A muscarinic inhibition of the noradrenaline release evoked by postganglionic sympathetic nerve stimulation

Properties of the peripheral opiate receptors in the cat nictitating membrane

The noradrenaline overflow and the contractile response elicited by nerve stimulation of the muscle were inhibited by 1 micron morphine in the cat nictitating membrane. This concentration of morphine did not modify the response of the muscle to exogenous noradrenaline. The inhibitory effect of morphine was increased by low Na+ (50 mM), whereas the capacity of naloxone as antagonist to morphine was higher with 150 mM than with 50 mM Na+. These results suggest that the peripheral opiate receptors which interact with noradrenergic neurotransmission could show a sodium allosteric transformation similar to that described for the brain opiate receptor. The effect of morphine was enhanced by manganese ion in the presence of normal Na+. The responses of the cat nictitating membrane to nerve stimulation were not altered in the presence of the protein modifying reagent DTNB [5,5'-dithiobis-(2-nitrobenzoic acid)] but the effect of morphine on the adrenergic neurotransmission was diminished by DTNB with 150 mM Na+. It is postulated that the affinity of the ligands for presynaptic receptors which regulate adrenergic neurotransmission might be modified during the physiological changes in ion concentration which accompany nerve depolarization.

Acetylcholine-norepinephrine interactions on potassium movements in the sinus node

Possible adrenergic mechanisms involved in acetylcholine (ACh) induced potassium movements in the sinus node have been investigated using a tracer potassium (42K) and a microelectrode technique. The ACh-induced increase in 42K uptake was enhanced by propranolol and was unaffected by phentolamine. Reserpinization neither prevented the ACh-induced increase in 42K uptake nor the enhanced effect in the presence of propranolol. In reserpinized preparations, ACh-induced 42K uptake was the same before and after norepinephrine (NE) administration, but was greater after acute reserpinization. NE alone induced an increase in 42K uptake but the simultaneous administration of ACh and NE provoked an increase in 42K uptake similar to that obtained with ACh alone. When both neuromediators and atropine were given simultaneously, an increase in 42K uptake did not occur. Thus, the ACh induces an increase in 42K uptake independently of NE. ACh antagonizes the NE effect on 42K uptake independently of a muscarinic interaction.

Presynaptic receptor systems on the noradrenergic neurones of rat brain

A simple volume averaging model was shown to be inadequate for predicting effective computed tomographic attenuation values of mixtures of bone or air with soft tissue. Computed tomographic values derived from scanning stratified sub-slice thickness objects were shown theoretically and experimentally to have a non-linear dependency on relative fractional content and a surprisingly large dependency on spatial extent of the stratified substances. The mode was applied to the problem of computed tomographic values in thin, flat structures such as pools of blood in the subarachnoid space. The results show only small deviations from simple volume averaging theory for layers of low contrast substances such as blood and soft tissue, but potentially large deviations for layers of substances with high contrast differences such as bone, air, and tissue. This phenomenon explains certain artifacts and demonstrates rather fundamental problems in the accuracy of analytic reconstruction techniques. It may justify postprocessing correction or iterative approaches.

Neural and psychologic mechanisms and the problem of sudden cardiac death

Brain stimulation can provoke a variety of arrhythmias and lower the ventricular vulnerable threshold. In the animal with acute myocardial ischemia such stimuli suffice to provoke ventricular fibrillation. Vagal neural traffic or adrenal catecholamines are not the conduits for this brain-heart linkage. Accompanying increases in heart rate or blood pressure are not prerequisites for the changes in cardiac excitability. Increased sympathetic activity, whether induced by neural or neurohumoral action, predisposes the heart to ventricular fibrillation. Protection can be achieved with surgical and pharmacologic denervation or reflex reduction in sympathetic tone. With acute myocardial ischemia, augmented sympathetic activity accounts for the early surge of ectopic activity frequently precipitating ventricular fibrillation. Asymmetries in sympathetic neural discharge may also contribute to the genesis of serious arrhythmias. The vagus nerve, through its muscarinic action, exerts an indirect effect on cardiac vulnerability, the consequence of annulment of concomitant adrenergic influence, rather than of any direct cholinergic action on the ventricles. There exist anatomic, physiologic as well as molecular bases for such interactions. Available experimental evidence indicates that environmental stresses of diverse types can injure the heart, lower the threshold of cardiac vulnerability to ventricular fibrillation and, in the animal with coronary occlusion, provoke potentially malignant ventricular arrhythmias. Available evidence indicates that in man, as in the experimental animal, administration of catecholamines can induce ventricular arrhythmia, whereas vagal activity exerts an opposite effect. Furthermore, in certain subjects diverse stresses and various psychologic states provoke ventricular ectopic activity.

