The antagonism of cocaine to the action of choline 2,6-xylyl ether bromide at sympathetic nerve endings

Choline 2,6-xylyl ether bromide (TM10) prevents contraction of the nictitating membrane and of the spleen in response to sympathetic nerve stimulation. It was confirmed that this effect was due to prevention of the release of noradrenaline (“sympathin”) at the nerve endings. The intravenous injection of cocaine before choline xylyl ether blocked the action of the latter drug. However, when atropine was injected before the cocaine the effects of large doses of choline xylyl ether were not completely blocked. After block had been produced by choline xylyl ether the injection of cocaine partially restored both the secretion at the sympathetic nerve endings and the responses of the organs supplied by the nerves. Phenoxybenzamine did not restore the secretion at the nerve endings after it had been inhibited by choline xylyl ether, indicating that the latter did not cause an increased uptake of sympathin by the receptors.     

Evidence for a competitive antagonism of guanethidine by dexamphetamine

After guanethidine had blocked the response of the cat nictitating membrane to sympathetic nerve stimulation, dexamphetamine restored the responses to all frequencies of stimulation. Dexamphetamine antagonized the sympathetic nerve block by guanethidine in the isolated sympathetically innervated rabbit ileum; the evidence suggests that the antagonism was competitive. Dexamphetamine antagonized the sympathetic nerve block by guanethidine in the isolated hypogastric nerve-vas deferens preparation of the guinea-pig. Doses of dexamphetamine, larger than those required to antagonize the blocking action of guanethidine, abolished the responses of the nictitating membrane, ileum and vas deferens to nerve stimulation. Dexamphetamine did not influence the depletion of noradrenaline by guanethidine in the heart and spleen of rabbits. The hypothesis is advanced that both dexamphetamine and guanethidine act on the store of noradrenaline at sympathetic nerve endings.     

Effects of drugs on partially and completely denervated skeletal muscle of the frog

The sensitivity of completely denervated and the innervated end-plate regions of partially denervated frog's sartorius muscles to stimulation by drugs has been investigated. The drugs were tested on muscles isolated 12 to 39 days following denervation. On the basis of the results obtained most of the drugs could be put into one of two main groups. The first group included choline, tetramethylammonium, carbachol and decamethonium; these drugs readily stimulated non-denervated muscles, and chronic denervation, either partial or complete, greatly increased the sensitivity of the muscles. The second group included pilocarpine and carbamate of (2-hydroxypropyl) trimethyl ammonium chloride (Bethanechol, U.S.P.). The muscles were relatively insensitive to stimulation by these two drugs. They could stimulate the muscles if applied in high enough concentrations, but the responses thus obtained could not be distinguished from responses produced by similar concentrations of sucrose and chronic denervation did not increase the sensitivity of the muscles to stimulation by these two drugs. Methacholine, like the drugs in the second group, was unable to stimulate the muscles in low concentrations, but chronic denervation rendered the muscles sensitive to stimulation by this drug. Tetraethylammonium presented a completely different pattern of activity which was consistent with a hypothesis that in low doses this drug stimulates skeletal muscle by causing a release of acetylcholine from nerve endings in the muscle.     

On the mechanism of the adrenergic nerve blocking action of bretylium

Summary Bretylium and tetracaine when perfused at increasing concentrations inhibited and finally blocked discharges elicited by acetylcholine and KCl in adrenergic nerve endings of the isolated perfused heart and less regularly in the isolated perfused spleen of the cat.There was a direct correlation between the inhibition of the effects of sympathetic nerve stimulation and the inhibition of KCl-induced discharges by bretylium and tetracaine in the heart with regard to both intensity and time course. Acetylcholine-induced release of noradrenaline in the heart was somewhat more resistant to the action of bretylium than acetylcholine-induced antidromic discharges in cardiac adrenergic nerves.Bretylium and tetracaine inhibited and blocked discharges in sinus nerve afferents elicited by rises in the perfusion pressure and by injection of KCl.The concentration for complete block in the perfused carotid sinus preparation was 800–1,000 times higher for bretylium than for tetracaine, whereas bretylium was equally or more potent (depending on the time of interaction) than tetracaine in the isolated perfused heart. Moreover, the effect of bretylium in the heart and spleen developed slowly, increased continuously during the perfusion period and was very resistant to washing out. In contrast, tetracaine achieved its maximum effect on heart, spleen and carotid sinus preparations very rapidly and was readily washed out. The time course and reversibility of the effect of bretylium on baroreceptor afferents were not drastically different from those of tetracaine.All observations may be explained by the fact that bretylium is a very weak local anaesthetic causing, however, a marked selective local anaesthesia of adrenergic nerve terminals because of its high accumulation in these endings. The adrenergic nerve blocking effect of bretylium seems to be adequately explained by its stabilizing effect on the membrane of the nerve terminals.Preliminary results of this report have been communicated to the German Pharmacological Society in Mainz at the Spring Meeting 1969 (Haeusler, G., W. Haefely and A. Huerlimann, 1969).     

