A comparative study of the beta-adrenoceptors of guinea-pig, rabbit and rat trachea.

The sensitivity to isoprenaline of beta inhibitory adrenoceptors of tracheal chain preparations from guinea-pig, rabbit and rat was compared. For this purpose the tracheal chain preparations were contracted either with KCl or pilocarpine. Isoprenaline (1.0 X 10(-7)M--6.4 X 10(-6)M) caused concentration-related relaxation of guinea-pig trachea, whereas the rabbit and the rat trachea remained insensitive even at very high concentrations of isoprenaline (2.5 X 10(-3)M). Phenoxybenzamine (2.6 X 10(-6)M) potentiated or unmasked the relaxant effects of isoprenaline respectively in the guinea-pig or the rabbit and the rat trachea contracted with KCl. Phentolamine was ineffective in potentiating or unmasking the relaxant effects of isoprenaline. The potentiating or unmasking effects of phenoxybenzamine were absent in reserpinized preparations and lidocaine treated preparations from both guinea-pig and rat. Normetanephrine (5.4 X 10(-7)M) potentiated responses to isoprenaline only in the guinea-pig trachea. Even a 10-fold higher concentration of normetanephrine was ineffective in the rabbit or the rat rachea.