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At least 19 recordsLinked to original sources
PubMed1975

The influence of temperature increase, elevation of extracellular h+-concentration, and of triiodothyronine on the actions of phenylephrine, histamine, and beta-sympathomimetic drugs on rabbit aortic strips.

In the isolated preparation from the rabbit thoracic aorta, the affinities of the vasoconstrictor agents phenylephrine and histamine, as well as of the vasodilator beta-sympathomimetic drugs isoprenaline, fenoterol (TH 1165a), terbutaline, and salbutamol under the conditions of temperature increase, triiodothyronine and decrease of extracellular pH were investigated. It was observed that (1) a temperature increase from 25 degrees to 42 degrees C significantly indreased the maximal tension evoked by histamine, whereas that induced by the alpha-sympathomimetic drug phenylephrine was not altered significantly; the maximal relaxation caused by beta-sympathomimetic drugs either at 25 degrees or at 42 degrees C did not differ from one another; (2) the affinities of histamine, phenylephrine and of the beta-sympathomimetic drugs isoprenaline, fenoterol, terbutaline, and salbutamol each were comparable at either 25 degrees or 42 degrees C; the rank order of efficacy of the beta-sympathomimetic drugs is isoprenaline greater than fenoterol greater than salbutamol greater than terbutaline; (3) a decrease of the pH from 7.37 to 7.15 diminished the affinities of histamine and of the beta sympathomimetic drugs whereas that of the alpha-adrenergic drug phenylephrine was not altered. A further decrease of the pH to 6.8 diminished additionally the affinity of histamine and of isoprenaline, and especially that of the other beta-sympathomimetic drugs to such an extent that in the latter case complete dose-response curves could not be determined any more; (4) pretreatment of the animals with 0.4 mg/kg of triiodothyronine (T3) for two days, which strongly depressed the tension induced by either histamine or phenylephrine, did not alter the affinity of both drugs; T3 in vitro (10(-6) M) only diminished the affinity of histamine but left that of phenylephrine unaltered; pretreatment for two days with 0.2 mg/kg of T3 yielded a significant diminution of the pD2-values for two beta-sympathomimetic drugs investigated, namely isoprenaline and fenoterol; also the administration of T3 in vitro in a final concentration of 10(-6) M resulted in a diminution of the affinity of both beta-sympathomimetic drugs; (5) the results obtained show that also on the aorta beta-adrenoceptor stimulants are dependent on the metabolic state while alpha-adrenoceptor stimulants are not.

Adrenergic beta-Agonists↗
PubMed1992

The effect of various sympathomimetics on the regional circulations in hyperdynamic sepsis.

Because sepsis is characterized by a depression in vascular reactivity, we hypothesized that changes in organ blood flows (Q) would differ between the nonseptic and septic state during the infusion of sympathomimetics. Therefore we examined the (sepsis x organ Q) interaction during the infusion of five sympathomimetics in 36 mature, awake sheep before and after cecal ligation and perforation produced hyperdynamic sepsis. A 3-hour infusion of dobutamine, norepinephrine, dopamine, dopexamine, or salbutamol was compared with that of placebo during both nonseptic and septic studies; drug infusion was titrated to an increase in cardiac index of greater than 20%. Increased plateau infusion doses of norepinephrine (+305%), salbutamol (+275%), dopamine (+70%), and dobutamine (+49%) were required to achieve predefined treatment guidelines during the septic versus nonseptic study. Few differences in the regional effects of individual sympathomimetics were found in the nonseptic study, although infusion of sympathomimetics was accompanied by a redistribution of systemic Q toward the heart and away from the brain, kidney, small intestine, liver, and pancreas. In the septic study, however, the sympathomimetic infusions were not accompanied by the redistribution of Q away from small intestine and liver that was demonstrated in the nonseptic study. Therefore (1) the depressed vascular reactivity in hyperdynamic sepsis altered the dose profile of exogenous sympathomimetics required to augment systemic Q, and (2) the (sepsis x sympathomimetic) interaction was characterized by a depression in the anticipated redistribution of organ Q from "nonvital" to "vital" circulations.

