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The benzonaphthyridine derivative, AH 21-132, has non-specific relaxant effects in isolated airways smooth muscle. The action of AH 21-132 in trachealis muscle is not antagonised by propranolol but AH 21-132 is slightly potentiated by epithelium removal. Electrophysiological recording from guinea-pig trachealis shows that AH 21-132-induced relaxation is accompanied by suppression of electrical slow waves and by cellular hyperpolarisation. Unlike theophylline, AH 21-132 does not cause spasm of cooled (12 degrees C), indomethacin-treated trachealis muscle, nor does it act as an antagonist at adenosine A1 receptors. AH 21-132 does not depress the Ca2+ sensitivity or responsiveness of Triton X-100 skinned trachealis fibres. In tracheal relaxant concentrations, AH 21-132 selectively inhibits cAMP phosphodiesterase (PDE) compared with cGMP-PDE. The (-)-enantiomer of AH 21-132 is more potent than its (+)-enantiomer both in causing tracheal relaxation and in inhibiting cAMP-PDE. When tested on PDE isoenzymes separated from bovine trachealis and guinea-pig cardiac ventricles, AH 21-132 exhibits selectivity as an inhibitor of the isoenzyme types III and IV. AH 21-132 increases the trachealis content of cAMP and cGMP, but only in concentration greater than that required fully to suppress the mechanical tone of the tissue. AH 21-132 has bronchodilator activity in anaesthetised, ventilated guinea-pigs when administered intraduodenally, intravenously or by inhalation. Inhaled AH 21-132 also provides bronchodilatation in healthy human volunteers in whom bronchoconstriction has been induced by inhaled methacholine.
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