Abstract
Effects of acetylcholine (ACh) and norepinephrine on atrial contractility and pacemaker activity were investigated in isolated and blood-perfused canine atrium preparations with a support dog which were suspended in the blood-filled bath kept at 37°C. The drug was given in two forms of administration, i.e., intraarterial injection into the cannulated sinus node artery or direct administration into the bath. ACh administered into the bath produced a significant decrease in the developed tension from a concentration of 10-5 g, 'ml and norepinephrine produced a significant increase in the developed tension from 3×10-6 g/ml. An injection via the sinus node artery resulted in 300 and 100 times greater response to ACh and norepinephrine, respectively, in the tension development. In atrial pacemaker activity, ACh given into the bath did not produce a dose-related decrease while norepinephrine produced a dose-related increase frequently accompanied by an irregularity of rhythmicity.
