‘Anticholinergic’ action of aminophylline in isolated guinea-pig atria

In isolated guinea-pig atria, aminophylline, 2 × 10⁻⁴ g/ml, markedly reduced the negative chronotropic responses to vagal stimulation. In a concentration of 5 × 10⁻⁴ g/ml, it almost abolished the bradycardia produced by exogenous acetylcholine, but not the negative inotropic responses to the parasympathetic transmitter. The transmission in the superior cervical ganglion of the cat was not affected by injections of aminophylline into the lingual artery. This methyl xanthine appears to exert its ‘anticholinergic’ effect postsynaptically on the cardiac pacemaker cells. The mechanism of this action may be related to the cyclic 3′,5′-AMP system.     

Impulse interval-dependent effect of sympathetic nerve stimulation on evoked acetylcholine release from the rabbit perfused atria preparation

The aim of the present study was to explore possible prejunctional effects mediated by impulse activity of sympathetic terminals on evoked acetylcholine release in an organ innervated by the autonomic ground plexus. Rabbit atria were isolated with the extrinsic right vagus and sympathetic nerves intact and perfused with Tyrode solution. Acetylcholine overflow was determined after labelling of the transmitter stores with [¹⁴C]choline and fractionation of the radioactivity on cation exchange columns. The overflow of endogenous noradrenaline was measured by HPLC and electrochemical detection.The vagus nerve was stimulated at 2 Hz for 3 min four times at intervals of 10 min. During the second stimulation the postganglionic sympathetic nerves were stimulated (2 Hz, 3 min) in such a way that the impulses preceded the vagus stimuli by a fixed time interval which was varied in different experiments (0, 7, 19, 50, 132, and 350 ms). Evoked acetylcholine release was significantly enhanced when the vagus was excited 7, 19 and 50 ms after the sympathetic nerves but it was unaltered at the 132 or 350 ms intervals, and when both nerves were stimulated simultaneously. Noradrenaline release was similar (about 6 ng per stimulation period) in all experimental groups. When sympathetic nerve stimulation had little effect in releasing noradrenaline (<2.0 ng per stimulation period), facilitation of acetylcholine release at the 19 ms pulse interval was absent. The resting outflow of acetylcholine was unaffected by sympathetic nerve stimulation.The experiments show a facilitation of evoked acetylcholine release by sympathetic activity. As revealed by the pulse-to-pulse method this effect is confined to a relatively brief interval immediately following the excitation of the noradrenergic terminal, and is unlikely to be mimicked by exogenous drug application.     

Co-transmitter mediated facilitation by sympathetic nerve stimulation of evoked acetylcholine release from the rabbit perfused atria preparation

Rabbit atria were isolated with the extrinsic right sympathetic and vagus nerves attached and perfused with Tyrode solution. Acetylcholine overflow was determined after labelling of the transmitter stores with [¹⁴C]choline and fractionation of the radioactivity on cation exchange columns. Sympathetic nerve stimulation (SNS, 2 Hz, 3 min) carried out together with vagus nerve stimulation (VNS, 2 Hz, 3 min), but each SNS pulse preceding a vagal one by 19 ms, caused a facilitation of acetylcholine overflow of about 60% versus independent controls in the absence of SNS. Antagonists of putative neurotransmitters were tested to find out the prejunctional mediator involved in the facilitation.The facilitation was not significantly reduced by prazosin, rauwolscine, idazoxan, or propranolol, excluding mediation by - or \-adrenoceptors. However, guanethidine abolished evoked noradrenaline release and facilitation, suggesting that it is due to a compound co-released with noradrenaline from postganglionic noradrenergic nerves. Pretreatment of rabbits with reserpine which reduced noradrenaline content of atria and SNS evoked overflow by 94% did not affect the facilitation of acetylcholine release which, due to the cardiostimulatory action of SNS being absent, resulted in enhanced depression of atrial force. We conclude that the facilitation is due to release of a reserpine-resistant co-transmitter from sympathetic nerves.Possible mediation of the facilitation by ATP through P_2X- or P_2Y-purinoceptors was excluded by ineffectiveness of , \-methylene ATP-preperfusion, of suramin and cibacron blue, respectively. However, the selective A₂ adenosine receptor antagonist CP 66,713 reduced the facilitation by 25% whereas DPCPX (A₁-selective) had no effect. Of the non-subtype-selective antagonists only 8-phenyltheophylline but not XAC decreased the facilitation by 40%. Mediation of the facilitation by tachykinin-, angiotensin-, opioid, AMPA/kainate-, M₁ muscarinic, 5-hydroxytryptamine 5-HT_1–4-receptors, or by nitric oxide, was excluded by administration of respective antagonists or inhibitors. Thus, whilst adenosine seems to be responsible for about one-fourth of the effect of sympathetic nerve stimulation, the major part of the facilitation of acetylcholine release remains unexplained.Under conditions of a low rate of evoked acetylcholine overflow prazosin enhanced the facilitatory action of SNS, suggesting an ₁-adrenoceptor mediated prejunctional inhibition of acetylcholine release.     

