Bronchopulmonary effects of dopamine in the guinea-pig

The effects of dopamine (DA) were studied in vivo on the pulmonary airway resistance of the unanaesthetized guinea-pig, and in vitro, on the isolated trachea of the same animal. DA relaxes the isolated trachea previously contracted by ACh, but only at very high concentrations (10(-4) to 10(-3) M) and appears to be approximately 7000 times less active than isoprenaline (ISO). The relaxation caused by DA is: (1) diminished by pretreatment with cocaine or reserpine indicating an indirect effect linked to a release of endogenous catecholamines; (2) increased by phentolamine, suggesting a masked contracting alpha-adrenergic component; (3) completely abolished by propranolol through competitive antagonism. In vivo, DA reduces the increase in pulmonary airway resistance provoked by ACh. Its action, which is mainly beta-adrenergic, is, nevertheless, 1.4 x 10(5) times less than that of ISO. Thus, dopaminoreceptors do not seem to exist in the tracheobronchial smooth muscle of the guinea-pig. DA only has partial beta-adrenergic agonistic action at that site.