The inhibitory action of noradrenaline and adrenaline on acetylcholine output by guinea-pig ileum longitudinal muscle strip

1. Noradrenaline and adrenaline reduce the output of acetylcholine by the guinea-pig ileum longitudinal strip by up to 80%, both in resting conditions and after stimulation. The effect is graded with dose, and is detectable with noradrenaline 2 x 10(-7) g/ml. Adrenaline is approximately 4 times as active as noradrenaline, and its action after being washed out is more persistent.2. If resting output is high, both amines have a proportionately greater effect and their action, as dosage is increased, is to reduce resting output to a basal level, relatively constant from strip to strip, of about 10 ng/g/min.3. With stimulation, the effect of the amine is greater at low frequencies, when the output per volley is high, than at high frequencies. The effect is reduced by increasing the number of shocks delivered. There thus appears to be a basal output per volley, of the order of 1-2 ng/g/volley, which can be reached either by relatively rapid stimulation, by prolonged stimulation, or by treatment with these amines.4. If noradrenaline is applied during continued stimulation at 40/min, the depression of acetylcholine output during its presence is followed by an augmented output when the drug is withdrawn. The magnitude of this "overshoot" increases with the duration of noradrenaline exposure.5. Phenylephrine 4 mug/ml. and amphetamine 20 mug/ml. reduced the acetylcholine output, but isoprenaline 1 mug/ml., dopamine 1 mug/ml. and methoxamine 10 mug/ml. were ineffective.6. Phenoxybenzamine reduced the resting output and increased the stimulation output. Of the two other blocking agents examined, phentolamine had no effect on either resting or stimulation output and ergotamine transiently reduced stimulation output. The effect of phenoxybenzamine was not due to a reaction with either adrenoceptive or muscarinic receptors.7. Phenoxybenzamine, phentolamine and ergotamine abolished the effect of adrenaline and noradrenaline on both resting output and on output in response to stimulation.8. In strips obtained from animals treated with reserpine and guanethidine, a rise in resting acetylcholine output and in stimulation output at low frequencies was found. In these conditions, noradrenaline was still effective.9. Reducing the hydroxytryptamine content of the strips by treatment with p-chloro-(+/-)-phenylalanine did not significantly affect acetylcholine output.10. It is concluded that acetylcholine output by the nervous networks of the longitudinal strip is under the normal control of the sympathetic by a species of presynaptic inhibition mediated by alpha receptors. This implies that for a tissue under dual autonomic control, withdrawal of sympathetic control will lead to a parasympathetic response which is not only unopposed but also itself enhanced.     

The role of β-adrenoceptors in the responses of the hepatic arterial vascular bed of the dog to phenylephrine, isoprenaline, noradrenaline and adrenaline

1 The sympathetically-innervated hepatic arterial vascular bed of the dog was perfused from a femoral artery. Hepatic arterial blood flow and perfusion pressure were recorded continuously, and the hepatic arterial vascular resistance (HAVR) calculated from these measurements.

2 Intra-arterial injections of phenylephrine caused dose-dependent rises in HAVR, indicating hepatic arterial vasoconstriction, at all doses above threshold. No secondary reductions in HAVR followed these responses.

3 Intra-arterial injections of isoprenaline caused only dose-dependent reductions in HAVR at doses above threshold.

4 Intra-arterial injections of noradrenaline typically caused an initial increase in HAVR which was followed at all but the highest doses by a secondary, delayed, reduction in HAVR.

5 Intra-arterial injections of adrenaline, like those of noradrenaline, resulted in hepatic arterial vasoconstriction followed by hepatic arterial vasodilatation.

6 On a molar basis, the most potent hepatic arterial vasoconstrictor was noradrenaline, followed by adrenaline and phenylephrine.

7 The maximum reductions in HAVR caused by adrenaline (mean reduction = 21.9%) and noradrenaline (16.9%) were significantly smaller than those due to isoprenaline (_P < 0.001).

8 Propranolol attenuated the hepatic arterial vasodilator responses due to isoprenaline, and the secondary falls in HAVR following intra-arterial adrenaline and noradrenaline.