Effect of atropine and acetylcholine on nerve stimulated output of noradrenaline and dopamine-beta-hydroxylase from isolated rabbit and guinea pig hearts

The role of a cholinergic muscarinic inhibitory mechanism in sympathetic neurotransmission was investigated in isolated rabbit and guinea pig hearts with intact sympathetic nerves. The effect of varying frequencies of stimulation (2.5, 5 and 10 Hz) on the concentration of noradrenaline (NA) and dopamine-beta-hydroxylase (DBH) released into the perfusate was investigated. Stimulation in the presence of atropine sulfate (3.4 micrometer) resulted in an augmented outflow of NA at all three frequencies while DBH outflow was significantly incrreased only at 5 and 10 Hz. d-Tubocurarine (2.0 micrometer) attenuated the augmenting effect of atropine on NA release at all frequencies of stimulation whereas it negated the significant effect of atropine on DBH release. Nerve stimulation in the presence of acetylcholine (0.55 micrometer) resulted in a significant decrease in the concentrations of NA and DBH in the perfusate. It is suggested that atropine augments NA outflow in part by blocking an "intrinsic" muscarinic inhibitory mechanism. Acetylcholine's inhibitory effect on NA release is reflected in a similar decrease in DBH release and, therefore, may function in vivo via an effect on exocytosis at the adrenergic nerve ending.

Studies on the possible contribution of a peripheral presynaptic action of clonidine and dopamine to their vascular effects under in vivo conditions

The functional consequences of drug-induced stimulation under in vivo conditions of alpha-adrenoceptors and dopamine receptors at vascular adrenergic nerve endings (presynaptic receptors) was studied in the autoperfused hindquarters or hindlegs of cats anaesthetized with urethane. The changes in perfusion pressure in response to electrical stimulation of the lumbar sympathetic chain were taken as a measure of noradrenaline release from the vascular adrenergic nerves. Presynaptic inhibitory alpha-adrenoceptors and dopamine receptors were activated by clonidine and dopamine, respectively. According to in vitro experiments these two drugs are more potent stimulants of peripheral presynaptic than postsynaptic receptors. The lowest frequency of stimulation of the lumbar sympathetic chain which yielded a reproducible pressor response was 4 HZ for the autoperfused hindquarters and 1 HZ for the hindlegs; Clonidine was tested over a wide dose range (1-100 mug/kg i;v). A reduction of the stimulation-induced pressor response in the autoperfused hindquarters or hindlegs was observed only after the rather high dose of 100 mug/kg of clonidine. The inhibition was marked at low frequencies of stimulation (1-4 HZ) and weak or absent at high frequencies (16 and 32 HZ). The dose of clonidine (100 mug/kg) which proved to be effective at presynaptic receptors produced a transient increase in blood pressure and in perfusion pressure of the hindquarters and hindlegs and virtually abolished spontaneous sympathetic nervous activity. In spinal cats, the clonidine-induced increases in blood pressure and perfusion pressure were very pronounced and of rather long duration. Thus, under in vivo conditions clonidine showed no selectivity for presynaptic alpha-adrenoceptors in a blood-perfused vascular bed, and its presynaptic action was negligible as compared to its powerful central sympatho-inhibitory effect. Dopamine was constantly infused into the auto-perfused hindquarters or hindlegs at increasing rates until a vasoconstriction due to stimulation of vascular (postsynaptic) alpha-adrenoceptors occurred. The monoamine did not inhibit the stimulation-induced increases in perfusion pressure of the autoperfused hindquarters or hindlegs and, thus, an effect on presynaptic receptors was not found. The results underscore the importance of in vivo experiments for assessing the therapeutic significance of drug-induced stimulation of presynaptic receptors.

