Effect of dopamine on airways conductance in normals and extrinsic asthmatics

Reversing Cholinergic Bronchoconstriction by Common Inotropic Agents: A Randomized Experimental Trial on Isolated Perfused Rat Lungs

BACKGROUND

The ability of inotropic agents to alter airway reactivity and lung tissue mechanics has not been compared in a well-controlled experimental model. Therefore, we compared the potential to alter lung tissue viscoelasticity and bronchodilator effects of commonly used inotropic agents in an isolated perfused rat lung model.

METHODS

After achieving steady state lung perfusion, sustained bronchoconstriction was induced by acetylcholine (ACh). Isolated rat lungs were then randomly allocated to 6 groups treated with either saline vehicle (n = 8) or incremental concentrations of inotropes (adrenaline, n = 8; dopamine, n = 7; dobutamine, n = 7; milrinone, n = 8; or levosimendan, n = 6) added to the whole-blood perfusate. Airway resistance (Raw), lung tissue damping (G), and elastance were measured under baseline conditions, during steady-state ACh-induced constriction and for each inotrope dose.

RESULTS

No change in Raw was observed after addition of the saline vehicle. Raw was significantly lower after addition of dopamine (maximum difference [95% CI] of 29 [12-46]% relative to the saline control, P = .004), levosimendan (58 [39-77]%, P < .001), and adrenaline (37 [21-53]%, P < .001), whereas no significant differences were observed at any dose of milrinone (5 [-12 to 22]%) and dobutamine (4 [-13 to 21]%). Lung tissue damping (G) was lower in animals receiving the highest doses of adrenaline (difference: 22 [7-37]%, P = .015), dobutamine (20 [5-35]%, P = .024), milrinone (20 [6-34]%, P = .026), and levosimendan (36 [19-53]%, P < .001) than in controls.

CONCLUSIONS

Although dobutamine and milrinone did not reduce cholinergic bronchoconstriction, they reversed the ACh-induced elevations in lung tissue resistance. In contrast, adrenaline, dopamine, and levosimendan exhibited both potent bronchodilatory action against ACh and diminished lung tissue damping. Further work is needed to determine whether these effects are clinically relevant in humans.

The effects of dopamine on the respiratory system: Friend or foe?

Dopamine (DA) is an immediate precursor of noradrenaline that has stimulatory or inhibitory effects on a variety of adrenergic receptors. DA is primarily used in the management of circulatory shock for its combined vasopressor and inotropic effects, but it may also exert significant effects on the respiratory system Although the respiratory effects of intravenous DA attract less attention than its hemodynamic effects, there is evidence that DA affects ventilation, pulmonary circulation, bronchial diameter, neuromodulation of sensory pulmonary nerves and lung water clearance. Through these complex mechanisms, DA may exert beneficial as well as detrimental effects on respiration. DA may have beneficial effects on the respiratory system by decreasing oedema formation and improving respiratory muscle function, but can also have deleterious effects, by inhibiting ventilation. Hence, DA may be beneficial in lung oedema, but harmful in cases of difficult weaning from mechanical ventilation. DA should be used with caution in patients with heart failure during weaning from mechanical respiration; however, critically ill patients with chronic obstructive pulmonary disease (COPD) do not show this negative effect of DA on ventilatory drive.

The actions of dopamine on the airways

BACKGROUND

Dopamine exerts inhibitory and excitatory effects on different systems. In the lungs, dopamine modulates respiratory functions through carotid bodies and modulates pulmonary blood vessel tone, alveolar liquids, and bronchial exchange, and possibly participates in the regulation of airways diameter. It has been reported that dopamine has no acute effect on human airways in normal subjects or those with asthma background. However, inhaled or infused dopamine decreased histamine-induced bronchoconstriction in both normal and asthmatic subjects. We have examined the possible modulating effect of dopamine on bronchial diameter by administering inhaled dopamine and the DA2 dopaminergic blocker metoclopramide to subjects with various degrees of bronchial tone.

METHODS

We examined 56 volunteers. Arterial blood pressure and heart rate were determined in every subject. By means of spirometry, we measured forced vital capacity, forced expiratory volume in the first second, maximal forced expiratory flow, and forced expiratory flow at 50% of vital capacity, before and after each treatment. By inhalation with a nebulizer, we administered dopamine (0.5 microg/kg/min) to 10 healthy subjects, 10 subjects with asthma without acute bronchospasm, and 16 subjects with acute asthma attack; intravenous metoclopramide (7 microg/kg/min) was administered to 10 healthy subjects and 10 subjects with asthma without acute bronchospasm. For ethical reasons, metoclopramide was not used in subjects with acute asthma attack.

STATISTICS

non-parametric Wilcoxon tests for paired samples, ANOVA tests, and Bonferroni multiple comparison tests were performed.

RESULTS

Inhaled dopamine increased forced expiratory volume in the first second, forced vital capacity, maximal forced expiratory flow, and forced expiratory flow at 50% of vital capacity in the acute asthma attack group, but there were no modifications in the healthy group or in the asthma without acute bronchospasm group. Metoclopramide did not induce changes in respiratory parameters in healthy individuals or in those with asthma without acute bronchospasm.

CONCLUSIONS

Inhaled dopamine was able to induce bronchodilatation when the bronchial tone was already increased by acute asthma attack, but it did not modify the resting bronchial tone in normals or in asthmatics without acute bronchospasm. DA2 blockade with metoclopramide did not modify resting bronchial tone either. We suggest that dopamine exerts a modulatory effect on bronchial tone of human airways depending on the degree of existing basal tone.