The effects of metoclopramide on acetylcholine release and on smooth muscle response in the isolated guinea-pig ileum

Summary The effects of metoclopramide on smooth muscle contraction and on release of acetylcholine were studied in the guinea-pig myenteric plexus longitudinal muscle preparation. Acetylcholine was determined either as endogenous acetylcholine, or as labelled transmitter from strips preloaded with ³H-choline.Metoclopramide caused an increase in resting tension of longitudinal muscle as well as an increase in resting output of either endogenous or labelled acetylcholine. Tetrodotoxin abolished the metoclopramide-evoked increase in transmitter release. The increase in smooth muscle tension was clearly related to the increase in resting output. The effects of metoclopramide on both longitudinal muscle contraction and resting release of labelled acetylcholine were prevented in the presence of a concentration of 5-hydroxytryptamine (5-HT) that desensitized 5-HT receptors. This suggests that metoclopramide stimulates neuronal 5-HT receptors and, thereby, facilitates acetylcholine release.Metoclopramide augmented the twitch-like contractions induced by field stimulation at 0.1 Hz. Contractions elicited at 1 Hz were only slightly enhanced. Similarly, metoclopramide facilitated only the release of labelled acetylcholine evoked by electrical stimulation at 0.1 Hz, but not that at 1 Hz. The facilitatory effetts of metoclopramide on twitch height and evoked release could not be attributed to a blockade of presynaptic inhibitory -adrenoceptors, dopamine or muscarine receptors.     

Prostaglandin release from rat stomach following vagal stimulation or administration of acetylcholine

The rat isolated stomach was perfused via the coeliac artery and prostaglandin-like material released from the mucosa into the lumen (luminal fraction) and from blood vessels and surrounding serosal wall (serosal fraction) was bioassayed using rat fundus strips. Prostaglandin F_2α was identified by chromatographic and mass spectrometric techniques. The amount of prostaglandin F_2α released upon their vagal nerve stimulation or acetylcholine infusion increased significantly over basal levels, in association with increased gastric peristaltic activity. The quantity of prostaglandin released into the luminal fraction greatly exceeded the amount liberated into the serosal samples and this increase was dependent upon the rate of stimulation. Infusion of either atropine or hexamethonium markedly reduced both the prostaglandin output and the gastric contraction elicited by stimulation. Pretreating the stomach with indomethacin considerably decreased prostaglandin outflow. The role of endogenous prostaglanding F_2α in relation to parasympathetic neurotransmission is discussed.     

Muscarinic inhibition of acetylcholine release from a novel in vitro preparation of the guinea-pig trachea

Summary An isolated preparation of the guinea-pig trachea is described which allows the simultaneous measurement of acetylcholine release and smooth muscle contraction. Incubation of the epithelium-free preparation with [³H]choline resulted in the formation of [³H]acetylcholine. Electrical stimulation caused the release of [³H]acetylcholine and a contractile response. Tetrodotoxin and omission of calcium from the medium abolished both the evoked release and contractions.The muscarinic agonists oxotremorine, carbachol and pilocarpine concentration-dependently inhibited the electrically evoked acetylcholine release and contracted the tracheal smooth muscle. Pre- and postsynaptic EC₅₀ values for a given agonist were not different. Atropine (100 nmol/l) significantly faciliated the evoked acetylcholine release. A concentration of 10 nmol/l atropine did not change the evoked release but antagonized the inhibitory effect of oxotremorine. It is concluded that presynaptic muscarine autoreceptors inhibit the release of acetylcholine from parasympathetic nerves of the guinea-pig trachea.Send offprint requests to G. D'Agostino at the above address     