The effects of methacholine and calcium deprivation on the release of the false transmitter, alpha-methyladrenaline, from the isolated rabbit heart

1. Anaesthetized rabbits were infused for 20 min with 85 mug-kg-1-min-1(+/-)-alpha-methyladrenaline. The hearts dissected 15 min after the infusion contained 1.49 mug/g alpha-methyladrenaline; the endogenous noradrenaline content was correspondingly decreased. 2. Hearts from alpha-methyladrenaline-infused animals were isolated with the right sympathetic nerves intact and perfused. Ventricular rate, right atrial and right ventricular tensions were recorded using the transverse method. 3. Electrical stimulation (10 Hz, 1 ms, 1 min) of sympathetic nerves, perfusion with the nicotinic drug, p-aminophenethyltrimethylammonium (PAPETA) or perfusion with 54 mM KCl (high K+) solution evoked an output of both alpha-methyladrenaline and noradrenaline. The ratio of the amines in the perfusates was similar to that found in the hearts after termination of the experiments or in non-perfused hearts. 4. Methacholine perfused before and during sympathetic nerve stimulation, PAPETA or high K+ inhibited the release of both false transmitter and noradrenaline. These effects were reversed by atropine. Similarly, lowering the calcium chloride concentration of the medium from 1.8 to 0.1 mM decreased amine outputs. This was reversed by washing. 5. Tyramine evoked a preferential release of the false transmitter that was not altered by methacholine or calcium deprivation. 6. These experiments whow that the muscarinic inhibition of neuronal noradrenaline release and the requirement of calcium ions for its liberation by depolarizing stimuli can be extended to a false transmitter amine. It is suggested that the proportion of alpha-methyladrenaline to noradrenaline occurring in the perfusate during administration of tyramine reflects the relative concentrations of the amines in the axoplasm.

Cardiac norepinephrine output during carotid body stimulation

In a series of 7 dogs, selective stimulation of the carotid body receptors by hypoxic blood produces an increase of coronary flow and greater release of norepinephrine from the heart; the increase of coronary flow is less marked and the release of norepinephrine is increased after vagotomy. Myocardial norepinephrine content is decreased by carotid body stimulation.

Effect of vagal stimulation on the overflow of norepinephrine into the coronary sinus during cardiac sympathetic nerve stimulation in the dog

In anesthetized dogs with the chest open, supramaximal stimulation of the left cardiac sympathetic nerves at 2 and 4 Hz produced an increase of 40-50% in ventricular contractile force (CF) and of 40-65% in coronary sinus blood flow. At these frequencies of stimulation, norepinephrine (NE) overflow into the coronary sinus was 29.8 +/- 5.1 (SE) and 54.9 +/- 13.2 ng/min, respectively. Concurrent, supramaximal vagal stimulation, at a frequency of 15 Hz, had no significant effect on coronary sinus blood flow, but caused a 25% reduction in CF and a 30% decrease in NE overflow. The changes in CF and NE overflow evoked by vagal stimulation were prevented by atropine. These results are consistent with the hypothesis that there are muscarinic receptors on the postganglionic sympathetic terminals in the walls of the ventricles. Acetylcholine released during vagal stimulation combines with these receptors, causes a reduction in the liberation of NE, and thereby attenuates the positive inotropic response.

The effect of vagus nerve stimulation upon vulnerability of the canine ventricle: role of sympathetic-parasympathetic interactions

The effect of vagus nerve stimulation (VNS) upon ventricular vulnerability was studied in 30 mongrel dogs subjected to varying levels of adrenergic stimulation. Vulnerability was assessed both by determining the minimum current required to produce ventricular fibrillation (VF threshold) and by plotting VF threshold throughout the vulnerable period (VF zone). Chloralose-anesthetized animals were studied by means of sequential pulses applied to the apex of the right ventricular endocardium. Testing was carried out in closed-chest dogs, in open-chest dogs with and without left stellate ganglion stimulation (LSGS), and in open- and closed-chest dogs pretreated with propranolol. In the absence of adrenergic stimulation. VNS was without significant effect on either the VF threshold or the VF zone under closed- or open-chest conditions. During LSGS, however, VNS was associated with a 93 +/- 22% (mean +/- SE) increase in VF threshold (P less than 0.01) and constriction of the VF zone. Vagus nerve stimulation combined with LSGS raised VF threshold to the control value, but not beyond. After beta-adrenergic blockade with propranolol, VNS was without effect on VF threshold in either open- or closed-chest animals. It is concluded that augmented sympathetic tone is a precondition for a VNS-induced elevation in VF threshold. The vagal effect is indirect and is expressed by opposing the effects of heightened adrenergic tone on ventricular vulnerability.

