Effects of six beta-blockers on the isolated guinea-pig trachea

The effects of six beta-blocking agents (propranolol, metoprolol, atenolol, pindolol, medroxalol and labetalol) selected on the basis of their intrinsic sympathomimetic properties, membrane-stabilizing activities and selectivity for beta 1-adrenoceptors were investigated on the isolated guinea-pig trachea. Tracheal spirals were either allowed to remain under resting tension or were precontracted with histamine 2 X 10(-6) M or 2 X 10(-5) M. Pindolol, medroxalol and labetalol exerted a moderate relaxant effect on tracheal muscle. This effect was inhibited by propranolol, but not significantly modified by indomethacin. Propranolol and metoprolol exerted a moderate contracturant effect on the preparations. The effect was inhibited by indomethacin but not by FPL 55712. Atenolol had no effect on tracheal muscle. This in vitro study provided data concerning two properties of beta-blockers--intrinsic sympathomimetic and membrane-stabilizing activities--which are involved in the bronchomotor response to these drugs in vivo.