9 Propranolol did not modify the vasoconstrictor responses to phenylephrine.

10 Both adrenaline and noradrenaline were more potent hepatic arterial vasoconstrictors after propranolol than in the absence of β-adrenoceptor blockade. The potentiation of the vasoconstrictor effects of adrenaline was statistically significant.

11 After propranolol, adrenaline was a more potent hepatic arterial vasoconstrictor than noradrenaline.

12 Since the β-adrenoceptors in the hepatic arterial vasculature were not blocked by atenolol, but were stimulated by salbutamol, it is concluded that they are predominantly of the β₂-type.

13 The vasoconstrictor actions of phenylephrine, noradrenaline and adrenaline were all antagonized by the systemic administration of phentolamine, all three dose-response curves being shifted to the right.

14 The results are discussed with regard to the possible control of the hepatic arterial vasculature by naturally-occurring catecholamines.

     

Pharmacological characterization of the presynaptic α-adrenoceptors regulating cholinergic activity in the guinea-pig ileum

1 The presynaptic α-adrenoceptors located on the terminals of the cholinergic nerves of the guineapig myenteric plexus have been characterized according to their sensitivities to α-adrenoceptor agonists and antagonists.     

α- and β-Adrenoceptors in the detrusor muscle and bladder base of the pig and β-adrenoceptors in the detrusor muscle of man

1 The presence and type of adrenoceptors in the smooth muscle of the pig and human urinary bladder was assessed on the basis of the relative potency of α- and β-adrenoceptor agonists and antagonists.

2 In isolated, carbachol-contracted bladder strips from the pig detrusor muscle the relaxing potency of isoprenaline was four times that of salbutamol and ritodrine and thirty times that of noradrenaline.

3 Propranolol caused a parallel shift to the right of the noradrenaline dose-response curve which was not changed by phentolamine.

4 Propranolol and butoxamine showed, in contrast to practolol, a dose-dependent antagonism of the response to isoprenaline. A pA₂ value of 9.2 ± 0.2 and 6.8 ± 0.2 (mean ± s.e. mean) for the first two antagonists was calculated.

5 In the bladder base of the pig, propranolol caused a parallel shift to the right and phentolamine a shift to the left of the dose-response curve to noradrenaline.

6 In the human detrusor muscle the potency and maximum effect of isoprenaline and salbutamol were less than those in the pig detrusor muscle. The potency of isoprenaline was sixty times that of salbutamol.

7 Whereas a parallel shift to the right of the dose-response curve to isoprenaline was obtained with propranolol, no antagonism was obtained with butoxamine or practolol.

8 The results are interpreted as indicating the presence of β₂-adrenoceptors in the detrusor muscle of the pig and β-adrenoceptors with neither β₁- nor β₂-characteristics in the detrusor muscle of man. An indication of the presence of α-adrenoceptors in the bladder base but not in the detrusor muscle of the pig was obtained.

     

The effect of adrenaline, noradrenaline and isoprenaline on the guinea-pig uterus

The actions of adrenaline, noradrenaline and isoprenaline were compared on the isolated guinea-pig uterus. In uteri from immature animals (200 to 350 g) adrenaline caused relaxation which changed to a biphasic effect and finally to a contraction in the course of an experiment (6 to 8 hr). Noradrenaline always caused contraction and isoprenaline relaxation. In uteri from oestrogen-treated animals adrenaline and noradrenaline caused contraction and isoprenaline caused relaxation. Isoprenaline potentiated the contraction produced by adrenaline and reversed the adrenaline relaxation to a contraction. The change of the pharmacological action of adrenaline was not related to the Na⁺ and K⁺ content of the uterus, which remained constant throughout an experiment involving repeated application of the amines. Nor could it be related to a change in the glycogenolytic effect of adrenaline estimated by determinations of total glycogen of the muscle which, however, may not reflect a momentary change in rate of breakdown.     

