In voltage-clamped guinea-pig Purkinje fibers, removal of acetylcholine (ACh) in the presence of isoproterenol produced a rebound increase in the slow inward current. This effect of ACh was abolished by atropine. These results are consistent with the hypothesis that cessation of the stimulation of myocardial muscarinic receptors transiently leads to enhanced activity of the β-receptor/slow channel system in this tissue.