A potent factor in extracts of the skin of the Australian frog, Pseudophryne coriacea--III. Potentiation of contractions elicited in avian and mammalian isolated skeletal muscle preparations by direct and indirect electrical stimulation

Extracts of the skin of the Australian frog Pseudophryne coriacea displayed a striking potentiating effect on contractions evoked in isolated skeletal muscle preparations of mammals (phrenic nerve diaphragm) and birds (chick biventer cervicis and semispinalis muscles) by indirect and direct electrical stimulation. There was both a conspicuous increase in the amplitude of the twitch, up to 10-fold, and a remarkable prolongation of the duration of the twitch. The effect was dose- and frequency-dependent. In the presence of the extract, fusion of twitches after tetanic stimulation occurred earlier. No tachyphylaxis upon repeated stimulation by the extract was observed and the response to large doses persisted, declining slowly, for hours. These effects must be ascribed to an alkaloid related in structure to pumiliotoxin B. Response to the extract of Pseudophryne coriacea by indirectly-stimulated preparations was potentiated by physostigmine and blocked by tubocurarine and alpha-bungarotoxin, demonstrating that in these preparations the extract acted pre-synaptically to facilitate the release of acetylcholine from motor nerve endings. However, the extract of Pseudophryne coriacea displayed equally potent effects in directly stimulated preparations, insensitive to physostigmine and to blockers of nicotinic acetylcholine receptors, indicating a direct action on the skeletal muscle. It is suggested that, like pumiliotoxin B, the Pseudophryne coriacea alkaloid may interfere in the regulation of calcium channels in both nerve and muscle fibres.     

Labelled decamethonium in cat muscle.

1 Tritium-labelled decamethonium was infused intravenously in 12 cats at final rates of 1.3-4.2 nmol kg-1 min-1 to produce a steady plasma concentration which ranged between 0.21-1.3 mumol/l in different experiments. Muscle contractions were elicited by nerve stimulation and the potential at the end-plate regions of superficial fibres was recorded by extracellular electrodes. 2 Under these conditions, it was not possible to obtain a steady degree of neuromuscular block. The initial decrease in muscle contractions was followed by recovery towards the original value although the concentration of decamethonium in the plasma remained constant, or in some cases rose. The initial depolarization of the end-plate region also waned during the constant infusion of the drug. 3 Once the twitch tension had returned to control values during infusion of the drug, prolongation of the infusion for a total of four hours did not produce a secondary neuromuscular block. 4 Scintillation counting showed that during infusion of labelled decamethonium the radioactivity of the muscles increased progressively with time. The uptake was less in the soleus muscle than in the fast-contracting flexor longus digitorum and extensor longus digitorum muscles. Muscles which had been denervated 12-13 days previously showed a greater uptake of labelled drug than control muscles from the contralateral limb. 5 The labelled drug was localized by autoradiography of frozen sections of leg muscles following intra-arterial injection of decamethonium. Grain counts in individual fibres showed that small amounts of decamethonium had entered the muscle fibres along their entire length, and there was increased uptake of the drug into the cell in the region of the end-plate. 6 The mechanisms underlying the waning of the pharmacological response during constant application of depolarizing drugs are discussed.     

