A Comparison of β-Adrenergic Receptors and In Vitro Relaxant Responses to Isoproterenol in Asthmatic Airway Smooth Muscle

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Abstract
In previous reports, we have documented decreased in vitro airway smooth muscle responses to isoproterenol (ISO) in fresh postmortem trachea and bronchus from subjects with fatal asthma. One hypothesis to explain this finding is a decrease in beta-adrenergic receptor (beta AR) numbers on airway smooth muscle. We have now examined the autoradiographic distribution and density of beta AR using [125I]iodocyanopindolol on sections of airway smooth muscle adjacent to those studied functionally. The results have been compared with "normal" trachea and bronchi obtained from persons dying suddenly of nonpulmonary causes. In both trachea and bronchi, there was a 2.8-fold and 2.5-fold increase in specific grain counts, respectively, over smooth muscle from asthmatic airways (n = 6) compared with that determined in normal airways (n = 4, P less than 0.01, unpaired t test). The affinity of the beta AR for the agonist ISO, as determined by competitive binding experiments with increasing concentrations of (-)-ISO on tissue sections, was increased in asthmatic bronchi (IC50 = 80 +/- 13 nM; n = 3) compared with normal bronchi (IC50 = 562 +/- 144 nM; n = 4, P less than 0.05). We conclude that beta AR-mediated relaxant abnormalities in airway smooth muscle in fatal asthma cannot be explained by a decrease in receptor number and, surprisingly, beta AR expression is increased.

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