic properties of single nicotinic acetylcholine receptor channels in rat sympathetic neurones

1. The unitary conductance of nicotinic acetylcholine (ACh) receptor channels in rat sympathetic neurones has been studied. Conductance estimates varied from 26-48 pS with a mean of 36.8 pS in 1 mM-Ca2+. The main conductance level varied from patch to patch and the presence (or absence) of additional conductance levels also varied. 2. The channels showed large open channel noise and experiments with 300 mM-NaCl in the patch pipette substantially increased the open channel noise. The appearance of detectable step-like transitions within this noise strongly suggested the existence of closely spaced discrete levels. 3. Removal of divalent cations from the external solution increased the unitary channel conductance. Altering the main permeant ion in divalent-free solutions gave the following conductance sequence: K+ (93 pS) greater than Cs+ (61 pS) greater than Na+ (51 pS) greater than Li+(23 pS). 4. Replacement of Na+ by Cs+ in the external solution considerably reduced the current evoked by ACh in whole-cell recordings and the channel-opening frequency in outside-out patches. 5. The kinetic properties of channels activated by ACh and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) were also studied. At low concentrations of ACh and DMPP the gap distributions were complex and best fitted by the sum of four exponential components. Individual activations (bursts) were interrupted by the two shortest closed periods the briefer of which had time constants of 36 microseconds for ACh and 67 microseconds for DMPP. 6. The distribution of burst lengths had two components for each agonist, each component making up about 50% of the total area under the distribution. For ACh, the time constant of the longer component (12.2 ms) was similar to the decay time constant of excitatory postsynaptic potentials (EPSCs) at similar temperature and potential. For DMPP the time constant of the longer component was 17.6 ms. 7. The relative number of brief gaps per long burst was much larger for ACh than for DMPP. Therefore the corrected mean open time for ACh (0.86 ms) was much shorter than that for DMPP (2.3 ms). 8. In terms of receptor mechanism, the values of the channel opening equilibrium constant (beta/alpha) estimated from these numbers (ACh, 23; DMPP, 25) suggest that both agonists are efficaceous. 9. DMPP is a potent blocker of the channel with an equilibrium dissociation constant (KB) of around 50 microM and blockage gaps of around 1 ms duration. ACh also blocks the channel but with a higher KB of around 470 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of nicotine-induced relaxation of the bovine retractor penis muscle by compounds known to have ganglion-blocking properties

1. The relative potency in blocking the nicotine-induced relaxation of the bovine retractor penis muscle (BRP) was estimated for 12 drugs known to have ganglion-blocking properties. 2. The order of potency of the drugs studied was mecamylamine greater than chlorisondamine greater than pentolinium greater than propantheline greater than (+)-tubocurarine greater than hexamethonium greater than emepronium greater than tetraethylammonium greater than glycopyrrolate greater than decamethonium greater than butylscopolamine greater than scopolamine. 3. The results conform well to those obtained with other pharmacological methods used for the estimation of ganglion-blocking activity. 4. It is concluded that blockade of the nicotinic relaxation of the BRP can be used as an alternative method for quantitative assessment of ganglion-blocking activity. 5. Advantages of this technique are that it discriminates well between antinicotinic and antimuscarinic activity and that it satisfies most or all ethical and economical demands. 6. It is also possible that this method has certain value in predicting whether a drug has enough ganglion-blocking activity to be likely to cause impotence.

