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

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Abstract
1 Responses to dopamine were examined in the guinea-pig isolated lung parenchymal strip. 2 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. 3 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. 4 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. 5 It is concluded that dopamine produced both relaxation of lung parenchymal strip due to direct activation of β-adrenoceptors and contraction mediated through direct and indirect (catecholamine release) actions at α-adrenoceptors. There is no evidence in favour of the existence of specific dopamine-receptors in this preparation.