The action of substances which block sympathetic postganglionic nervous transmission

The substances which block sympathetic postganglionic transmission, xylocholine, bretylium and guanethidine, also block neuromuscular and sympathetic ganglionic transmission. To see if these last properties were related to the sympathetic blocking property, phenyltrimethylammonium, which blocks the neuromuscular junction (Riker, 1953), was used. It blocked the inhibition of the rabbit ileum produced by stimulating the periarterial nerves in the mesentery, though with higher concentrations the effect of stimulation was initially increased. The action was not modified by the presence of hyoscine. The blocking action was exerted on the response to stimulation of the highest frequency first, and on the response to stimulation of the lowest frequency last. This relation of block to stimulus frequency is similar to that at the neuromuscular junction when tubocurarine is used. Nine compounds have now been shown to block responses to sympathetic postganglionic stimulation, and seven of these are onium compounds. They are, however, mon-onium compounds, and not bis-onium compounds like hexamethonium and decamethonium, so that they can probably enter the postganglionic fibre, which bis-onium compounds (having a charged group at each end of the molecule) may not be able to do. Since these mon-onium compounds have some blocking action at neuromuscular junctions and at sympathetic ganglia, their block of postganglionic transmission may be essentially similar to that by hexamethonium at ganglia and to that by decamethonium at neuromuscular junctions. It is known that acetylcholine releases noradrenaline from sympathetic postganglionic terminations, and xylocholine and bretylium block this release in the vessels of the rabbit ear and in the rabbit isolated atria.     

Modification by lidocaine of the discharges of primary endings of muscle spindles in cat tenuissimus muscle.

The effects of lidocaine (1--20 microgram/ml) on afferent discharge patterns of primary endings of muscle spindles in cat tenuissimus muscle were investigated. Discharge from the endings, recorded in Ia afferent axons, was evoked by ramp stretch of the muscle, stimulation of single static or dynamic fusimotor axons or by a combination of stretch and fusimotor stimulation. Spontaneous discharge of the endings at the initial length of the muscles was reduced by 2--5 microgram/ml and abolished by 10--15 microgram/ml lidocaine. The static but not the dynamic discharge elicited by muscle stretch was blocked by concentrations of 10--15 microgram/ml. The same concentrations abolished static and dynamic fusimotor influences on primary ending discharge. However, in one experiment where the spindle was microscopically observed, fusimotor stimulation still resulted in contraction of the intrafusal muscle even when fusimotor stimulation failed to elicit changes in discharge response patterns of the primary endings. These findings indicate that lidocaine interferes with the encoding mechanism prior to block of impulse conduction in either the fusimotor or afferent axons.     

Effect of ethanol on the spontaneous transmitter release by nerve endings in the process of maturation

Ethanol stimulates the spontaneous transmitter release from motor nerve endings, as shown by the increase of miniature end plate potential (m.e.p.p.) frequency at the neuro-muscular junction. The stimulation of acetylcholine spontaneous quantal release by ethanol is greater in regenerating than in mature nerve endings. The different effects of ethanol on regenerating nerve endings may be related to changes of chemical-physical membrane properties.     

The actions of bretylium: adrenergic neurone blocking and other effects

Bretylium caused a specific and lasting depression of many excitatory and inhibitory responses evoked by electrical stimulation of the peripheral sympathetic nervous system, probably by impairing conduction of impulses in adrenergic neurones with consequent failure of noradrenaline and adrenaline release. This effect, which will be referred to as the adrenergic neurone blocking action, was preceded by weak sympathomimetic effects. In the presence of bretylium the effects of adrenaline and noradrenaline were increased, as after sympathectomy. Concentrations producing blocking of adrenergic neurones did not prevent the release of adrenaline and noradrenaline from the adrenal medulla by splanchnic nerve stimulation or by the injection of dimethylphenylpiperazinium iodide, nor did they cause antiparasympathetic or parasympathomimetic effects. No action on the central nervous system has been detected. Curare-like neuromuscular block occurred with 10 to 30 times the amount required to block the response to adrenergic nerve stimulation alone and was accompanied by signs of temporary synaptic block in autonomic ganglia. Adrenergic nerve trunks and sensory nerves in the skin were readily blocked for long periods by topical application of bretylium, whereas the phrenic nerve of the rat was not. Bretylium had little effect on gastrointestinal propulsion or on the sensitivity of smooth muscle to acetylcholine, 5-hydroxytryptamine, adrenaline, or noradrenaline, but moderate amounts depressed the peristaltic reflex and the sensitivity of the guinea-pig ileum to histamine. Bretylium caused postural hypotension in the cat in doses which had little effect on the supine blood pressure. Experiments on the nictitating membrane indicated that compensation for the effects of bretylium on low rates of stimulation of postganglionic sympathetic nerves could be attained by a small increase in the rate of stimulation, whereas compensation for its effects on high rates required an increase in the rate of stimulation beyond physiological limits.     

