Relaxant effects of theophylline and clenbuterol on tracheal smooth muscle from horse and rat in vitro.

A comparison between the relaxant effects of clenbuterol and theophylline on horse tracheal smooth muscle has been made in vitro. Rat tracheal smooth muscle was also investigated as a reference. The tracheal preparations were initially contracted with carbachol since the smooth muscle did not spontaneously develop tone. The response of the carbachol-contracted preparations to theophylline was the same in the two species. The response to clenbuterol varied. In only five out of eleven horses were the tracheal smooth muscles sensitive to clenbuterol (mean pD2 = 7.92 M). In the remaining six horses the tracheal smooth muscles were insensitive to clenbuterol (mean pD2 = 3.59 M), yet the preparations responded well to theophylline with complete relaxation. All rat tracheal preparations were insensitive to clenbuterol.