Animals↗
PubMed1992

Interaction of sepsis and sepsis plus sympathomimetics on myocardial oxygen availability.

The ability to regulate myocardial blood flows (Q) in accord with changing myocardial O2 needs may be depressed in sepsis. This could be an important concern when sympathomimetics are used to augment systemic oxygen delivery (QO2) in this syndrome as increased myocardial O2 needs may accompany an infusion of this class of drugs. Therefore after measuring the effect of sepsis on myocardial O2 metabolism, we then infused various sympathomimetics to evaluate the sepsis+sympathomimetic interaction on myocardial QO2. We measured Q to the left (LV) and right (RV) ventricles by the radioactive microsphere technique in 36 unanesthetized mature sheep, before and during the infusion of dopamine, dobutamine, dopexamine, norepinephrine, salbutamol, or placebo. Randomly selected for infusion, these drugs were titrated to augment the thermodilution-derived cardiac index (CI) by greater than 20%. This study was repeated 24-48 h after cecal ligation and perforation had resulted in a hyperdynamic septic state [change (delta) in CI = sepsis - baseline = +54%; P less than 0.01]. During the septic study, both Q-LV (delta = +80%; P less than 0.01) and Q-RV (delta = +84%; P less than 0.01) were increased above baseline values; the augmented Q to both LV and RV was directly correlated with the arterial perfusion pressure (PA) x CI product and the mean pulmonary artery pressure (PPA) x CI product, respectively. Only 23% of study animals demonstrated net transmyocardial lactate production during the septic study. When the infusion of sympathomimetics was accompanied by an increase in the PPA x CI and PA x CI products, a further increase in both Q-RV and Q-LV, respectively, occurred. Also, neither the ventricular endocardial-to-epicardial flow ratios nor transmyocardial lactate extraction were modified by the sympathomimetics infusion. We conclude that the septic response to infection in this animal model was not accompanied by significant abnormalities in the metabolic regulation of myocardial QO2 (R. E. Cunnion, G. L. Scher, and M. M. Parker, Circulation 73: 637-644, 1986).

Animals↗
PubMed1992

Effects of L-tyrosine on mixed-acting sympathomimetic-induced pressor actions.

We previously reported the ability of L-tyrosine (L-TYR) to potentiate the anorectic activity of various mixed-acting sympathomimetics including [R*S*]-(+/-)-norephedrine [phenylpropanolamine (PPA)], [1R,2S]-(-)-ephedrine (EPH), and [S]-(+)-amphetamine (AMPH) in hyperphagic rats. Included in those studies was the attenuation of L-TYR's effect when coadministered with L-valine, a large neutral amino acid that competes with L-TYR for uptake into the brain, suggesting a central locus for the action of L-TYR. Additional studies demonstrated the inability of L-TYR to alter the peripherally mediated PPA-, EPH-, and AMPH-induced increases in gastrointestinal transit time and retention and intrascapular brown adipose tissue thermogenesis. Because the mixed-acting sympathomimetics are known to increase blood pressure, these studies examined the ability of L-TYR to influence the pressor responses to PPA, EPH, and AMPH (0.03-1 mg/kg) in urethane-anesthetized rats. Each of the mixed-acting sympathomimetics significantly increased mean arterial, systolic, and diastolic blood pressures when administered alone, but no potentiation by L-TYR was observed. These results demonstrate the inability of L-TYR to potentiate the peripheral vasopressor effects of PPA, EPH, and AMPH. These data, in conjunction with our previous findings, suggest that the potentiation by L-TYR of the mixed-acting sympathomimetics is largely restricted to centrally mediated responses.

Amphetamine↗
PubMed1992

Neonatal periventricular-intraventricular hemorrhage after maternal beta-sympathomimetic tocolysis. The March of Dimes Multicenter Study Group.

