Abstract
Effects of adrenergic agonists on 4 different preparations (trachea, bronchus, bronchiole and parenchymal strip) of rat airway smooth muscle were studied. Phenylephrine, an alpha-agonist, contracted the trachea, bronchus and parenchymal strip but had no effect on the bronchiole. Similarly adrenaline, both a beta- and an alpha-agonist, contracted the trachea, bronchus and parenchymal strip, but neither contracted nor relaxed the bronchiole. These results suggest that the rat trachea and bronchus contain alpha receptors but the bronchiole does not. Alpha response of the parenchymal strip preparation seems to originate from the vascular smooth muscle. Isoprenaline, a nonselective beta-agonist, could not elicit any response on resting airways, but caused a relaxation of all four preparations after being contracted by carbacholine. Expressed in % of maximal carbacholine contractions, the relaxations were: 21% in the trachea, 45% in the bronchus, 92% in the bronchiole and 141% in the parenchymal strip. This shows that beta response is weak in the large central airways but becomes stronger towards the small peripheral airways, when measured against cholinergic contraction.
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