The influence of cholinoceptor activity on the mitotic rate in the crypts of Lieberkühn in rat jejunum

1. The influence of cholinoceptor stimulation and cholinoceptor blockade on the mitotic rate in epithelial cells lining the crypts of Lieberkühn in rat jejunum was studied. 2. Cholinoceptor stimulation, either by injection of carbachol or by inhibition of acetylcholinesterase enzymes resulted in an increase in the mitotic rate. This increase in mitotic rate was blocked by tubocurarine, but not by atropine or hexamethonium. Atropine by itself increased the mitotic rate whereas tubocurarine alone or hexamethonium along decreased the mitotic rate. 3. The influence of atropine on crypt cell proliferation appears to involve an adrenergic mechanism since it is blocked by phentolamine.

Muscarinic inhibition of potassium-induced noradrenaline release and its dependence on the calcium concentration

1. Noradrenaline release from the isolated rabbit heart was evoked by perfusion with a medium containing 135 mM potassium and 17 mM sodium ions (high K+-low Na+). 2. The noradrenaline output in response to high K+-low Na+ was dose-dependently decreased by methacholine (0.625-320 muM) and this effect was reserved by atropine 1.44 mM. 3. Lowering the calcium concentration of high K+-low Na+ from 1.8-0.1125 mM decreased the noradrenaline output by 85%. The effect of methacholine, expressed as % inhibition of noradrenaline release, was potentiated by lowering of the calcium concentration. 4. Both at normal and lowered calcium concentrations the inhibitory action of methacholine was larger from 0-5 than from 5-10 min after perfusion with high K+-low Na+. 5. Perfusion of hearts with media containing high K+-low Na+ or normal K+-low Na+ caused noradrenaline outputs somewhat smaller than those after high K+-low Na+. The release from 0-5 min was both calcium-dependent and inhibited by methacholine. 6. High K+ and/or low Na+ solutions caused an increase in coronary perfusion pressure which was little affected by the noradrenaline released simultaneously. 7. It is concluded that activation of muscarine receptors at the terminal adrenergic fibre decreases the availability of calcium for transmitter release.

Prejunctional effects of a purified toxin from the scorpion Tityus serrulatus: release of 3H-noradrenaline and enhancement of transmitter overflow elicited by nerve stimulation

The effects of a purified fraction of the venom of the Brazilian scorpion, Tityus serrulatus, were studied in isolated guinea-pig atria previously labelled with 3H-noradrenaline. Exposure to 0.3 and 1.0 mug/ml of the scorpion toxin resulted in a long lasting positive chronotropic effect which was concentration-dependent. The increase in atrial rate coincided with an enhancement in spontaneous outflow of radioactivity. The increase in outflow of radioactive products elicited by exposure to 1.0 mug/ml of the scorpion toxin was approximately 3-fold. 3H-noradrenaline accounted for 60% of the total increase in outflow of radioactivity elicited by the scorpion toxin and the 3H-deaminated glycol (3,4-dihydroxyphenyl-glycol) represented the main metabolite formed, accounting for approximately 35% of the total release. 20 min after exposure to 1.0 mug/ml of the scorpion toxin the overflow of the labelled transmitter elicited by accelerans nerve stimulation (4 Hz, during 60 sec. supramaximal voltage) was increased 8-fold. This effect of the scorpion toxin appears to be unrelated to inhibition of neuronal uptake, block of alpha-adrenoceptors or stimulation of beta-adrenoceptors. Consequently, in addition to releasing noradrenaline, the scorpion toxin enhances transmitter overflow elicited by nerve stimulation through a prejunctional effect which appears to reflect a nove mechanism of action.