OBJECTIVE: Our objective was to determine if the rate of periventricular-intraventricular hemorrhage is increased in the offspring of women who received a beta-sympathomimetic agent as part of the management of preterm labor. STUDY DESIGN: This retrospective study consists of 2827 women who were delivered of a singleton, live infant free of congenital neurologic anomalies between 25 and 36 completed weeks of gestation during a multicenter preterm birth prevention trial. The data were analyzed, adjusting for type of tocolytic agent, race, infant sex, gestational age, birth weight, health care center, route of delivery, indication for delivery, intrapartum fetal distress, respiratory distress syndrome, and neonatal sepsis. RESULTS: The overall incidence of periventricular-intraventricular hemorrhage in this population was 5.6%. In a univariate analysis in which no adjustment was made for potentially confounding variables, beta-sympathomimetic tocolysis was found to be associated with nearly a fourfold increase in the incidence of periventricular-intraventricular hemorrhage when compared with the use of either magnesium sulfate or no tocolytic agent. The results of a multivariate regression analysis revealed that beta-sympathomimetic agents were associated with a statistically significant increase in the overall incidence of periventricular-intraventricular hemorrhage (odds ratio 2.47, 95% confidence interval 1.34 to 4.56, p = 0.004) and a similar, but not significant, increase in the incidence of grades 3 and 4 periventricular-intraventricular hemorrhage (odds ratio 2.50, 95% confidence interval 0.96 to 6.48, p = 0.06). CONCLUSION: beta-Sympathomimetic tocolytic therapy may be associated with a more than twofold increase in the incidence of neonatal periventricular-intraventricular hemorrhage.

Birth Weight↗
PubMedDate not supplied

Distinguishing sympathomimetic amines from amphetamine and methamphetamine in urine by gas chromatography/mass spectrometry.

Derivatives of seven commonly used sympathomimetic amines and two "designer amines" were isolated from urine, separated chromatographically from amphetamine and methamphetamine, and determined by mass spectrometry with selected ion monitoring. The drugs included ephedrine, propylhexedrine, pseudoephedrine, phenylpropanolamine, hydroxynorephedrine, phenylephrine, phentermine, methylenedioxyamphetamine (MDA), and methylenedioxy methamphetamine (MDMA). The drugs were liquid extracted from urine and derivatized by either heptafluorobutyric anhydride (HFBA) or 4-carbethoxyhexafluorobutyryl chloride (4-CB). Because the base peak ions for ephedrine, pseudoephedrine, propylhexedrine, MDMA, and phentermine are identical to methamphetamine (e.g. 254 amu for HFBA) and those for phenylephrine, hydroxynorephedrine, phenylpropanolamine, and MDA are identical to amphetamine (e.g. 240 amu for HFBA), a table of selected ions was developed for all 11 drugs that distinguished amphetamine and methamphetamine from the sympathomimetic amines with either HFBA or 4-CB. The distinguishing ions rely on the ring structure of the different drugs and fragmentation associated with that structure. The 4-CB reagent partially derivatized the hydroxy-containing sympathomimetic amines, while the HFBA completely derivatized all the sympathomimetic amines. Furthermore, false positive results for the 4-CB reagent were found only for methamphetamine (20-2250 ng/mL of methamphetamine) when high concentrations (greater than 5 micrograms) of ephedrine or pseudoephedrine were present in the specimen. These results are related to a combination of injection port temperature and cleanliness of the injection port sleeve.

Amphetamine↗
PubMedDate not supplied

A study of the relationship between cardiac beta-adrenoceptor blockade and intrinsic sympathomimetic activity in rats depleted of catecholamines.