Dopaminergic modulation of human bronchial tone

BACKGROUND

Dopamine exerts inhibitory and excitatory effects on different systems. Its effect on human bronchial tone is controversial. It has been reported that dopamine has no acute effect on human airways from normal subjects or those with asthma background. However, inhaled or infused dopamine decreased histamine-induced bronchoconstriction in both normal and asthmatic subjects.

METHODS

We examined the possible modulating effect of dopamine on bronchial diameter by administering inhaled dopamine and the DA(2) dopaminergic blocker metoclopramide (MTC) to subjects with various degrees of bronchial tone. We examined 50 volunteers. Arterial blood pressure and heart rate were determined in each subject. By means of spirometry, we measured forced vital capacity (FVC), forced expiratory volume in the first second (FEV(1)), maximal forced expiratory flow (FEF(max)), and forced expiratory flow at 50% of vital capacity (FEF(50)), before and after each treatment. By inhalation with a nebulizer, we administered the following: a) dopamine (0.5 microg/kg/min) to 10 healthy subjects, 10 subjects with asthma without acute bronchospasm (AWAB), and nine subjects with acute asthma attack (AAA), and b) intravenous (i.v.) metoclopramide (7 microg/kg/min) was administered to 10 healthy subjects and 11 subjects with AWAB. For ethical reasons, MTC was not used in subjects with acute asthma attack. Non-parametric Wilcoxon test for paired samples, ANOVA test, and Bonferroni multiple comparison test were performed. Inhaled dopamine increased FEV(1) and FVC, FEF(max), and FEF(50) in the AAA group, but there were no modifications in the healthy group or in the AWAB group. Metoclopramide did not induce changes in respiratory parameters in healthy individuals or in those with AWAB.

CONCLUSIONS

Inhaled dopamine is able to induce bronchodilatation when the bronchial tone is already increased by acute asthma attack but did not modify the resting bronchial tone in normal subjects or in asthmatics without acute bronchospasm. Additionally, DA(2) blockade with metoclopramide did not modify resting bronchial tone. Dopamine exerts a modulatory effect on the bronchial tone of human airways depending on the degree of preexisting tone.

The relationship between exercise-induced asthma and plasma catecholamines

In asthmatics, exercise (E1) is known to induce bronchoconstriction; a second period of exercise (E2) within 30 to 90 min induces a lesser degree of bronchoconstriction. The reason for the "refractory" bronchoconstrictor response to E2 is unclear. We studied 6 normal subjects (Group 1) and 12 stable asthmatics to examine the possible relationship between the refractory period and sympathoadrenal function. The asthmatics were further divided into 2 groups, based on the absence (Group 2, n = 5) or presence (Group 3) of an exercise-induced bronchoconstrictor response. In each subject/patient, after control measurements of spirometry and venous blood sampling, a standard level of treadmill exercise was performed, and blood and spirometry were obtained at 1, 5, 15, and 25 min after the end of exercise. A second period of treadmill exercise, of the same level and duration, was then performed, and blood sampling and spirometry were repeated at the same time intervals after the end of exercise. There was no significant (p less than 0.1) difference in baseline plasma epinephrine (Ep), or norepinephrine (Np) levels; however, plasma dopamine (Dp) levels were significantly (p less than 0.05) higher in Group 3. In Groups 1 and 2, FEV1 did not change significantly after the first (E1) or second (E2) exercise periods, but there were significant (p less than 0.01) changes in Group 3 (maximal percent changes in FEV1, delta FEV1%, E1 = -14.4%, E2 = -8.8%), and the delta FEV1% after E2 was significantly (p less than 0.01) less than after E1.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of dopamine on tracheal smooth muscle

Dose-response curves to dopamine were obtained on guinea-pig, dog and human tracheal smooth muscle. Dopamine produced a relaxation of the guinea-pig tracheal chain, and this relaxation was completely blocked by propranolol. The potency of dopamine as a beta-agonist was 1/10 000 that of isoprenaline, 1/250 that of adrenaline and 1/50 that of noradrenaline. In human and dog tracheal smooth muscle, dopamine induced a contraction which could be entirely abolished by alpha-adrenoceptor antagonists. As an alpha-agonist, the potency of dopamine was 1/20 that of adrenaline and noradrenaline. Our data also show that a dopamine induced contraction is greatly potentiated if the smooth muscle specimen has been previously slightly contracted with histamine. We conclude that there are few, if any, specific dopaminergic receptors in the airways. Dopamine acts on both alpha- and beta-adrenoceptors, but in humans and dogs, its effect is predominantly on alpha-adrenoceptors.

Pharmacological analysis of the responsiveness of guinea-pig lung parenchymal strip to dopamine

Responses to dopamine were examined in the guinea-pig isolated lung parenchymal strip. Complete cumulative concentration-response curves to dopamine exhibited a biphasic pattern with a small initial contraction at concentrations below 10(-5) M followed by a dose-dependent relaxation at higher concentrations. Phentolamine (10(-5) M) completely abolished the contractile component and enhanced sensitivity and maximal relaxation to dopamine. In the presence of phentolamine, propranolol antagonized the dopamine-induced relaxation (pA2 = 8.54 +/- 0.07). In the presence of propranolol (10(-6) M), dopamine produced a dose-related contraction displaced to the right by phentolamine. Incubation with haloperidol (10(-5) M) did not modify the characteristics of the concentration-response curve to dopamine. Pretreatment with reserpine abolished the contraction to dopamine without affecting its relaxant response. Cocaine significantly increased the pD2 value of dopamine in the presence of propranolol. It is concluded that dopamine produced both relaxation of lung parenchymal strip due to direct activation of beta-adrenoceptors and contraction mediated through direct and indirect (catecholamine release) actions at alpha-adrenoceptors. There is no evidence in favour of the existence of specific dopamine-receptors in this preparation.