Effect of calcium on the relationship between frequency of stimulation and release of noradrenaline from the perfused spleen of the cat

The sympathetic nerves of the cat spleen were stimulated electrically (20 v, 1 msec, 7 sec) with increasing frequencies (1-30 Hz) during perfusion of the spleen with normal or low-calcium Krebs solution. Noradrenaline (NA) output per stimulus was measured. In spleens perfused with normal Krebs solution, the NA output per stimulus at 5Hz was barely detectable, while the output at 30 Hz was about 5-fold greater than at 5 Hz. Reduction of the calcium concentration in the Krebs solution to 0.2-0.5 mM, while reducing outputs at both frequencies, resulted in an output at 30 Hz that was nearly 8 to 10 times that at 5 Hz. In spleens pretreated with phenoxybenzamine (PBZ) the NA output was increased at all frequencies, and the maximal output occurred at 5 Hz, whereas the output at 30 Hz was only one-third of the maximal output. Reduction of the calcium concentration of the perfusion solution to 0.5 mM, while reducing outputs from PBZ-treated spleens, increased the output at 30 Hz by about 5-fold over the output at 5 Hz. The muscarinic inhibition of stimulation-evoked release of NA by acetylcholine was much more pronounced at low than at high frequency of stimulation. The increase in the release of NA per stimulus with increase in the frequency of sympathetic nerve stimulation is explained on the assumption that the specific entry of calcium ions required for transmitter release is also frequency-dependent.

Inhibition by acetylcholine of adrenergic neurotransmission in vascular smooth muscle

Changes in the isometric tension of isolated strips of cutaneous, femoral, mesenteric, pulmonary, and muscle arteries and veins were recorded at 37°C in an organ bath. Acetylcholine (5 x 10 -8 and 10 -7 g/ml) caused relaxation of strips from the saphenous veins, the femoral veins, and all of the arteries after contraction by norepinephrine released from nerve terminals by electrical stimulation (2-5 Hz); in the pulmonary and mesenteric veins, acetylcholine caused a further increase in tension. Pulmonary artery and mesenteric vein strips were incubated with [ 3 H] norepinephrine and mounted for superfusion (3 ml/min) and isometric tension recording. Electrical stimulation increased the tension and the total radioactivity released in both preparations. Acetylcholine (2 x 10 -7 g/ml) depressed the contractions of the pulmonary artery strips but augmented those of the mesenteric vein strips; it diminished the efflux of radioactivity in both, indicating that acetylcholine inhibits adrenergic neurotransmission. In the absence of sympathetic stimulation, acetylcholine (5 x 10 -10 -10 -5 g/ml) caused all vein strips to contract; the most common reaction in artery strips was a slight relaxation (at 10 -9 -10 -8 g/ml) followed by a contraction (at 5 x 10 -8 -10 -5 g/ml). During contractions caused by norepinephrine, acetylcholine caused a further increase in tension in vein strips but a relaxation in artery strips. Atropine abolished the effects of acetylcholine. The results of this study suggest the presence in vascular smooth muscle of both excitatory and inhibitory cholinergic receptors.

Effects of intracoronary infusions of acetylcholine and nicotine on the dog heart in vivo

1. In anaesthetized dogs intracoronary infusions of high doses of nicotine and acetylcholine increased myocardial contractile force and this could be prevented by pre-treatment with desmethylimipramine or phenoxybenzamine.2. The inotropic effect of nicotine was brief and subsided during the continuing infusion of the drug. The infusion of nicotine did not reduce the inotropic effects of cardiac sympathetic nerve stimulation.3. The motropic effect of intracoronary acetylcholine often fluctuated during prolonged infusions and was not altered by pretreatment with atropine. Acetylcholine infusions reduced the inotropic responses produced by cardiac sympathetic nerve stimulation and led to a substantial transient reduction in the associated pressor responses. Intracoronary acetylcholine also reduced the pressor and inotropic effect of intravenous noradrenaline. The attenuation of these adrenergic cardiovascular responses by acetylcholine was prevented by atropine.