Autoregulation of acetylcholine release in isolated hippocampal nerve endings

The existence of presynaptic autoreceptors modulating acetylcholine release from central cholinergic nerve endings was investigated by using rat hippocampal synaptosomes in a superfusion system. The presence of exogenous acetylcholine, carbachol or oxotremorine in the superfusion fluid produced a dose-dependent inhibition of the release of [³H]acetylcholine elicited by 15 mM KCl in synaptosomes prelabeled with tritiated choline. The inhibition was counteracted by atropine. Another well known muscarinic agonist, bethanechol, had no effect on [³H]acetylcholine release. Our results indicate that central cholinergic nerve terminals possess autoreceptors of the muscarinic type for the control of acetylcholine release. Moreover, differences seem to exist between pre- and postsynaptic muscarinic receptors in the central nervous system.     

Increase by NO synthase inhibitors of acetylcholine release from guinea-pig myenteric plexus

The effects of nitric oxide (NO) synthase inhibitors on the electrically evoked release of [³H]acetylcho-line were studied in guinea-pig myenteric plexus preparations preincubated with [³H]choline. N^G-monomethyl-l-arginine (EC₅₀ 5.3 mol l^–1) and N^G-nitro-l-arginine (EC₅₀ 1.3 mol l^–1) concentration-dependently increased the evoked release of [³H]acetylcholine without affecting the basal outflow. The facilitatory effect of N^G-monomethyl-l-arginine was prevented by l-arginine but not by d-arginine. The results suggest that endogenous NO inhibits the depolarisation-evoked release of acetylcholine. Correspondence to: H. Kilbinger at the above address     

Comparison of corelease of noradrenaline and ATP evoked by hypogastric nerve stimulation and field stimulation in guinea-pig vas deferens

Contractions and overflow of tritium and ATP elicited by hypogastric nerve stimulation (HNS) and field stimulation (FS) were studied in the guinea-pig isolated vas deferens preincubated with [³H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique.HNS and FS elicited contraction, tritium overflow and ATP overflow. HNS at supramaximal current strength produced smaller responses than did FS at supramaximal current strength (210 pulses/7 Hz). Supramaximal HNS and submaximal FS were used in the remainder of the study. Prazosin (0.3 mol/l) reduced contractions and the overflow of ATP elicited by both HNS and FS; the evoked overflow of tritium was not changed (210 pulses/7 Hz). Combined administration of prazosin (0.3 mol/l) and suramin (300 mol/l) abolished contractions and reduced the overflow of ATP elicited by both HNS and FS slightly more than did prazosin alone; tritium overflow again was not changed (210 pulses/7 Hz). Contractions, tritium overflow and ATP overflow increased with the frequency of both HNS and FS (from 7 to 25 Hz; 210 pulses); the increase in ATP overflow with frequency was more marked than the increase in tritium overflow. The preferential increase of ATP overflow with the frequency of HNS and FS persisted in the combined presence of prazosin (0.3 mol/l) and suramin (300 mol/l).The study confirms for HNS, a more physiologic way of sympathetic nerve stimulation, several observations previously obtained with FS. First, HNS-evoked ATP release is detectable as an overflow of ATP into the superfusion fluid. Second, a large part of the HNS-evoked release of ATP is postjunctional in origin, due to activation of post-junctional ₁-adrenoceptors and presumably P₂-purinoceptors. Third, the average neural release of ATP per pulse facilitates with the frequency of stimulation to a greater extent than the average release of noradrenaline per pulse.     

