Isoprenaline-induced facilitation of the Eustachian tube opening

Isoprenaline was given i.v. to rats and its ability to change the air pressure necessary to open the Eustachian tube (ET) was studied. Air was injected once a minute into the middle ear via a tube glued to a hole in the tympanic bulla. The rate of pressure increase was 0.7 kPa/sec. The air pressure at which the ET opened was in 10 rats 3.4 +/- 0.3 (SE) kPa. Isoprenaline in a dose of 1 microgram/kg decreased this pressure opening level (Pol) by 0.14 +/- 0.03 kPa and with 10 micrograms/kg by 0.22 +/- 0.03 kPa. These effects were reduced by propranolol. Isoprenaline did not act via released adrenaline. A surface tension lowering substance, Tween 20, instilled into the middle ear, decreased Pol by about 0.9 kPa. It is suggested that isoprenaline evokes a decreased Pol via secretion of surface tension lowering substances.