Withdrawal of acetylcholine elicits Ca2+-induced delayed afterdepolarizations in cat atrial myocytes.
BACKGROUND
Recent experiments in atrial myocytes indicate that withdrawal of cholinergic agonist can directly increase Ca2+ influx via L-type Ca2+ current and stimulate Ca2+ uptake into the sarcoplasmic reticulum (SR), thereby increasing intracellular Ca2+. Overload of cellular Ca2+ within the SR can initiate various types of atrial dysrhythmias. The present study was designed to determine whether withdrawal of acetylcholine (ACh) can elicit Ca2+-induced delayed afterdepolarizations (DADs) in atrial myocytes.


RESULTS
A nystatin perforated-patch 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 micromol/L) increased action potential duration, shifted plateau voltage toward positive, and generated DADs that initiated spontaneous action potentials. Voltage-clamp analysis revealed that withdrawal of ACh elicited a rebound stimulation of L-type Ca2+ current (I(Ca,L)) (+45%) and Na/Ca exchange current (I(NaCa)) (+16%) and the appearance of transient inward current (I(ti)) and spontaneous [Ca2+]i transients. Each of these changes induced by withdrawal of ACh was abolished by Rp-cAMPs (50 to 100 micromol/L) or H-89 (2 micromol/L), inhibitors of cAMP-dependent protein kinase A. Ryanodine (1 micromol/L) abolished I(NaCa) and the appearance of I(ti) without decreasing the rebound stimulation of I(Ca,L) elicited by withdrawal of ACh.


CONCLUSIONS
Withdrawal of ACh can elicit cAMP-mediated stimulation of Ca2+ influx via I(Ca,L) and uptake of SR Ca2+. As a result, cellular Ca2+ overload causes enhanced SR Ca2+ release and the initiation of DADs. These mechanisms may generate triggered and/or spontaneous atrial depolarizations elicited by withdrawal of vagal nerve activity.
Wang
Y G
YG
Loyola University of Chicago, Stritch School of Medicine, Department of Physiology, Maywood, Ill 60153, USA.
Hüser
J
J
Blatter
L A
LA
Lipsius
S L
SL
D000109:Acetylcholine / Q000494:pharmacology*; D000200:Action Potentials / Q000187:drug effects*; D000818:Animals; D016275:Atrial Function; D002118:Calcium / Q000378:metabolism*; D002415:Cats; D005260:Female; D006325:Heart Atria / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D008297:Male; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism
D000109:Acetylcholine; D002118:Calcium