1. The intrinsic sympathomimetic activity of a range of beta-adrenoceptor antagonists and its relationship to beta-adrenoceptor blockade was studied in pentobarbitone-anaesthetized, vagotomized rats which had been depleted of catecholamines by pretreatment with syrosingopine. Dichlorisoprenaline, practolol, oxprenolol, pindolol and acebutolol, produced dose-dependent positive chronotropic responses in this preparation. 2. The relationship between the dose requirements for this intrinsic sympathomimetic activity and beta-adrenoceptor-blocking activity was not the same for all drugs: (i) dichlorisoprenaline and practolol had intrinsic activity at all beta-adrenoceptor-blocking doses; and (ii) oxprenolol, pindolol and acebutolol had predominantly beta-adrenoceptor blockade at the lower dose levels and agonist activity only became significant at high doses relative to those producing beta-adrenoceptor blockade. 3. The positive chronotropic response to both practolol and pindolol was observed in rats which had been pithed and was antagonized by propranolol (0.1-3.0 mg/kg, i.v.), indicating that beta-adrenoceptors were involved. 4. It was concluded that the intrinsic sympathomimetic activity of beta-adrenoceptor antagonists was not a simple property as it was described by the relationship between the dose requirements for intrinsic sympathomimetic activity and for beta-adrenoceptor blockade as well as the degree of partial agonist activity.

Adrenergic beta-Antagonists↗
PubMed1977

Effect of acetylated derivatives of some sympathomimetic amines on the isolated auricles and tracheal chain of the guinea-pig.

The effects of acetylation of sympathomimetic amines, tyramine, amphetamine, ephedrine, phenylephrine, orciprenaline, and salbutamol, and their O- and N-acetyl derivatives and the effects of reserpine or physostigmine pretreatment on the isolated auricles and tracheal chain of guinea-pigs have been studied. All the parent drugs relaxed the tracheal chain and had a positive inotropic and chronotropic effect on the isolated auricles; only amphetamine, on the contrary, contracted the tracheal chain. O-acetylation of these sympathomimetic amines generally decreased less chronotropic than iontropic action on the isolated auricles. O-acetylation of tyramine however: actually increased the positive chronotropic activity of drug. As a rule, O-acetylation also decreased the beta-adrenergic effect of these compounds on the tracheal chain, but not so markedly as on the isolated auricles. N-acetylation generally abolished the adrenergic effects of these sympathomimetic amines on the isolated auricles and decreased those effects on the tracheal preparation. N,O-triacetylation of salbutamol abolished the stimulating effect of the parent drug on the auricles but increased the relaxant activity on the trachea. Physostigmine antagonized the effects of O-acetyltyramine and O-triacetylorciprenaline but not those of tyramine and orciprenaline on the trachea preparation. It is concluded that among the sympathomimetic amines acetylation may be utilized for the development of specific bronchodilators and O-acetylation for inducing drug latentiation.

Acetylation↗
PubMed1977

Effect of acetyl derivatives of some sympathomimetic amines on the blood pressure of the rat.

The effect of five sympathomimetic amines and some of their acetyl derivatives on the blood pressure of the rat was determined on the left carotid artery. After pretreatment with chlorisondamine (1 mg/kg subcutaneously) the blood pressure rise by sympathomimetic amines and their acetyl derivatives was compared with that of adrenaline. If the potency of adrenaline is specified as 100, the potencies of the other drugs are phenylephrine (metaoxedrinum, NFN) 37, tyramine 1.1, O-acetyltyramine 0.52, amphetamine 0.50, O-diacetylphenylephrine 0.25, ephedrine 0.23, O-acetylephedrine 0.02, N-acetylphenylephrine 0.01. The effects of N-acetyltyramine, N-acetylephedrine and N-acetylamphetamine are even weaker. Reserpine 5.0 or 0.05 mg/kg intraperitoneally 24 hours before the experiment increased the blood pressure rise by the directly acting sympathomimetic amines and their acetyl derivatives, but decreased the effects of the indirectly acting drugs. After treatment with phenoxybenzamine (2 mg/kg intraperitoneally), adrenaline exhibited the greatest blood pressure decrease and the effects of the other drugs in descending order: orciprenaline, O-acetyltyramine, phenylephrine, ephedrine, amphetamine, O-diacetylphenylephrine and O-acetylephedrine. Tyramine did not show any blood pressure decrease. The blood pressure decrease by sympathomimetic amines and by their acetyl derivatives was probably due to beta-receptor stimulation because it was prevented by propranolol. The N-acetyl derivatives recembled their parent drugs with regard to the immediate onset and short duration of their effects. The O-acetyl derivatives exhibited slower onset and longer duration of effect than their parent drugs. Physostigmine-pretreatment diminished the rise in blood pressure by O-acetyltyramine, but the effect of tyramine remained unchanged.

