Evidence for prejunctional and postjunctional antagonism of the sympathetic neuroeffector junction by acetylcholine in canine cardiac Purkinje fibers

This study determined whether acetylcholine inhibited changes in action potential duration and developed tension in canine Purkinje fibers produced by norepinephrine released during field stimulation more than it inhibited changes produced by superfusion with norepinephrine. Field stimulation increased action potential duration measured at 75% of repolarization by 9.6 ms and increased active tension by 35.5%. Acetylcholine (10(-6) M) almost completely inhibited changes in action potential duration and developed tension during field stimulation. Norepinephrine (10(-8) M to 5 X 10(-8) M) superfusion during normal pacing increased developed tension by 40.6%. Acetylcholine (10(-6) M) reduced this increase to 19.7%. Thus, acetylcholine inhibited the increase in tension during field stimulation by 90.7%, whereas it inhibited equivalent increases in tension produced by superfusion with norepinephrine by only 51.4% (p less than 0.01). These data suggest that acetylcholine antagonizes the effects of adrenergic stimulation by pre- and postjunctional inhibitory actions in canine cardiac Purkinje fibers.