Rectification of currents activated by nicotinic acetylcholine receptors in rat sympathetic ganglion neurones

1. The inward rectification of the whole-cell current evoked by acetylcholine (ACh) and other nicotinic agonists in rat sympathetic ganglion neurones has been studied using patch-clamp recording techniques. The selective nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) (20 microM) induced an average peak current of -367 pA at -50 mV but no detectable outward current at +50 mV. Similar observations were made with ACh and carbachol. 2. The current-voltage relation of the whole-cell response induced by DMPP was linear in the negative voltage range; however, there was no detectable outward current in the voltage range 0 to about +70 mV. Above +70 mV an outward current became clearly detectable. Rapid depolarizing jumps in the holding potential failed to reveal any rapidly decaying outward current. 3. The rectification was not alleviated by changing the main permeant cation, by removal of divalent cations from the intracellular or extracellular solutions or by altering the pH buffer in the extracellular solution from HEPES to Tris. 4. Intracellular magnesium ions can block the channel. This effect increases with depolarization, but dissociation outwards (i.e. permeation by Mg2+) appears to relieve the block at more extreme positive potentials. This effect alone, or in combination with the voltage dependence of the burst length, is unlikely to be able to account for the whole-cell rectification in intact cells, much less that seen in cells perfused with Mg2(+)-free intracellular medium. 5. When the reversal potential was shifted to approximately -50 mV (by the use of impermeant cations) nicotinic agonists produced small outward currents in the membrane potential range -20 to +10 mV while shifting it to about +40 mV produced small inward currents in the potential range 0 to +20 mV. The rectification therefore appears to be independent of the direction of current flow and is maximum at a potential positive to 0 mV. 6. At positive potentials the receptors desensitized much less than at negative potentials in the continued presence of agonist. Thus, exposure of the cells to a steady application of 30 microM-ACh produced no detectable response if the cell was at a positive potential, but when the cell was stepped to a negative potential in the continued presence of ACh (at a time when much of the ACh current would be expected to have desensitized), ACh induced a large inward current. The onset of the ACh current had a time constant of 10 ms. It then decayed with a time constant of 790 ms as desensitization developed.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison between mechanisms of action of different nicotinic blocking agents on rat submandibular ganglia

The blocking properties of tubocurarine, decamethonium, hexamethonium and trimetaphan on nicotinic agonists applied by repetitive ionophoretic pulses were examined in rat submandibular ganglion cells using a two-microelectrode voltage-clamp technique at 30 degrees C. Hexamethonium, a proposed slowly dissociating, open-channel blocker at concentrations of 2-20 microM did not produce a 'use-dependent' run-down of responses, but its antagonism was clearly dependent on membrane potential. The voltage-dependent reduction of agonist response by hexamethonium was not dependent on the nature of agonist used. Similar results were obtained with acetylcholine (ACh) and carbamylcholine (CCh) ionophoresis. (+)-Tubocurarine (5 microM) and decamethonium (10 microM) produced 'use-dependent' run-down of agonist responses which became more pronounced at higher frequency and as the cell was hyperpolarized, consistent with open-channel blockade. In contrast, trimetaphan (2.5 microM), a receptor antagonist did not cause 'use-dependent' run-down of responses. Hence, the antagonism produced by hexamethonium, unlike tubocurarine and decamethonium, could not be accounted for in terms of open-channel blockade but requires an alternative mechanism, the nature of which is discussed.

Immunochemical and pharmacological distinctions between curaremimetic neurotoxin binding sites of central, autonomic, and peripheral origin

Comparative pharmacological and immunochemical studies were conducted on alpha-bungarotoxin binding sites from rat brain or muscle, Torpedo electric tissue, or the TE671 or PC12 clonal cell lines. Characteristic distinctions were observed in the pharmacological profile of drugs competing for toxin binding to different tissues. Differences also were found in the proportion of toxin binding sites (membrane-bound or detergent-solubilized) that are immunologically reactive with either monoclonal antibodies directed against nicotinic acetylcholine receptors from the electric organ of Torpedo or polyclonal antisera raised against nicotinic receptors from the electric organ of Electrophorus. These results suggest that toxin binding sites are structurally heterogeneous. Structural heterogeneity of nicotinic acetylcholine receptors, neurotoxin binding sites, or both, may contribute to the manifestation of nicotinic receptor functional heterogeneity and may explain the apparent discrepancy at some sites between toxin binding activity and toxin functional potency.