Constitutive activity of H3 autoreceptors modulates histamine synthesis in rat brain through the cAMP/PKA pathway

We previously described that agonist-activated histamine H3 autoreceptors inhibit the stimulation of histamine synthesis mediated by calcium/calmodulin- and cAMP-dependent protein kinases (CaMKII and PKA respectively) in histaminergic nerve endings. In the absence of an agonist H3 receptors show partial constitutive activity, so we hypothesized that suppression of constitutive activity by an inverse agonist could stimulate these transduction pathways. We show here that the H3 inverse agonist thioperamide increases histamine synthesis in rat brain cortical slices independently from the amounts of extracellular histamine. Thioperamide effects were mimicked by the inverse agonists clobenpropit and A-331440, but not by the neutral antagonist VUF-5681. In contrast, coincubation with VUF-5681 suppressed thioperamide effects. The effects of thioperamide were completely blocked by the PKA inhibitor peptide myristoyl-PKI14-22, a peptide that did not block depolarization stimulation of histamine synthesis. In addition, thioperamide effects required depolarization and were impaired by blockade of N-type calcium channels (mediating depolarization), but not by CaMKII inhibition. These results indicate that constitutive activity of H3 receptors in rat brain cortex inhibits the adenylate cyclase/PKA pathway, and perhaps also the opening of N-type voltage sensitive calcium channels, but apparently not CaMKII.     

Pharmacology of the venom of Conus geographus

Conus geographus venom has a direct action on vertebrate skeletal musculature. When exposed to venom extracts, all mammalian skeletal muscles tested became refractory to 30 V electrical stimulation. Mammalian smooth and cardiac musculature, barnacle musculature and ‘slow’ fibres of amphibian skeletal musculature were not obviously affected by venom extracts. Compound action potentials in amphibian sciatic nerve were not modified by prolonged exposure to venom extracts and conduction in the sciatic nerve did not appear to be affected. Transmembrane resting potentials in fibres from the frog sartorius were maintained essentially unchanged in the presence of the venom. Although it was not possible to induce action potentials in fibres from paralysed musculature, no evidence was obtained that sodium ion movements across membranes are affected by the venom. It was possible to overcome the block of direct electrical stimulation induced by the venom (25 μg per ml) by increasing voltage strength to 90–100 V or by using tetanic stimulation at 30 V. The material in the venom active against muscle was dialysable and stable to heating at 100°C.     

The action of guanethidine with particular reference to the sympathetic nervous system

It has been suggested that guanethidine can release and then deplete postganglionic sympathetic nerve endings of noradrenaline. However, no release of noradrenaline from postganglionic nerve endings or from the adrenal medulla by guanethidine was found by direct experiment. Although release of noradrenaline from postganglionic sympathetic nerve endings in response to nerve stimulation was rapidly reduced and finally abolished by guanethidine, the drug did not appear to affect the release of catechol amines from the adrenal medulla in response to splanchnic nerve stimulation. The nature of the action of guanethidine is discussed, and it is concluded that it blocks the effect of postganglionic sympathetic nerve stimulation by interfering with the synthesis of transmitter and that it also has a direct sympathomimetic effect.     