Contraction of isolated guinea-pig ileum by urotensin II via activation of ganglionic cholinergic neurons and acetylcholine release

Urotensin II and its receptor are expressed in the gastrointestinal tract of mice, but the effects of urotensin II on the gastrointestinal functions have not been established. In the present study, we investigated the effects of human urotensin II on a segment of the guinea-pig ileum. The addition of urotensin II induced contraction of the ileum in concentration-manner (-log EC(50) value was 8.13+/-0.21). The response by urotensin II was extracellular CaCl(2)-dependent and easily desensitized. Like nicotine, the contraction induced by 100 nM urotensin II was inhibited by treatment with atropine, hexamethonium, D-tubocurarine, tetrodotoxin or hemicholinium-3, and enhanced by physostigmine. Treatment with omega-conotoxin GVIA (an inhibitor of N-type Ca(2+) channels, 300 nM) inhibited 100 nM urotensin II- and 4 microM nicotine-, but not 3 microM acetylcholine-, induced contraction. Both urotensin II and nicotine stimulated [(3)H]choline release in a tetrodotoxin-sensitive manner from the prelabeled slices of the ileum. These findings suggest that urotensin II stimulated acetylcholine release from the ganglionic cholinergic neurons and thus stimulated contraction via muscarinic acetylcholine receptors in the guinea-pig ileum. Urotensin II receptor system in the myenteric neurons may regulate the gastrointestinal functions.     

Inhibitory effects of procaine on the contractile responses of the guinea-pig Taenia caecum to acetylcholine,substance P and potassium chloride

Summary The effect of procaine on the contractile responses to acetylcholine, substance P and KCl was investigated using the isolated guinea-pig taenia caecum. In normal Tyrode solution (37°C), procaine (10–100 mol/l) caused a parallel shift to the right of only the dose-response curve of acetylcholine (pA₂ value, 5.11). The pA₂ value of procaine against acetylcholine was not significantly affected by increasing the Ca concentration in the bathing solution from 0.9 to 7.2 mmol/l. On the other hand, a high concentration of procaine (10 mmol/l) caused a transient contraction of the taenia caecum, but completely suppressed contractions to all concentrations of the agonists. In K-depolarized preparations, procaine (1–10 mmol/l) shifted the dose-response curve for the CaCl₂-induced contraction to the right. Substance P (3 mol/l)-induced contraction of the taenia caecum incubated with Ca-free EGTA (0.1 mmol/l) solution (20°C) was markedly reduced by procaine (10 mmol/l). Using the single sucrose-gap technique, it was found that procaine (10 mmol/l) produced a membrane depolarization and increases in both amplitude and frequency of spontaneous spike discharge. These potential changes were still observed even after the procaine-induced contraction had disappeared. The spike discharges and contraction caused by procaine were abolished in the presence of a Ca-entry blocker, verapamil (10 mol/l). From these observations, it is concluded that at low concentrations procaine acts as a competitive antagonist of muscarinic receptors in the guinea-pig taenia caecum while high concentrations of procaine may depress the contractile responses to acetylcholine, substance P and KCl by inhibiting the Ca-induced Ca release from the intracellular store site or by reducing the transmembrane Ca influx during depolarization.     

The effects of vasodilating drugs and vasoconstrictor nerve stimulation on oxygen uptake in skeletal muscle

Oxygen uptake was studied in skeletal muscle of cat, dog, fox and hare during i.a. infusion of acetylcholine, isoproterenol, and histamine. All agents produced similar effects in the different species. After a brief initial increase, the calculated oxygen uptake returned to near resting levels and remained so throughout the infusion period.

During acetylcholine infusion and concomitant vasoconstrictor nerve stimulation, the oxygen uptake decreased compared to resting levels although blood flow was increased. During motor nerve stimulation, the oxygen uptake increased considerably and acetylcholine infusion did not change the magnitude of that increase.

It is concluded that acetylcholine, isoproterenol, and histamine do not influence the capillary surface area to such an extent as to cause major changes in the oxygen uptake of the resting skeletal muscle. On the other hand, sympathetic vasoconstrictor nerve stimulation decreased oxygen uptake, even during increased blood flow, indicating that under those circumstances the capillary surface area is considerably decreased.     