Amphetamines↗
PubMed1992

Highly potent beta-2 sympathomimetics convert to less potent partial agonists as relaxants of guinea pig tracheae maximally contracted by carbachol. Comparison of relaxation with receptor binding and adenylate cyclase stimulation.

Beta-adrenergic agonists are among to the most potent dilators of airway smooth muscle available and act as functional antagonists of a variety of contractile stimuli. In order to elucidate their loss of relaxant potency in dependence on antagonistic stimuli tracheal relaxation by the beta-2 sympathomimetics (+/-)-salbutamol (+/-)-fenoterol, and (+/-)-formoterol was compared to (-)-isoprenaline in guinea pig tracheae partially and maximally precontracted by carbachol. In partially precontracted tracheae, salbutamol, fenoterol and formoterol exerted maximum relaxation with low EC50 of 20, 5.6 and 0.29 nmol/l, respectively. In maximally precontracted tracheae, however, salbutamol, fenoterol and formoterol were only partial agonists for relaxation with different intrinsic activities (0.62, 0.62 and 0.77, respectively) and increased EC50 (120, 50 and 3.6 nmol/l, respectively). A reduction of relaxant potency by increased muscarinic stimuli was also observed for beta-1 adrenoceptors stimulated by (-)-noradrenaline after blockade of beta-2 adrenoceptors. In order to investigate if the reduced relaxant potency of beta-2 sympathomimetics was caused by a reduced spare receptor capacity or a reduced intrinsic activity for stimulation of adenylate cyclase (AC), we performed experiments in membranes from lung and tracheal cells. In radioligand binding, beta-2 sympathomimetics recognized the high-affinity state (57%) of the beta-2 adrenoceptor with a lower effectiveness than (-)-isoprenaline, which exhibited a 100-fold higher affinity for high over low-affinity states. Dissociation constants for the low-affinity state matched EC50 for AC stimulation. Intrinsic activities (%) for AC stimulation were significantly lower for salbutamol (67%), fenoterol (67%) and formoterol (89%) than for (-)-isoprenaline (100%), indicating that the reduced relaxation potency of the beta-2 sympathomimetics of maximally precontracted tracheae is caused by a reduced intrinsic activity for AC stimulation. It might be speculated that formoterol could improve drug therapy of asthma due to its high binding affinity and its high intrinsic activity for relaxation.

Adenylyl Cyclases↗
PubMed1991

L-tyrosine fails to potentiate several peripheral actions of the sympathomimetics.

We have recently reported the ability of L-tyrosine (L-TYR) to potentiate the anorectic activity of various mixed-acting sympathomimetics including phenylpropanolamine (PPA), l-ephedrine (EPH) and d-amphetamine (AMP). Included in those studies was the attenuation of L-TYR's effect when coadministered with L-valine, a large neutral amino acid which competes with L-TYR for uptake into the brain, suggesting a central locus for the action of L-TYR. To determine to what extent L-TYR can potentiate peripheral actions, we investigated the effects of L-TYR with either PPA (20 mg/kg), EPH (20 mg/kg) or AMP (1.75 mg/kg) on gastric transit, gastric retention and intrascapular brown adipose tissue thermogenesis. In each of the paradigms studied, PPA, EPH and AMP significantly increased the expected sympathomimetic-mediated response, but no potentiation of L-TYR was observed. These results are consistent with the hypothesis that L-TYR potentiates the anorectic activity of the mixed-acting sympathomimetics largely via an action at a central locus.

Adipose Tissue, Brown↗
PubMed1975

Tolerance to sympathomimetic bronchodilators in guinea-pig isolated lungs following chronic administration in vivo.