The channel-blocking action of methonium compounds on rat submandibular ganglion cells

The effects of drugs of the polymethylene bis-trimethylammonium (methonium) series on the characteristics of the synaptic currents (e.s.cs) recorded from voltage-clamped rat submandibular ganglion cells have been studied. The drugs studied were from C4 to C10 (decamethonium). All of the drugs except C4 shortened the initial decay phase of the e.s.c.; C9 and C10 produced an additional slowly decaying component. These effects were interpreted in terms of an open channel block mechanism, the calculated rate constants for association with the open channel at -80 mV being fairly similar (5.9 X 10(6) to 18.1 X 10(6)M-1S-1) for all of the compounds except C4, which had no effect on the e.s.c. decay. All of the compounds produced use-dependent block when tested with short trains of stimuli at 10 Hz, or with trains of ionophoretic pulses of acetylcholine, consistent with their channel blocking property. Tubocurarine had a similar effect, but not trimetaphan or mecamylamine. Recovery from use-dependent block with short chain methonium compounds, up to C8, was very slow in the absence of agonist, being incomplete even after several minutes. With C9 or C10 or tubocurarine, recovery from use-dependent block was complete within a few seconds. With C6 recovery in the absence of agonist was unaffected by membrane potential, but could be accelerated by applying acetylcholine with the cell depolarized to -40 mV. This persistent block was ascribed to the ability of the blocking molecule to become trapped by closure of the channel. With C9 and C10 it is assumed that their presence inhibits channel closure, so they can escape without the help of agonist. When use-dependent block is avoided by leaving the ganglion unstimulated during equilibration with the blocking drug, the first e.s.c. elicited shows no appreciable reduction of amplitude, though with C6, C7 or C8 subsequent responses elicited at 0.1 Hz become progressively more blocked. Even at 1 mM, C6 does not prevent acetylcholine from opening ionic channels. It is concluded that all of the effects on e.s.c. amplitude can be interpreted in terms of channel block, there being no evidence of any receptor blocking action.

An analysis of the anatomical basis for the mechanical response to motor nerve stimulation of the rat vas deferens

1. An anatomical basis was sought for the biphasic motor nerve response of the rat vas deferens. The motor nerve pathway to the tissue was stimulated at different points between the vertebral outflow and the intramural fibres, in the pithed rat and in isolated tissues, to examine the possibility of two anatomically separate groups of neurones. Different preparations of the isolated tissue were devised to detect whether different groups of smooth muscle fibres contributed to the two phases.2. The fibres mediating both phases of the response arose from the upper lumbar vertebral outflows. Both phases were elicited by pre- or post-ganglionic stimulation and could be depressed by hexamethonium. In the pithed rat or with hypogastric nerve stimulation in the isolated tissue, however, the initial ;twitch' phase was relatively resistant to such blockade.3. When the rat vas deferens was perfused through the lumen in situ or in vitro, the perfusion pressure response to motor nerve stimulation exhibited two phases similar to those of the longitudinal contractile response.4. Isolated rat vasa were bisected into portions, each of which was stimulated and longitudinal tension was recorded. The proportions of the two phases of the response varied along the length of the tissue. At the prostatic end the total response was relatively weak with a dominant ;twitch' and at the epididymal end the two phases were comparable in magnitude. The distribution of adrenergic nerve terminals within the muscle layers also varied along the length of the rat vas deferens.5. The effects of drugs were investigated on the motor responses of the above preparations. The ;twitch' phase was relatively susceptible to blockade by reserpine and lysergic acid diethylamide and the ;secondary' phase to phentolamine with both equally sensitive to guanethidine. Each phase had similar susceptibilities to blockade irrespective of which part of the tissue was involved.6. It was concluded that two types of nerve-muscle transmission may be involved in the rat vas deferens with the proportion of each varying along the length of the tissue but both displaying pharmacological characteristics of adrenergic fibres.