The presence of β-adrenoceptors in the guinea-pig seminal vesicle

1. The preparation of longitudinal smooth muscle strips from guinea-pig seminal vesicles is described.2. Isoprenaline and salbutamol inhibited contractions produced by parasympathomimetic agents.3. The inhibitory action of isoprenaline was blocked by low concentrations of propranolol and butoxamine. It was concluded that beta(2)-adrenoceptors were present in the tissue.4. The inhibitory action of isoprenaline was not apparent when adrenaline, noradrenaline, hypogastric nerve or transmural stimulation were used to contract the tissue.     

Distribution of α- and β-adrenoceptors in human urinary bladder

1 The distribution of alpha- and beta-adrenoceptors in isolated preparations of human bladder neck and detrusor muscle has been studied.2 Adrenaline caused contraction of the bladder neck which was blocked by phenoxybenzamine but unaltered by propranolol.3 Isoprenaline caused relaxation of the bladder neck which was blocked by propranolol. High concentrations caused contraction which was enhanced by propranolol but blocked by phenoxybenzamine.4 Detrusor muscle was relaxed by isoprenaline and this effect was blocked by propranolol. Phenylephrine caused relaxation of detrusor which was unaffected by phenoxybenzamine; in some cases contraction was produced in the presence of propranolol.5 It is concluded that the bladder neck contains mainly alpha-receptors and the detrusor mainly beta-receptors but both regions posses both types of adrenoceptor.     

Actions of phenylephrine on β-adrenoceptors in guinea-pig trachea

1. Phenylephrine produced relaxation of the isolated guinea-pig tracheal chain preparation, its potency being 1/5 that of noradrenaline on normal tissues.2. The potentiation of phenylephrine by cocaine (10(-5)M) was only slight. Thus on cocaine-treated tissues phenylephrine was 1/45 as potent as noradrenaline.3. The dose-response lines to phenylephrine were shifted in a parallel manner by propranolol 10(-8)M and 10(-7)M, suggesting that the relaxations were mediated through beta-adrenoceptors.4. Phenylephrine had a lower intrinsic activity than the catecholamines and produced multiphasic dose-response lines at the higher doses used in the presence of propranolol (10(-6)M). These observations have been explained by the evidence obtained that phenylephrine is a partial agonist with beta-adrenoceptor blocking activity.5. From experiments using alpha-adrenoceptor blocking drugs, it has been concluded that stimulation of alpha-adrenoceptors has little influence on the beta-adrenoceptor relaxation to phenylephrine on the guinea-pig tracheal chain preparation.     

Inhibition of rabbit intestine mediated by α- and β-adrenoceptors

1. The effects of some alpha- and beta-adrenoceptor agonists and antagonists were studied on isolated segments of rabbit intestine in an attempt to characterize the two types of inhibitory response produced by sympathomimetic amines.2. Phenylephrine, an alpha-adrenoceptor agonist, produced an inhibition of rapid onset, from which recovery occurred despite the continued presence of the drug. On washout there was an overshoot in contraction height. Isoprenaline, a beta-adrenoceptor agonist, produced an inhibition of slow onset which was maintained throughout the presence of the drug and there was no overshoot on washout.3. Adrenaline resembled phenylephrine more closely than it resembled isoprenaline, in that it showed more affinity for alpha-adrenoceptors, whereas noradrenaline, and the transmitter released on periarterial nerve stimulation, behaved more like isoprenaline, although both types of receptor were affected.4. Adenosine-5'-triphosphate produced an inhibition resembling that produced by an alpha-adrenoceptor agonist, whereas the dibutyryl analogue of cyclic adenosine 3',5'-monophosphate (cyclic 3',5'-AMP) produced an inhibition resembling that produced by a beta-adrenoceptor agonist.5. In critical concentrations theophylline augmented and imidazole inhibited beta-adrenoceptor mediated responses, as well as responses to dibutyryl cyclic AMP. However, additional actions of theophylline and imidazole were also demonstrated.6. Responses mediated by alpha-adrenoceptors, but not those mediated by beta-adrenoceptors, were blocked by membrane stabilizers, quinidine being the most potent of those studied.7. The results are discussed in relation to the possible mechanisms of action of alpha- and beta-adrenoceptor agonists.     

