Effect of noradrenaline on the action of nicotine and tyramine on isolated atria.

Both nicotine (in the presence of atropine) and tyramine cause a rise in the rate of isolated rabbit atria. When noradrenaline is allowed to act on the atria for 20 to 30 min and then removed from the bath by repeated changes of the bath fluid, the action of nicotine and of tyramine is greatly increased. The first addition of noradrenaline to the bath often has a much smaller effect on the response to nicotine or to tyramine than have later additions. Sometimes the greater effect after the addition of nicotine persists for 2 or 3 hr. When reserpine is added to the bath and left in contact with the atria for 1 to 2 hr, and is then removed from the bath, the effect of noradrenaline on the atrial rate is much reduced in size and duration. Bretylium abolishes the action of nicotine but increases the action of tyramine.