Atropine is a clinically relevant
anticholinergic drug, which blocks inhibitory effects of the parasympathetic
neurotransmitter acetylcholine on heart rate leading to
tachycardia. However, many cardiac effects of
atropine cannot be adequately explained solely by its antagonism at
muscarinic receptors. In isolated mouse ventricular cardiomyocytes expressing a Förster resonance energy transfer (FRET)-based cAMP biosensor, we confirmed that
atropine inhibited
acetylcholine-induced decreases in cAMP. Unexpectedly, even in the absence of
acetylcholine, after
G-protein inactivation with
pertussis toxin or in myocytes from M2- or M1/3-
muscarinic receptor knockout mice,
atropine increased cAMP levels that were pre-elevated with the β-
adrenergic agonist isoproterenol. Using the FRET approach and in vitro
phosphodiesterase (PDE) activity assays, we show that
atropine acts as an allosteric PDE type 4 (
PDE4) inhibitor. In human atrial myocardium and in both intact wildtype and M2 or M1/3-receptor knockout mouse Langendorff hearts,
atropine led to increased contractility and heart rates, respectively. In vivo, the
atropine-dependent prolongation of heart rate increase was blunted in PDE4D but not in wildtype or PDE4B knockout mice. We propose that inhibition of PDE4 by
atropine accounts, at least in part, for the induction of
tachycardia and the arrhythmogenic potency of this drug.