WITHDRAWAL OF ACETYLCHOLINE ELICITS CA2-INDUCED DELAYED AFTERDEPOLARIZATIONS IN CAT ATRIAL MYOCYTES()

Background Recent experiments in atrial myocytes indicate that withdra wal of cholinergic agonist can directly increase Ca2+ influx via L-typ e Ca2+ current and stimulate Ca2+ uptake into the sarcoplasmic reticul um (SR), thereby increasing intracellular Ca2+. Overload of cellular C a2+ within the SR can initiate various types of atrial dysrhythmias. T he present study was designed to determine whether withdrawal of acety lcholine (ACh) can elicit Ca2+-induced delayed afterdepolarizations (D ADs) in atrial myocytes. Methods and Results A nystatin perforated-pat ch whole-cell method and fluorescence microscopy (indo 1) were used to measure electrical activities and intracellular free Ca2+ ([Ca2+](i)) , respectively. Withdrawal of ACh (1 mu mol/L) increased action potent ial duration? shifted plateau voltage toward positive, and generated D ADs that initiated spontaneous action potentials. Voltage-clamp analys is revealed that withdrawal of ACh elicited a rebound stimulation of L -type Ca2+ current (I-Ca,I-L) (+45%) and Na/Ca exchange current (I-NaC a) (+16%) and the appearance of transient inward current (I-ti) and sp ontaneous [Ca2+](i) transients. Each of these changes induced by withd rawal of ACh was abolished by Rp-cAMPs (50 to 100 mu mol/L) or H-89 (2 mu mol/L), inhibitors of cAMP-dependent protein kinase A. Ryanodine ( 1 mu mol/L) abolished I-NaCa and the appearance of I-ti without decrea sing the rebound stimulation of I-Ca,I-L elicited by withdrawal of ACh . Conclusions Withdrawal of ACh can elicit cAMP-mediated stimulation o f Ca2+ influx via I-Ca,I-L and uptake of SR Ca2+. As a result, cellula r Ca2+ overload causes enhanced SR Ca2+ release and the initiation of DADs. These mechanisms may generate triggered and/or spontaneous atria l depolarizations elicited by withdrawal of vagal nerve activity.