An experimental analysis of the tachycardia that follows vagal stimulation

1. Postvagal tachycardia, the transient increase in heart rate that follows the sinus bradycardia elicited by vagal stimulation, was investigated in thirty chloralosed cats. Maximum postvagal tachycardia was elicited by stimulation at frequencies of 20-60 Hz with train durations of 30-90 sec. A positive correlation was demonstrated between the magnitudes of postvagal tachycardia and of the preceding sinus bradycardia.2. Postvagal tachycardia was not affected by either spinal transection at C7 or by administration of propranolol (1.5 mg/kg I.V.), but was abolished by the administration of atropine (2.0 mg/kg I.V.).3. Postvagal tachycardia was observed to follow the vagal bradycardia induced reflexly either by administration of phenyldiguanide (100-300 mug I.V.) or by stimulation of the aortic depressor nerve.4. In the isolated atria-vagus preparations from six rabbits a positive correlation was demonstrated between the magnitude of postvagal tachycardia and of the preceding bradycardia elicited by vagal stimulation.5. Continuous intracellular recordings were obtained from four sinuatrial node pace-maker cells in the isolated atria-vagus preparation of the rabbit before, during and after vagal stimulation. During postvagal tachycardia the slope of the diastolic prepotential, the maximum diastolic potential, threshold potential and the overshoot were found to be increased; these changes are different from those found in pace-maker cells during adrenergic activation.6. These findings demonstrate that postvagal tachycardia is not mediated by sympathetic adrenergic mechanisms, but suggest that it is dependent upon the preceding vagal bradycardia and may be related to an increase in net sodium influx into pace-maker cells initiated by the hyperpolarization of the pace-maker cell membrane during and immediately after vagal stimulation.

Effects of several muscarinic agonists on cardiac performance and the release of noradrenaline from sympathetic nerves of the perfused rabbit heart

1. The effects of several muscarinic agonists on atrial tension development, ventricular rate and noradrenaline release from terminal sympathetic fibres evoked by electrical nerve stimulation (SNS) and 1,1-dimethyl-4-phenylpiperazinium (DMPP) were measured in isolated perfused rabbit hearts.2. Hexamethonium, in a concentration which almost abolished the release of noradrenaline by DMPP, had no effect on the release produced by SNS, confirming that the stimulation was postganglionic.3. The order of potency for inhibition of atrial tension development was N-methyl-1,2,5,6, tetrahydro-nicotinic acid prop-2-yne ester (MH-1)>oxotremorine > acetylcholine > methacholine > carbachol > furtrethonium > pilocarpine>4-(m-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium chloride (McN-A-343)>N-benzyl-3-pyrrolidyl acetate methobromide (AHR 602). All effects were abolished by atropine (1.4 x 10(-6)M).4. Each compound was more potent relative to acetylcholine in inhibiting ventricular rate than atrial tension. With the exception of carbachol, the order of potency was the same.5. Both AHR 602 and McN-A-343 facilitated the release of noradrenaline by SNS and inhibited that by DMPP. The effects were atropine-resistant and hence non-muscarinic.6. The muscarinic compounds (except AHR 602 and McN-A-343) each produce atropine-sensitive inhibition of noradrenaline release evoked both by SNS and DMPP although it is likely that furtrethonium and pilocarpine have additional non-muscarinic inhibitory activity against DMPP. The order of potency on both parameters and the potencies relative to acetylcholine were in good agreement with those for inhibition of atrial tension.7. The results suggest that similar muscarinic receptors mediate inhibition of atrial tension development, ventricular rate and neuronal noradrenaline release caused by SNS and DMPP.8. In terms of the two muscarinic sites known to be present in the superior cervical ganglion, the receptors of the terminal fibres mediating inhibition of noradrenaline release are more likely to correspond to those mediating hyperpolarization than to those mediating depolarization, for which AHR 602 and McN-A-343 show specificity.

Effects of McN-A-343 and DMPP on the uptake and release of 3 H-noradrenaline by guinea-pig atria

1. McN-A-343 4-(m-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium chloride and DMPP (N,N-dimethyl-N'-phenylpiperazinium iodide) inhibit the uptake of (+/-)-(3)H-noradrenaline by guinea-pig atria, being approximately as potent as cocaine in this respect.2. The inhibition of uptake produced by McN-A-343 or DMPP was not affected by atropine or hexamethonium in concentrations which antagonized actions on muscarinic and nicotinic receptors respectively.3. McN-A-343 in the presence of atropine had a positive inotropic action on atria, but this was not accompanied by efflux of radioactivity from atria previously incubated with (-)-(3)H-noradrenaline.4. In the presence of McN-A-343, responses of atria to noradrenaline were increased and those to tyramine were decreased.5. DMPP had positive inotropic and chronotropic actions on atria, and these effects were accompanied by an increase in efflux of radioactivity from atria previously incubated with (-)-(3)H-noradrenaline.

