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Abstract
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 μg/kg decreased this pressure opening level (Pol) by 0.14±0.03 kPa and with 10 μg/kg by 0.22±0.03 kPa. These effects were reduced by propranolol. Isoprenaline did not act via released adrenalin. 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.