1 Pretreatment of the guinea-pig subcataneously three times daily with either 5 mug/kg isoprenaline or adrenaline reduced the response of the isolated perfused histamine-constricted lung when challenged with either the same or a different sympathomimetic bronchodilator. The longer the animals were pretreated and the higher the dose of bronchodilator the greater was the degree of tolerance developed. 2 Tolerance was developed to aminophline in the same preparation when the guinea-pig had been pretreated with aminophyline or isoprenaline. Cross-tolerance also developed adrenaline when the guinea-pig had been pretreated with aminophyline. 3 Tolerance was still persistent in guinea-pigs one and two weeks after pretreatment three times daily with either isoprenaline of adrenaline (5 mu/kg s.c.) for seven days in the same preparation. Only after three weeks was the tolerance diminished. 4 These results suggest that asthmatic patients who use bronchodilators excessively may become refractory to the bronchodilator effect of these drugs. They also support the hypothesis that induced cross-resistance to endogenous sympathetic stimulation could lead to a deterioration of the asthmatic state in patients using the sympathomimetic bronchodilators and that this may explain the increase in asthma mortality rate. 5 A mechanism of tolerance to sympathomimetic bronchodilators is postulated.

Airway Resistance↗
PubMed1975

Sympathomimetic actions of reserpine administered during treatment with dopamine in the dog.

Reserpine injected intravenously during infusion with dopamine brought about sympathomimetic effects, but a second injection of reserpine after 120 minutes did not elicit such effects. This pressor effect was eliminated by phenoxybenzamine and this positive chronotrpic effect by propranolol. Reserpine induced similar but weak sympathomimetic effects after cocaine, while it induced no changes in blood pressure and heart rate after infusion with noradrenaline. Meanwhile, pressor response to dopamine was potentiated 24 hours after reserpine and was further potentiated after additional infusion with noradrenaline. The sympathomimetic actions induced by the concurrent administration of reserpine and dopamine may be attributable to a facilitation of those indirect mechanisms, prinicpally of endogenous catecholamine release.

Animals↗
PubMed1976

Centrally mediated cardiovascular responses to intracisternal injections of sympathomimetic amines in anesthetized rats.

The cardiovascular effects resulting from intracisternal (i.c.) injections of sympathomimetic amines were studied in alpha-chloralose-urethanized rats. Norepinephrine (0.5-5 mug i.c.) caused a typical rise in blood pressure with no significant change in heart rate and a fall in blood pressure with a bradycardia, which were completely blocked after treatment with phentolamine (10-50 mug i.c.) L-isoproterenol (0.05-0.5 mug i.c.) and trimetoquinol (0.5-3 mug i.c.), a beta-sympathomimetic agent, usually caused a fall in blood pressure with a tachycardia, which was reduced after treatment with propranolol (10-50 mug i.c.), but trimetoquinol was inclined to cause a rise in blood pressure with a tachycardia. Epinephrine (5 mug i.c.) showed both centrally mediated alpha- and beta-sympathomimetic effects. Tyramine (0.5-1 mg i.c.) caused mixed blood pressure responses presumably due to a release of norepinephrine and epinephrine, and these responses were partially blocked after treatment with phentolamine (100 mug i.c.) or propranolol (50 mug i.c.). These observations suggest that both alpha- and beta-sensitive adrenergic zones may exist on the vasomotor center of the pons and medulla in rats, and both norepinephrine and epinephrine might centrally play a physiological role as the neurotransmitters controlling blood pressure in rats.

Animals↗
PubMed1978

[Use of sympathomimetics in the treatment of asthma].

There are many sympathomimetic products available in treatment of asthma, such as ephedrin, isoproterenol, orciprenalin, salbutamol, terbutalin..., used by oral or injectable route and spray. In the spray form, the sympathomimetic products have immediate and intense activity but the easiness of their use leads to a dose multiplication or overdose ineluctably, and thus involves cardiac and respiratory accidents which can be fatal. Their prescription should be slightly different according to the clinical type of asthma in the patient. On the other hand the oral sympathomimetic products are less inclined to overdose.