Reversal of methylmercury-induced block of nerve-evoked release of acetylcholine at the neuromuscular junction

Acute bath application of micromolar concentrations of methylmercury (MeHg) blocks the nerve-evoked release of acetylcholine at the neuromuscular junction by presynaptic effects. The goal of the present study was to try to reverse this block of stimulus-evoked release. Experiments were conducted using the phrenic nerve hemidiaphragm of the rat and conventional intracellular microelectrode techniques. Myofibers were cut ("cut muscle") to prevent contractions elicited by stimulation of the phrenic nerve. End-plate potentials (EPPs) were recorded before and during MeHg application and during subsequent reversal attempts. MeHg (100 microM) blocked the EPP within 8-9 min of application. The time to block did not differ if Sr2+ (2 mM) was substituted for Ca2+ prior to exposure to MeHg. Washing the preparation with MeHg-free physiological saline at the time the EPP was blocked failed to reverse the block of synaptic transmission even during protracted washing. Increasing the extracellular [Ca2+] from 2 to 4 mM, or application of 4-aminopyridine (50 or 100 microM) failed to reverse block of the EPP. D-Penicillamine was also ineffective at reversing transmission block when applied at 0.4 mM; however, when applied at 1 mM D-penicillamine caused a return of EPPs in three of eight experiments within 5-20 min of wash. Longer periods of washing with D-penicillamine or use of higher concentrations of D-penicillamine were not effective in reversing transmission block in the refractory preparations. Increasing the intensity or duration of stimulation at the time of EPP block was uniformly successful in reversing MeHg-induced block; in 9 of the 10 preparations tested, EPPs could again be elicited from MeHg-blocked preparations merely by increasing the intensity and/or duration of stimulation, despite the continued presence of MeHg. Following reversal of transmission block by MeHg, continued exposure to MeHg resulted in a subsequent block of the EPP within approximately 3 min. During this subsequent block of the EPP by MeHg, increasing extracellular [Ca2+] or adding 4-aminopyridine did restore synaptic transmission. These results indicate that a temporary reversal of MeHg-induced block of synaptic transmission can be produced and that this effect does not require extracellular Ca2+ during the initial stages, but does seem to require extracellular Ca2+ during later stages.     

Angiotensin and peripheral sympathetic nerve activity

On the isolated vas deferens of the guinea-pig angiotensin potentiated strongly the height of contractions due to electrical stimulation of the hypogastric nerve; it did not affect the responses to noradrenaline and acetylcholine, nor did it elicit any contraction when given alone. Angiotensin likewise potentiated the responses of the cat spleen to nerve stimulation, but it also induced by itself strong contractions of the organ and reduction of the venous outflow. In experiments on the arterial blood pressure of anaesthetized and spinal cats, in which sympathetic postganglionic transmission was temporarily blocked by nicotine or tetramethylammonium, pressor responses to angiotensin were strongly reduced. As with some ganglion-stimulating drugs, the pressor responses, enhanced after a second series of nicotine injections, were reduced to the control level by hexamethonium. These findings indicate the involvement of peripheral sympathetic nerves in the action of angiotensin: the hypothesis is advanced that angiotensin acts at the peripheral nerve endings by promoting a greater output of noradrenaline.     

Cardiac vagolytic action of some neuromuscular blockers

Cardiac vagolytic effect of four commonly used neuromuscular blockers, (viz. D-tubocurarine, decamethonium, pancuronium and gallamine) was compared in mid-collicular decerebrate rats. The intravenous doses of neuromuscular blockers used (d-tubocurarine: 0.1 mg/kg; decamethonium: 2 mg/kg; pancuronium: 0.1 mg/kg; gallamine: 20 mg/kg) were sufficient to produce the paralysis of respiratory muscles. Bradycardia was induced by electrical stimulation of the vagus or by injecting dimethyl-phenyl-piperazinium (DMPP; a ganglionic stimulant). It was observed that d-tubocurarine and decamethonium were devoid of cardiac vagolytic action. On the other hand, pancuronium and gallamine inhibited significantly the bradycardia induced by electrical stimulation of the vagus or injection of DMPP; gallamine was found to have greater vagolytic action. The pressor responses to DMPP were not attenuated by pancuronium and gallamine indicating that in the dose administered, these agents did not block the ganllia. Bradycardia induced by the administration of acetylcholine in the left atrium was also attenuated by pancuronium and gallamine suggesting that the drugs produce cardiac vagolytic action by acting on the post-synaptic cholinergic receptors of the heart.     