The lack of effects of somatostatin on gastric responses induced by electrical vagal stimulation

The effects of somatostatin on ulcer formation, gastric acid secretion and histamine release were assessed during vagus nerve stimulation in rats. Direct electrical vagal stimulation significantly increased histamine release and acid output in gastric secretion but decreased mast cell counts in gastric glandular mucosa. Hemorrhagic ulceration on the gastric glandular mucosa was also observed. Somatostatin pretreatment (10 micrograms/kg) did not inhibit gastric ulcer formation, gastric acid secretion or histamine release induced by vagal stimulation. Cimetidine (an H2 blocker) pretreatment, however, significantly decreased gastric acid secretion as well as ulcer formation. The present study indicates the direct vagal stimulation increases gastric acid secretion and ulcer formation. These effects are partially histamine dependent. Somatostatin did not inhibit histamine release induced by vagal stimulation and reflects the inability of the drug to prevent ulcer formation and gastric output under these conditions in rats. However, the inhibition of basal gastric acid secretion produced by somatostatin might be useful clinically in humans.     

Non-cholinergic,non-adrenergic responses to nerve stimulation of different regions of the guinea-pig small intestine

Summary Non-cholinergic, non-adrenergic responses to nerve stimulation recorded from smooth muscles of the guinea-pig duodenum, jejunum, proximal and terminal ileum were investigated in an attempt to characterize these responses.In the presence of atropine (1–2 mol · l^–1) and guanethidine (10 mol · l^–1) coaxial stimulation induced in all regions of the guinea-pig small intestine an initial relaxation (primary relaxation) upon which contraction (primary contraction) appeared, followed by rebound contraction.Noradrenaline decreased the cholinergic smooth muscle twitches, predominantly at low stimulation frequencies, and had a similar effect on the non-cholinergic, non-adrenergic primary relaxation, primary and rebound contractions.ATP decreased the smooth muscle twitches; however, this agent had only a transient influence on the non-cholinergic, non-adrenergic responses of muscle (tension and membrane potential) to single stimuli. With higher stimulus frequencies ATP increased the primary relaxation and decreased the contraction phases. ATP also inhibited the post-tetanic inhibition induced by non-cholinergic, non-adrenergic nerve stimulation.In most of the muscle cells of the guinea-pig proximal and terminal ileum the non-cholinergic, non-adrenergic nerve stimulation generated i.j.p.s., while about 15–20% of the cells responded with e.j.p.s. During long-lasting stimulation (10s) the i.j.p.s. were sometimes interrupted by action potentials or by a gradual depolarization of the membrane. The i.j.p.s. were followed by a marked rebound depolarization accompanying the action potentials. Those cells which generated i.j.p.s. in response to field stimulation, were depolarized by ATP, while those cells, which generated e.j.p.s, were hyperpolarized by ATP.A reduction in the concentration of extracellular sodium chloride decreased both the primary and rebound contractions; the primary contraction was, however, more sensitive than was the rebound contraction.Theophylline increased the primary and rebound contractions with no marked influence on the primary relaxation, lowered the action potential threshold, increased the rebound depolarization and did not markedly influence the i.j.p.s.Quinidine enhanced the primary relaxation and inhibited the primary contraction in a concentration-dependent manner. Inhibition of the rebound contraction by quinidine was slight (less than 50%).The present results demonstrate that primary relaxation, primary and rebound contractions are associated with i.j.p.s and e.j.p.s., and rebound depolarization with action potentials, respectively; they are typical responses of various regions of the guinea-pig small intestine to activation of inhibitory and excitatory non-cholinergic, non-adrenergic nerves. The P₁ and P₂ receptors, proposed by Burnstock (1975), probably do not mediate the non-cholinergic, non-adrenergic postsynaptic responses of the guinea-pig small intestine. A possible physiological function of ATP as a mediator of non-cholinergic, non-adrenergic nerves of the guinea-pig small intestine is discussed.This study was supported in part by JSPS Japan     

Effects of methylxanthines and imidazole on the contractions of guinea-pig ileum induced by transmural stimulation.

Methylxanthines (10(-5) to 10(-3)M) were found to increase the amplitude of contractions of guinea-pig ileum induced by transmural stimulation but to inhibit those induced by acetylcholine or histamine. The order of the abilities of methylxanthines to augment the contractile responses was theobromine greater than caffeine greater than theophylline. When the contractions were completely suppressed by reduction of the calcium content in the medium or by addition of cyclic AMP, methylxanthines restored the responses effectively, just as does addition of calcium. Methylxanthines also accelerated the release of acetylcholine from the ileum associated with stimulation. Imidazole (3 X 10(-5) to 10(-3) M) had an essentially similar effect to methylxanthines in potentiating the contractile responses and in augmenting the release of acetylcholine. The present results indicate that the potentiating effects of methylxanthines and imidazole are due to an action on the nerve terminals, not on the postsynaptic membranes or contractile elements. Therefore, it si concluded that theit potentiating actions are due to facilitation of the movement of calcium in the nerve terminals on excitation, resulting in increased release of acetylcholine, and are not due to the effect of cyclic AMP formed as a result of their inhibitory actions on phosphodiesterase.     