Temperature modulation of α- and β-adrenoceptors in the isolated frog heart

1. The effects of adrenaline on the isolated frog's heart at 27 degrees C are not antagonized by phentolamine (1.5 x 10(-6)M) but are abolished at 7 degrees C.2. At 27 degrees C isoprenaline was more potent than noradrenaline, but at 7 degrees C noradrenaline was more potent than isoprenaline.3. Phenoxybenzamine (1.5 x 10(-5)M) or dibenamine (1.5 x 10(-5)M) at 7 degrees C abolished the work output induced by adrenaline. When the temperature was raised to 24 degrees C, adrenaline caused an increase in work output.4. It is concluded that in the isolated frog heart there are at least two pools of adrenoceptors, the availability of which can be governed by temperature.     

Effect of cocaine on the affinity of α-adrenoceptors for noradrenaline

1. Doses of cocaine which cause specific or unspecific supersensitivity in cat spleen did not alter the blocking effect of phenoxybenzamine on the responses of isolated cat spleen strips to noradrenaline.2. The same doses of cocaine did not increase the protection of alpha-adrenoceptors given by noradrenaline during a standard exposure to phenoxybenzamine.3. It is concluded that cocaine does not change the affinity of the alpha-adrenoceptor for noradrenaline, and therefore changes in affinity are not responsible for the potentiating action of cocaine.     

Distribution and types of adrenoceptors in the guinea-pig ileum: the action of α-and β-adrenoceptor agonists

1 Segments of guinea-pig ileum and the myenteric plexus-longitudinal smooth muscle preparation were used for a study of the actions of adrenaline, noradrenaline, isoprenaline, ephedrine and phenylephrine on the responses of coaxially stimulated ileum at different distances from the ileocaecal valve.2 The responses of the ileum to electrical stimulation were suppressed by adrenaline, nonadrenaline and ephedrine, while phenylephrine and isoprenaline inhibited them only partially.3 The twitch inhibition elicited by these adrenoceptor agonists was the same at all distances from the ileocaecal valve. There was no significant difference between their cumulative and non-cumulative concentration-response curves.4 Smooth muscle relaxation was induced only by isoprenaline and contraction only by phenylephrine at all distances from the ileocaecal junction. Adrenaline and noradrenaline evoked smooth muscle contraction in the terminal (0 to 20 cm), a concentration-dependent, biphasic response in the intermediate part (21 to 50 cm) and a relaxation in the proximal ileum (> 50 cm from the ilecocaecal valve). Ephedrine did not change significantly the smooth muscle tension in the terminal and the intermediate segments and induced smooth muscle relaxation in the proximal ones.5 Ouabain and a potassium-free solution did not appear to influence the prejunctional action of noradrenaline nor the amplitude of smooth muscle relaxation in the proximal ileum, whereas the concentration-contractor response curves were significantly depressed and shifted to the right by ouabain and in a potassium-free solution.6 The brief initial (phasic) contraction induced by acetylcholine was not influenced during the sustained increase or decrease in tension induced by catecholamines. On the contrary, the stimulatory catecholamine actions disappeared or were changed to smooth muscle relaxation by acetylcholine pretreatment. Potassium chloride pretreatment did not change the character of the adrenoceptor agonist action of the agonists studied.7 Since there is a similar prejunctional action at all distances from the ileocaecal valve and a different postjunctional effect of the adrenoceptor agonists at different distances from the ileocaecal junction, it could be suggested that in the guinea-pig ileum there are at least two alpha-adrenoceptors (inhibitory prejunctional-alpha(2), stimulatory postjunctional-alpha(1)), an inhibitory postjunctional beta-adrenoceptor and an as yet uncharacterized inhibitory postjunctional receptor.8 Based on the specific postjunctional action of phenylephrine and the prejunctional action of ephedrine in the guinea-pig ileum, these drugs could be used with success as ;specific' alpha(1)- and alpha(2)-adrenoceptor stimulants.     

