Binding of [3H][D-Ala2, MePhe4, Gly-ol5] enkephalin, [3H][D-Pen2, D-Pen5] enkephalin, and [3H]U-69,593 to airway and pulmonary tissues of normal and sensitized rats

The role of endogenous opioid peptides in the regulation of bronchomotor tone, as well as in the pathophysiology of asthma is uncertain. We have studied the binding of highly selective [H-3]labeled ligands of mu-([D-Ala(2), MePhe(4), Gly-ol(5)]enkephalin; DAMGO), delta ([D-Pen(2), D-Pen(5)]enkephalin; DPDPE), and kappa-(U-69,593) opioid receptors to membranes of trachea, main bronchus, lung parenchyma and pulmonary artery obtained from normal (unsensitized) and actively IgE-sensitized rats acutely challenged with the specific antigen. [H-3]DAMGO, [H-3]DPDPE and [H-3]U-69,593 bound to membranes of normal and sensitized tissues at a saturable, single high-affinity sire. The rank order of receptor densities in normal tissues was delta- greater than or equal to kappa- greater than or equal to mu-, with lung parenchyma exhibiting the greatest binding capacity for delta- and mu-receptors compared to the other regions examined. The K-d values showed small differences between ligands and regions tested. The mu- and delta-opioid receptor densities were decreased in sensitized main bronchus and lung parenchyma, respectively, compared to normal tissues. By contrast, kappa-opioid receptor density was augmented in sensitized lung parenchyma but an increase in K-d values was also observed. These differential changes in the density and affinity of opioid receptor types may be related to alterations in endogenous opioid peptides during the process of sensitization. (C) 1997 Elsevier Science Inc.