Counter effects of higenamine and coryneine, components of aconite root, on acetylcholine release from motor nerve terminal in mice

The counter effects of higenamine and coryneine, components of aconite root, on acetylcholine (ACh) release from motor nerve terminals in the mouse phrenic nerve-diaphragm muscle preparation were studied by a radioisotope method. Both nerve-evoked release and spontaneous release of [³H]-ACh from the preparation preloaded with [³H]-choline were measured. The change in the tetanic tension of muscle was simultaneously recorded in the same preparation. Higenamine (10 μM) augmented both the nerve-evoked and spontaneous ACh releases, and the muscle tension. The effects were inhibited by pretreatment with propranolol (10 μM), a β-adrenoceptor antagonist. Coryneine reduced the nerve-evoked release of ACh, accelerated the decay of tetanic tension (tetanic fade) at 30 μM, and it depressed the peak amplitude of tetanic tension at a higher concentration of 100 μM. These results suggest that of the two components contained in aconite root, higenamine increases ACh release via activation of β-adrenoceptor, and conversely coryneine depresses ACh release by preferentially acting at motor nerve terminal.     

A presynaptic excitatory M1 muscarine receptor at postganglionic cardiac noradrenergic nerve fibres that is activated by endogenous acetylcholine

Summary Rabbit atria were isolated with the extrinsic right vagus and sympathetic nerves intact and perfused with Tyrode solution. Noradrenaline overflow evoked by sympathetic nerve stimulation (SNS) at 3 Hz for 3 min was determined before, during, and after vagus nerve stimulation (VNS), also at 3 Hz and for 3 min. The VNS pulses preceded the SNS pulses by 3, 100 and 233 ms. Acetylcholine overflow was determined after labelling of the transmitter stores with [¹⁴C]choline.Pirenzepine 80 nmol/l failed to alter the muscarinic inhibition of noradrenaline overflow when the vago-sympathetic impulse intervals were 3 and 233 ms. At an interval of 100 ms VNS did not significantly inhibit noradrenaline overflow in the absence of pirenzepine but produced an inhibition in the presence of the drug. When the pirenzepine concentration was varied (0.4–300 nmol/l) the largest inhibition of noradrenaline overflow was observed at 5.7 nmol/l whereas 300 nmol/l fully antagonized the inhibition. Acetylcholine overflow evoked by VNS was not altered by pirenzepine 0.4–300 nmol/l.AF-DX 116 (11-[{2[oi(diethylamino)methyl]-1-piperidinyl}-acetyl]-5,11-dihydro-6H-pyrido-[2,3-b]-[1,4]benzodiazepine-6-one), an M₂ receptor selective antagonist, concentration-dependently (100–800 nmol/l) inhibited the decrease of tension development elicited by VNS. At the 100 ms vago-sympathetic impulse interval noradrenaline overflow was enhanced in the presence of AF-DX 116 400 and 800 nmol/l. However, already 100 nmol/l of the drug caused a maximum (fourfold) increase of acetylcholine overflow.It is concluded that acetylcholine released onto noradrenergic nerve fibres causes a small facilitation of noradrenaline overflow at a vago-sympathetic impulse interval of 100 ms. This response is mediated by an M₁ receptor and is superimposed on the well-known M₂ receptor mediated inhibition of noradrenaline release which is obtained at vago-sympathetic impulse intervals ranging between 3 and 233 ms. The M₂ autoreceptor regulating acetylcholine release is activated by lower synaptic concentrations of the transmitter than the M₂ heteroreceptor regulating noradrenaline release.Abbreviations SNS sympathetic nerve stimulation - VNS vagus nerve stimulation Send offprint requests to: E. Muscholl at the above address     