Pharmacology of AH 5158; a drug which blocks both α- and β-adrenoceptors

1. AH 5158 differs from conventional adrenoceptor blocking drugs in producing competitive blockade of both alpha- and beta-adrenoceptors.2. AH 5158 is 5-18 times less potent than propranolol in blocking beta-adrenoceptors. It resembles propranolol in its non-selective blockade of beta(1)-cardiac and beta(2)-vascular and tracheal adrenoceptors and in its lack of intrinsic sympathomimetic activity.3. AH 5158 is 2-7 times less potent than phentolamine in blocking alpha-adrenoceptors. AH 5158 itself is more active on beta- than alpha-adrenoceptors.4. Blockade of noradrenaline vasopressor responses by AH 5158 in anaesthetized dogs was dose-dependent up to 1 mg/kg but no further blockade was obtained with larger doses of AH 5158. ;Self-limiting' blockade was not observed in dogs pretreated with cocaine, or in untreated dogs if the vasopressor agent was oxymetazoline instead of noradrenaline. A possible cause of ;self-limiting' blockade is discussed.5. In doses higher than those required for either alpha- or beta-adrenoceptor blockade, AH 5158 produced effects on cardiac muscle that are attributable to membrane-stabilizing activity. This was manifested as a negative inotropic action in spinal dogs and in guinea-pig left atrial strips, as a negative chronotropic action in syrosingopine pre-treated dogs, and as an increase in the effective refractory period of guinea-pig left atrial strips. AH 5158 was 3-11 times less potent than propranolol in these tests.6. In open chest dogs AH 5158 resembled propranolol in reducing cardiac output, rate and contractility, effects which are attributable to beta-adrenoceptor blockade. The drug differed from propranolol in decreasing rather than increasing total peripheral resistance and in causing larger decreases in arterial blood pressure at equipotent beta-adrenoceptor blocking doses. These differences are attributable to the alpha-adrenoceptor blocking actions of AH 5158.7. In anaesthetized dogs, intravenously administered AH 5158 antagonized both catecholamine and ouabain-induced arrhythmias. Orally administered AH 5158 lowered systolic arterial pressure in conscious renal hypertensive dogs.8. These results show AH 5158 to possess a novel profile of activity. Possible uses of the drug in cardiovascular disorders such as hypertension, angina pectoris and cardiac arrhythmias are discussed.     

Central α- and β-adrenoceptors modifying arterial blood pressure and heart rate in conscious cats

1 In conscious unrestrained cats noradrenaline, alpha-methylnoradrenaline and clonidine, infused into the lateral cerebral ventricles (i.c.v.) caused dose-related falls in blood pressure and heart rate; both effects were abolished after i.c.v. phentolamine.2 In 12 out of 20 cats, i.c.v. isoprenaline and salbutamol when given caused dose-related pressor responses and tachycardias. These effects were abolished after i.c.v. beta-adrenoceptor blocking drugs but were unaffected by alpha-adrenoceptor blocking agents.3 In 5 out of 20 cats, i.c.v. isoprenaline regularly produced dose-related falls in blood pressure with associated tachycardias; both effects were abolished after i.c.v. beta-adrenoceptor blocking agents.4 Intracerebroventricular dopamine produced cardiovascular responses which were qualitatively similar to those produced by i.c.v. isoprenaline.5 Intracerebroventricular adrenaline produced complex responses in untreated animals but typical alpha-effects were obtained after prior i.c.v. treatment with a beta-adrenoceptor blocking agent and typical beta-effects after i.c.v. pretreatment with an alpha-adrenoceptor blocking agent.6 The cardiovascular changes produced by i.c.v. beta-adrenoceptor agonists were abolished after systemic administration of hexamethonium or bethanidine.7 The results are discussed in the light of the mode of action of beta-adrenoceptor stimulants and beta-adrenoceptor blocking agents in the treatment of hypertension.     