Effect of preganglionic nerve stimulation on sensitivity of the superior cervical ganglion to nicotinic blocking agents

1 Periodic stimulation (every 10 min) of the cervical sympathetic nerve increased the ganglionic block by low concentrations of chlorisondamine (CHL) in the superior cervical ganglion of the cat when compared to the contralateral unstimulated side.2 Periodic stimulation of the postganglionic nerve was ineffective in increasing the block.3 Ganglionic block by low concentrations of mecamylamine had the same stimulus dependency, but ganglionic block by any dose of hexamethonium was not influenced by nerve stimulation.4 Physostigmine infused together with CHL increased the rate of onset of block produced by CHL. Atropine had no apparent effect on the development of ganglionic block by CHL.5 Repeated intra-arterial injections of 1,1-dimethyl-4-phenylpiperazinium into the circulation of the superior cervical ganglion increased the magnitude of block produced by CHL. Similar injections of methacholine had no effect on ganglionic block produced by CHL.6 The results are interpreted to indicate that activation of ganglionic nicotinic receptors increased the affinity of receptors for CHL and mecamylamine.

Response of the rat ileum, uterus and vas deferens to carbachol and acetylcholine following repeated daily administration of a cholinesterase inhibitor

1. Daily i.p. administration, for eight days, of the cholinesterase inhibitor disulfoton to rats produced mild to moderate signs of intoxication (tremors, incontinence and diarrhoea) but no deaths.2. Segments of ileum taken from the treated rats were subsensitive to carbachol but the vas deferens and the uterus did not exhibit any change in sensitivity to carbachol.3. The sensitivity to acetylcholine was increased in the ileum and vas deferens but not in the uterus.4. Acetylcholinesterase activity was 60-70% inhibited in all three tissues.

Neuromuscular blocking and ganglion blocking activities of some acetylcholine antagonists in the cat

The potencies of tubocurarine, gallamine, pancuronium, benzoquinonium, hexamethonium and mecamylamine in blocking neuromuscular transmission in the soleus muscle, and in blocking contractions of the nictitating membrane evoked by preganglionic sympathetic stimulation have been compared in cats under chloralose anaesthesia. On a molar basis, pancuronium was about 8 times and benzoquinonium about 2·5 times more potent than tubocurarine in blocking the soleus muscle; gallamine was less than half as potent, mecamylamine about 128 times and hexamethonium about 380 times less potent. In blocking the superior cervical ganglion, mecamylamine was about 17 times more, tubocurarine was about 5 times more and pancuronium about twice as potent as hexamethonium. Benzoquinonium was about half as potent as hexamethonium, and gallamine about 5 times less potent. The results emphasize that the shorter distance between charged centres, as in hexamethonium, reduces affinity for muscle receptors but does not necessarily enhance affinity for ganglion receptors, and from the point of view of deductions concerning the configuration of the ganglionic receptor, the ganglion blocking potencies of some neuromuscular blocking drugs should be taken into account.

Analysis of the mechanism of action of some ganglion-blocking drugs in the rabbit superior cervical ganglion

1. Mechanisms of action of hexamethonium, mecamylamine and (+)-tubocurarine on the rabbit superior cervical ganglion were investigated by intracellular recording techniques.2. In concentrations up to 1 mM, none of these drugs affected the resting membrane potential nor altered the excitability of the postganglionic neurone to direct or antidromic stimulation.3. Post-tetanic potentiation of the excitatory postsynaptic potential (e.p.s.p.) was inhibited by mecamylamine (10-100 muM) but not affected by either hexamethonium (5-100 muM) or (+)-tubocurarine (10-50 muM).4. The decline in amplitude of successive e.p.s.ps in a train (40 Hz) was not influenced by hexamethonium or (+)-tubocurarine but was greatly exaggerated in the presence of mecamylamine; desensitization of the receptors for acetylcholine was excluded as a possible explanation for this latter finding.5. Mecamylamine depressed the quantal content of e.p.s.ps in a train, with the exception of the first e.p.s.p. which had an increased quantal content.6. Reduction in quantal content was attributed to a substantial fall in the size of the store of quanta of transmitter immediately available for release and to a reduction in the rate of mobilization of acetylcholine into that store; mecamylamine also caused a simultaneous increase in the fractional release.7. Hexamethonium and (+)-tubocurarine had no effect on transmitter release.8. The time-course of presynaptic effects of mecamylamine was similar to the duration of its postsynaptic blocking action.9. It is concluded that inhibition of ganglionic transmission by mecamylamine is due to both presynaptic and postsynaptic inhibitory actions; in contrast, hexamethonium and (+)-tubocurarine reduce transmission solely by their postsynaptic actions.

