Isolated canine atrial plateau fibers were treated with acetylcholine or norepinephrine to note the effects on the transmembrane potential. Acetylcholine, 1.0 or 2.0 mug/ml, consistently reduced the slope of inherent phase 4 depolarization. Increases in maximum diastolic potential and rising velocity occurred along with a decrease in overshoot. The plateau phase disappeared. Pretreatment with atropine, 1.0 mug/ml, prevented these responses, and alone this drug had no discernible effect. Norepinephrine consistently increased the slope of phase 4 depolarization. Frequently plateau fibers generated action potentials by the normal pacemaker mechanism. "Arrhythmias" characterized by spontaneous excitations were induced in 92% of the norepinephrine experiments. Norepinephrine also enhanced the plateau phase of the action potential and decreased the rising velocity and overshoot. Racemic propranolol, 1.0 mug/ml, blocked all the above effects including arrhythmias. Dextropropranolol, 1.0 mug/ml, did not block effects produced by norepinephrine. Acetylcholine, applied to fibers under treatment with norepinephrine, reduced the slope of norepinephrine-induced phase 4 depolarization and terminated induced arrhythmias.
- Copyright © 1975 by American Physiological Society