The effects of bretylium on C fibre excitation and noradrenaline release by acetylcholine and electrical stimulation

1. The effects of bretylium on the excitation of postganglionic adrenergic C fibres by acetylcholine and the release of noradrenaline by acetylcholine and electrical stimulation of the splenic nerves have been studied using the in situ and cross perfused cat spleen.2. Close arterial injections of acetylcholine (10-200 mug) evoked a brisk asynchronous discharge in fine filaments of the splenic nerve which reduced the height of the orthodromic C fibre compound action potential.3. Hexamethonium abolished both the excitation of C fibres and release of noradrenaline by acetylcholine, whereas the liberation of noradrenaline by electrical stimulation of the splenic nerves remained unchanged.4. Bretylium (0.5 and 1.0 mg) given close arterially blocked the output of noradrenaline and contractions of the spleen that occurred in response to nerve stimulation (30 c/s) but had much less effect on the responses to acetylcholine.5. Bretylium (2-4 mg) given close arterially blocked the output of noradrenaline and contractions of the spleen caused by both nerve stimulation (30 c/s) and acetylcholine.6. The close arterial injection of (+)-amphetamine sulphate (100 mug) after bretylium (2-4 mg) partially restored the output of noradrenaline and contractions of the spleen to both nerve stimulation and acetylcholine.7. The difference in the sensitivity to blockade by bretylium of the effects of nerve stimulation and the sympathomimetic effects of acetylcholine did not exist if the more "physiological" frequency of stimulation of 10 c/s was employed.8. The close arterial injection of acetylcholine (100 mug) caused a mean average fibre discharge frequency of 5.4 spikes/sec.9. Bretylium in amounts sufficient to completely block the sympathomimetic effects of acetylcholine did not alter the excitation of C fibres by acetylcholine.10. The significance of these results is discussed both in relation to the mode of action of bretylium and to the use of these differential effects of bretylium as evidence for the "cholinergic link" hypothesis.     

Actions of triethylcholine on neuromuscular transmission

The effects of the triethyl analogue of choline (triethyl 2-hydroxyethyl ammonium) on muscular activity have been studied in conscious rabbits, chicks, dogs and a cat. The contractions of the tibialis anticus and soleus muscles of cats under chloralose anaesthesia, and of the tibialis anticus muscle of rabbits under urethane anaesthesia and the isolated diaphragm preparation of the rat were also used. In conscious animals, triethylcholine caused a slowly developing muscular weakness which was more severe after exercise and which resembled the symptoms of myasthenia gravis. In nerve-muscle preparations triethylcholine had a selective action in reducing the contractions of muscles elicited by a high rate of nerve stimulation while leaving unaffected the contractions caused by slower rates of stimulation. During the paralysis of the tibialis muscle of the cat produced by triethylcholine, action potentials recorded from the motor nerve were unaffected and the muscle responded normally to injected acetylcholine and to direct electrical stimulation. The failure of neuromuscular transmission produced by triethylcholine was reversed by injection of choline, but anticholinesterases were ineffective. Choline reduced the toxicity of triethylcholine in mice. It is concluded that triethylcholine produces transmission failure at the neuromuscular junction by interfering with the ability of the nerve endings to synthesize acetylcholine. The possibility that triethylcholine is itself acetylated by the nerve endings and released as an inactive neurohormone is discussed. It was shown that triethylcholine was devoid of depolarizing action and curare-like blocking action. It possesses a transient ganglion blocking action of the tetraethylammonium-type as shown in experiments in which it caused a fall in blood pressure and blocked the response of the nictitating membrane to pre- but not to post-ganglionic stimulation of the cervical sympathetic nerve.     