Albuterol↗
PubMed1976

Sympathomimetic-induced effects in the soleus muscle of the guinea-pig.

Sympathomimetic amines reduced the peak tension, time to peak and time to half-relaxation of indirectly elicited twitches of the guinea-pig soleus muscle in vivo. Clonic contractions of the soleus muscle were depressed by the amines. On a molar basis salbutamol and orciprenaline were 5.12 and 80.0 times less potent than (+/-)-isoprenaline in producing these effects. Results obtained with the beta-receptor antagonists propranolol, practolol and H35/25 suggest that the depression in skeletal muscle contractility is due to beta2-adrenoreceptor stimulation. The effects of the amines on twitches and clonic contractions of the guinea-pig soleus muscle are qualitatively similar to those reported previously in the cat soleus muscle preparation, which has been used to assess the possible tremorogenic actions of sympathomimetic bronchodilators. On a quantitative basis the molar dose-ratios of the sympathomimetics used, and the effects of beta-receptor antagonists on the responses, are similar in the two species.

Adrenergic beta-Agonists↗
PubMed1978

Uropharmacology: VIII. Sympathomimetic agents.

Sympathomimetic drugs stimulate the receptors of the sympathetic nervous system. Although the bladder possesses sympathetic receptors, sympathomimetic drugs, in general, have little effect on bladder function. Their most useful clinical applications on the urinary tract are to increase or decrease bladder resistance.

Animals↗
PubMed1976

Mechanism of the indirect sympathomimetic effect of 5-hydroxytrypt-amine on the isolated heart of the rabbit.

1 Rabbit isolated hearts, perfused by the Langendorff technique, were used to investigate the indirect sympathomimetic effects of 5-hydroxytryptamine (5-HT). Comparisons were made with noradrenaline and with two indirectly acting sympathomimetic agents with entirely different mechanisms of action, tyramine and dimethylphenylpiperazinium (DMPP). 2 The cardiac stimulant effects of 5-HT, tyramine and DMPP were inhibited by propranolol and practolol and the pA2 values obtained were similar to those obtained with noradrenaline as the agonist. 3 Responses to 5-HT, tyramine and DMPP were greatly reduced on hearts from rabbits pretreated with 6-hydroxydopamine. Such hearts had less than 7% of their normal catecholamine concentration and no fluorescence characteristic of noradrenaline in the cardiac sympathetic nerves could be demonstrated. 4 Rapid, reversible and selective tachyphylaxis to 5-HT was demonstrated during perfusion with 5-HT. In hearts desensitized to DMPP by perfusion with DMPP, responses to 5-HT were also reduced. 5 Perfusion of hearts with colchicine inhibited stimulant responses to 5-HT and DMPP but had little effect on responses to noradrenaline or tyramine. 6 Desmethylimipramine enhanced cardiac stimulant responses to noradrenaline and to a lesser extent, those to 5-HT and DMPP. Responses to tyramine were consistently inhibited by desmethylimipramine. 7 Tetrodotoxin abolished responses of the heart to electrical nerve stimulation but left responses to noradrenaline, 5-HT and DMPP unaffected. 8 5-HT, tyramine and DMPP evoked 3H-release from hearts whose neuronal noradrenaline stores had been labelled by perfusion with [3H]-(-)-noradrenaline. The pattern of release evoked by 5-HT was similar to that of DMPP but differed from that of tyramine. 9 Reducing the calcium concentration in the Tyrode solution from 3.6 to 0.2 mEq/1 did not affect 3H-overflow after tyramine but greatly inhibited that evoked by 5-HT and DMPP. 10 The results confirm that the stimulatn effects of 5-HT on the rabbit isolated heart are the result of noradrenaline release. They further suggest that the site of the release is the terminal sympathetic nerve network. The mechanism of release shows more similarities to that of DMPP (calcium-dependent depolarization and exocytosis) than to that of tyramine (neuronal uptake and stoichiometric displacement).

Animals↗