Enhancement of spontaneous acetylcholine release from motor nerve terminal by calmodulin inhibitors

The role of calmodulin (CaM) in neuromuscular transmission in the rat diaphragm was electrophysiologically investigated using a microelectrode technique with two different CaM inhibitors, trifluoperazine (TFP) and N-(6-aminohexyl)-5-chrolo-1-naphthalene-sulfonamide HCl (W-7). These inhibitors in the perfusate decreased the amplitude of miniature endplate potentials and increased the frequency dose dependently without any changes occurring in the resting membrane potentials. These effect were abolished in a Ca²⁺-free perfusate. The acetylcholine (ACh) quantal size and content were not affected by the reagents. The effects of TFP and W-7 were thought to result from their specific inhibition of calmodulin. It is suggested that accumulated intracellular Ca²⁺ and cyclic AMP, due to inhibition of calmodulin, had enhanced the frequency of spontaneous ACh release from the nerve terminal, and the decrease in the amplitude might be attributed to inhibition of the postsynaptic action of CaM.     

Differences in the prejunctional effects of methacholine and pilocarpine on the release of endogenous acetylcholine from guinea-pig trachea

We investigated the effects of the full muscarinic acetylcholine receptor agonist methacholine and the partial and putatively M₂-selective agonist pilocarpine on endogenous acetylcholine release from guinea-pig trachea by use of high-performance liquid chromatography with electrochemical detection. Atropine-induced increases in acetylcholine release were used to monitor the system.Electrical field stimulation (8 V, 30 Hz, 0.5 ms for 5 min)-induced acetylcholine release in the presence of neostigmine, with or without preincubation with choline to maximally enhance acetylcholine output, was increased to about 225% by 0.3 M atropine, indicating functional autoinhibition. However, methacholine (10 M) did not affect the acetylcholine release, whereas it was enhanced to 166% by 30 M pilocarpine. When electrical field stimulation was applied at lower intensity (8 V, 16 Hz, 0.1 ms for 5 min) and in the absence of neostigmine, an increase by 0.3 M atropine (to 177%) but a decrease of the acetylcholine release by 10 M methacholine (to 65%) and 30 M pilocarpine (to 63%) were observed. These results clearly demonstrate (i) that inhibition of evoked endogenous acetylcholine release from prejunctional nerve terminals in guinea-pig trachea can only be demonstrated under conditions of low junctional concentrations of acetylcholine, and (ii) that pilocarpine, as a partial muscarinic agonist, behaves as an antagonist under high junctional concentrations of the neurotransmitter.     

Effects of (+)-tubocurarine on [3H]acetylcholine release from the rat phrenic nerve at different stimulation frequencies and train lengths

Summary The effect of (+)-tubocurarine (TC) on the release of [³H]acetylcholine from the rat phrenic nerve-hemidiaphragm preincubated with [3H]choline was investigated at different stimulation frequencies and train lengths.At 0.5 Hz (100 pulses) TC failed to modulate the evoked acetylcholine release. A slight (30%) inhibition was observed at 1 Hz (100 pulses). Release of acetylcholine evoked at 5, 25 and 50 Hz (100 pulses) or 100 Hz (200 pulses) was markedly reduced by TC. The degree of inhibition (60%) was similar between 5 Hz and 100 Hz. A concentration of 1 mol/l TC was a maximal effective concentration at 5 Hz whilst at all higher stimulation frequencies a 10-fold higher concentration was necessary for the maximal effect. When 300 pulses were continuously applied at 5 Hz or 50 Hz TC caused only a slight inhibition (20%). Additionally, the phrenic nerve was stimulated intermittently. Trains of 15 pulses were repeated 10 times with an interval of 3 s between each train. Under this latter stimulation condition TC failed to reduce acetylcholine release.It is concluded that nicotinic autofacilitation of acetylcholine release from the motor nerve operates at frequencies and stimulation conditions similar to the pattern of nerve activity under in vivo conditions. At least more than 15 pulses are required before the nicotinic autofacilitation becomes apparent. It appears unlikely that the TC induced fading of end-organ responses can only be attributed to a blockade of the presynaptic nicotine receptors. Send offprint requests to I. Wessler at the above address