Effects of quaternary ammonium compounds on choline transport in red cells

1. Neuromuscular blocking agents have been shown to be powerful inhibitors of choline transport in human erythrocytes. Ganglionic blocking agents were weaker inhibitors.2. The affinity of the choline transport site for alkyltrimethylammonium compounds was considerably higher than its affinity for alkyl-bis-(trimethylammonium) compounds of similar chain length. The affinity increases with increasing length of the alkyl chain in both series.3. Tetramethylammonium (TMA), ethyl- and propyltrimethylammonium appear to enter the cells on the choline carrier while the larger monoquaternary compounds, and the bisquaternary compounds, bind to the carrier but are unable to cross the cell membrane.4. Radioactively labelled carbachol, acetylcholine and decamethonium do not enter the cells on the choline carrier.5. Choline transport in erythrocytes from patients with myasthenia gravis is normal, suggesting that this disorder is not associated with a generalized defect of choline transport.

Observations on the smooth muscle and contractile activity of the common bile duct

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The electrically stimulated ileum of the guinea-pig for measuring acetylcholine antagonism at different sites

The transmurally stimulated guinea-pig ileum preparation was used to determine quantitatively the antagonism developed by hexamethonium and atropine against the emptying reaction and the longitudinal muscle response. Hexamethonium in concentrations of 1·5–6·0 μg/ml blocked the emptying reaction but larger doses, 8–10 μg/ml, failed to depress the longitudinal response to less than 50% of its original height. Atropine, on the other hand, in concentrations of 0·001–0·02 μg/ml, reduced the longitudinal response without affecting the emptying reaction. Thus, the preparation discriminates between acetylcholine antagonists acting at either the nicotinic or muscarinic site.

The activity of post-ganglionic sympathetic nerves to the uterus of the rabbit

1. The spontaneous activity recorded from the central ends of the uterine nerves of the rabbit is described. The discharges resembled those reported from other post-ganglionic sympathetic nerves; the individual units appeared to discharge remarkably regularly at frequencies between 4/sec and 1 per 3 sec.2. The resting discharge in late pregnant rabbits was usually more intense than in non-pregnant rabbits; otherwise no clear relation existed between variations in the discharges and in the sexual state.3. The discharge was reduced by stimulation of the central end of the depressor nerve and increased by asphyxia.4. Stimulation of either the preganglionic or the post-ganglionic nerve by a single shock evoked a long after-discharge in the post-ganglionic nerve. It is shown that this after-discharge is a property of some of the post-ganglionic neurones supplying the rabbit's uterus which does not depend on the integrity of the preganglionic nerves or of the spinal cord, or on transmission through the ganglia, and that it is not a general property of the sympathetic post-ganglionic neurones of the rabbit.5. Repetitive stimulation of the preganglionic nerve at low frequencies augmented the after-discharge in some cases and depressed it in others; stimulation at high frequencies invariably depressed the after-discharge. Repetitive stimulation of the post-ganglionic nerve at low or high frequencies depressed the after-discharge.6. If the normal preganglionic activity was interrupted abruptly by an anode block, the post-ganglionic discharge also stopped abruptly.7. A ganglion-stimulating action of hexamethonium is described.8. Adrenaline inhibited the post-ganglionic neurones with no sign of excitation. This effect was antagonized by Rogitine without affecting the resting discharge, the action of the depressor nerve or the after-discharge in any way.