Characterization of α-adrenoceptors in the myocardium

1. In the driven rabbit left atrium alpha-adrenoceptors mediate the inotropic effect of phenylephrine and the prolongation of functional refractory period by sympathomimetic amines.2. These receptors are highly sensitive to blockade by phentolamine and phenoxybenzamine.3. Prolongation of functional refractory period is greater with the secondary amines, phenylephrine, adrenaline and epinine, than with the primary amines, norphenylephrine, noradrenaline and dopamine.     

Comparison of pre-junctional α-adrenoceptors at the neuromuscular junction with vascular post-junctional α-receptors in cat skeletal muscle

1 Activation of pre-junctional α-adrenoceptors at the skeletal neuromuscular junction enhances acetylcholine release whereas activation of such receptors at autonomic nerve endings inhibits transmitter output. In the present study the characteristics of pre-junctional α-adrenoceptors at motor nerve terminals have been compared with post-junctional (vascular) α-adrenoceptors in the cat hind limb.

2 Reversal of partial (+)-tubocurarine blockade of contractions of the tibialis anterior muscle was used to monitor pre-junctional activity and increases in hindlimb vascular resistance to assess post-junctional actions at α-adrenoceptors.

3 Responses to intra-arterial injections of noradrenaline, adrenaline, phenylephrine, oxymetazoline, methoxamine and clonidine were monitored. Dose-response lines for all the compounds except clonidine were parallel. The latter agent produced only weak and inconsistent effects.

4 Ratios of the doses of the agents required to produce pre- and post-junctional effects indicated that oxymetazoline and adrenaline possessed some preferential activity at post-junctional sites, whereas the remaining agents were non-selective in their actions. If dose-ratios with respect to noradrenaline were compared at the two sites none of the compounds possessed a marked degree of selectivity.

5 In the presence of phentolamine or tolazoline, dose-response curves to the pre- and post-junctional effects of phenylephrine were shifted to a similar extent. Thymoxamine showed preferential activity as a pre-junctional α-receptor antagonist.

6 In comparing the results of this study with those of other authors, it is apparent that there are marked differences in the characteristics of pre-junctional α-receptors at the skeletal neuromuscular junction and at autonomic nerve endings. The pre- and post-junctional α-receptors in skeletal muscle show less divergence.

     

Reduced α1- and β2-adrenoceptor-mediated positive inotropic effects in human end-stage heart failure

1. alpha 1-Adrenoceptor (phenylephrine in the presence of propranolol) and beta 2-adrenoceptor (fenoterol)-mediated positive inotropic effects were investigated in human ventricular preparations isolated from five non-failing (prospective organ donors) and from eight explanted failing hearts with end-stage idiopathic dilative cardiomyopathy (NYHA IV). 2. For comparison, the nonselective beta-adrenoceptor agonist isoprenaline, the phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), the cardiac glycoside dihydroouabain, and calcium were studied. 3. Furthermore, the influence of IBMX on adenosine 3':5'-cyclic monophosphate (cyclic AMP) PDE activity as well as total beta-adrenoceptor density, beta 1- and beta 2-adrenoceptor subtype distribution, and alpha 1-adrenoceptor density were compared in nonfailing and failing human heart preparations. The radioligands (-)-[125I]-iodocyanopindolol for beta-adrenoceptor binding and [3H]-prazosin for alpha 1-adrenoceptor binding were used. 4. The inotropic responses to calcium and dihydroouabain in failing human hearts were unchanged, whereas the maximal alpha 1- and beta 2-adrenoceptor-mediated positive inotropic effects were greatly reduced. The inotropic effects of the other cyclic AMP increasing compounds, i.e. isoprenaline and IBMX, were also reduced to about 60% of the effects observed in nonfailing controls. The potency of these compounds was decreased by factors 4-10. 5. The basal PDE activity and the PDE inhibition by IBMX were similar in nonfailing and failing preparations. 6. The total beta-adrenoceptor density in nonfailing hearts was about 70 fmol mg-1 protein. In failing hearts the total number of beta-adrenoceptors was markedly reduced by about 60%.(ABSTRACT TRUNCATED AT 250 WORDS)