Studies on renal vasomotion

1. The present investigation was made on the left kidney of the dog. The animals were anaesthetized intravenously with pentobarbitone (30 mg/kg) and the kidneys were perfused with saline at room temperature (20 degrees -22 degrees C). The renal innervation was untouched.2. Stimulation of the left splanchnic major nerve at T10-T12, and of the renal nerves, consistently caused renal vasoconstriction.3. Repeated stimulation of both supradiaphragmatic vagi failed to induce any vasomotion in the kidney.4. The vasoconstrictor effect was not blocked by either nicotine or hexamethonium even in enormous doses (30,000 mug). This may indicate that renal ganglia do not exist, for these ganglion blockers would prevent transmission across the ganglia.5. Kidney perfusate, re-injected into the kidney after vasoconstriction induced by stimulation of the renal nerves, brought about a notable reduction in outflow. This effect was not observed when perfusate from a non-stimulated kidney was used. This points to the release of a vasoconstrictor substance after nervous stimulation.6. Acetylcholine (ACh) in concentrations ranging from 0.001 mug/ml. caused a reduction in renal outflow. Thresholds were extremely variable. Higher concentrations of ACh (100-1,000 mug/ml.) often induced vasodilatation. The vasoconstrictor effect of ACh was not blocked by atropine.7. Nicotine and hexamethonium (10,000-30,000 mug) induced blockade which elevated the threshold for ACh to values of 1,000 mug/ml.8. Noradrenaline (0.0001 mug/ml.) induced a strong renal vasoconstriction.9. Hydergine (5-10 ml. solutions in concentrations ranging from 15 to 30 mug/ml.) blocked the renal response to nerve stimulation. This suggests that the nature of the renal innervation is adrenergic.10. In diseased kidneys which show reduction of the lumen of the arterioles, the thresholds for ACh, nicotine and noradrenaline are greatly increased, which might explain why we failed to show any effect of these drugs on renal vasomotion in several kidneys, many of which were not examined histologically.11. The collision technique was applied in an attempt to discover the nature of the fibres activated by ACh. It was found that ACh greatly reduced the size of the action potentials generated by splanchnic stimulation. This would seem to indicate that these impulses are conducted antidromically by sympathetic postganglionic fibres.12. These findings are discussed in relation to the hypothesis that the renal innervation is chiefly adrenergic and that ACh acts as a sympathetic transmitter, liberating noradrenaline, and that this effect is blocked at postganglionic endings, or at some structure intervening between adrenergic nerve endings and the effector cells, or at sensory nerve endings.     

The action of 5-hydroxytryptamine on chemoreceptor discharges of the cat's carotid body.

1 Chemoreceptor discharges were recorded in vivo from fine filaments of the carotid sinus nerve containing a single or several active units; their frequency was used as an index of receptor activity. The effects of 5-hydroxytryptamine (5-HT) on chemoreceptors were studied in 26 adult cats. At times, sinus baroreceptor discharges were recorded from the carotid nerve and the effect of 5-HT on the discharges was examined. 2 Intra-carotid injections of 5-HT (2-20 mug) induced a sharp and brief increase in chemoreceptor discharges, followed by depression or block which lasted for several seconds. Repeated injections at short intervals, and a small dose after a large dose of 5-HT resulted in depressed or blocked response to 5-HT. 3 5-HT in high doses (10-20 mug, i.a.) slightly depressed the chemoreceptor discharges induced by either acetylcholine (ACh) or NaCN, when these substances were applied within 20 s after 5-HT. 5-HT (5-20 mug, i.a.) applied during asphyxia induced a further increase in chemoreceptor discharges, soon followed by block of the discharges lasting for several seconds. 4 Atropine or hexamethonium in high doses did not change the chemoreceptor response to 5-HT, while that to ACh was markedly depressed. 5 (+)-Lysergic diethylamide (LSD), methysergide or gramine did not alter the response to 5-HT, while LSD in low doses produced a marked increase in chemoreceptor discharges. 6 Acute and chronic treatment with reserpine (5-10 mg/kg, i.v.) of the animals did not change the sensitivity and the reactivity of the chemoreceptor to ACh and NaCN, while the chemoreceptor response to 5-HT was augmented, indicating an increase in the sensitivity of chemoreceptors to 5-HT. 7 5-HT in small doses (2-10 mug, i.a.) induced a marked increase in sinus baroreceptor discharges; subsequently discharges were depressed or blocked for several seconds. 8 The results are discussed in relation to possible mechanism of action of 5-HT on the chemoreceptors. It is concluded that the exogenous 5-HT probably acts directly on the chemosensory nerve endings and depolarizes them, but 5-HT contained in the carotid body does not play a significant role in the generation of chemoreceptor discharges.