Role of the -adrenoceptor in regulating noradrenaline overflow by nerve stimulation

1. A study of the actions of phenoxybenzamine on transmitter overflow, neuronal and extraneuronal uptake of noradrenaline and in causing alpha-adrenoceptor blockade was carried out using the isolated cat nictitating membrane preparation.2. Phenoxybenzamine increased transmitter overflow elicited by nerve-stimulation at 10 Hz in a concentration dependent manner in the range 10(-8) to 10(-5) g/ml.3. Neuronal uptake of [(3)H]-noradrenaline was not inhibited by concentrations lower than 10(-6) g/ml of phenoxybenzamine. With 10(-7) g/ml of phenoxybenzamine a significant increase in transmitter overflow was obtained, although neuronal uptake of noradrenaline was not affected. Higher concentrations of phenoxybenzamine (10(-6) and 10(-5) g/ml) inhibited the neuronal uptake of noradrenaline and further increased transmitter overflow.4. Extraneuronal uptake of [(3)H]-noradrenaline was inhibited only with the highest concentration of phenoxybenzamine tested (10(-5) g/ml) and therefore appears to be unrelated to the effects on transmitter overflow.5. There was a significant correlation between the degree of alpha-adrenoceptor block produced by phenoxybenzamine and the increase in transmitter overflow obtained by nerve stimulation.6. These results indicate that phenoxybenzamine, in addition to increasing overflow by preventing reuptake of noradrenaline, may increase transmitter release.7. The possibility that phenoxybenzamine acts on alpha-adrenoceptors in the adrenergic nerve terminal is discussed. These receptors would be involved in a negative feedback mechanism regulating transmitter release.

Modification of the evoked release of noradrenaline from the perfused cat spleen by various ions and agents

1. Cat spleens were perfused with Krebs-bicarbonate solution at a rate of about 7 ml./min at 33-35 degrees C. Noradrenaline release after splenic nerve stimulation at 10 or 30 Hz was measured. Effects of various ions and drugs on noradrenaline release were determined.2. Perfusion of phenoxybenzamine- and [(3)H]noradrenaline-treated spleens with 1, 2.5 and 5 mM cobalt or nickel-2 Krebs solution markedly reduced the release of noradrenaline by nerve stimulation. Lanthanum was the most potent inhibitor of noradrenaline release. Increasing the calcium concentration or adding tetraethylammonium chloride (TEA) partially counteracted the inhibitory effects of cobalt on release. Cobalt did not inhibit release induced by tyramine.3. Calcium did not cause spontaneous release of noradrenaline either when high concentrations were injected directly into the spleen or after first perfusing the spleen with calcium-free medium.4. Carbachol, protoveratrine and high potassium inhibit, whereas TEA, barium and rubidium enhance, the evoked release of noradrenaline.5. The relation of noradrenaline release to influx of calcium ions and its modification by various agents has been discussed.

Effects of the muscarinic agonist McN-A-343 on responses to sympathetic nerve stimulation in the rabbit ear artery

1. Observations were made on the effects of 4-(m-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium chloride (McN-A-343) on responses of isolated segments of the central artery of the rabbit's ear to sympathetic nerve stimulation and noradrenaline.2. With low frequencies of nerve stimulation (2-5 Hz), McN-A-343 caused a decrease in responses to sympathetic nerve stimulation. This effect of McN-A-343 was abolished by dexamphetamine or atropine. In the presence of atropine, McN-A-343 caused an increase in responses to sympathetic nerve stimulation.3. With high frequencies of nerve stimulation (10-20 Hz), McN-A-343 caused an increase in responses. This effect was not qualitatively changed in the presence of atropine.4. When McN-A-343 had an inhibitory effect on responses to sympathetic nerve stimulation, responses to noradrenaline were unaffected.5. It is suggested that McN-A-343 acts on muscarinic receptors through which noradrenaline release may be inhibited; it may also act on the cholinergic stage in adrenergic transmission postulated by Burn & Rand (1959).