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J Neurophysiol 82: 540-550, 1999;
0022-3077/99 $5.00
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The Journal of Neurophysiology Vol. 82 No. 2 August 1999, pp. 540-550
Copyright ©1999 by the American Physiological Society

Ca2+-Induced Ca2+ Release Activates Spontaneous Miniature Outward Currents (SMOCs) in Parasympathetic Cardiac Neurons

Laura A. Merriam, Fabiana S. Scornik, and Rodney L. Parsons

Department of Anatomy and Neurobiology, University of Vermont College of Medicine, Burlington, Vermont 05405

Merriam, Laura A., Fabiana S. Scornik, and Rodney L. Parsons. Ca2+-Induced Ca2+ Release Activates Spontaneous Miniature Outward Currents (SMOCs) in Parasympathetic Cardiac Neurons. J. Neurophysiol. 82: 540-550, 1999. Mudpuppy parasympathetic cardiac neurons exhibit spontaneous miniature outward currents (SMOCs) that are thought to be due to the activation of clusters of large conductance Ca2+-activated K+ channels (BK channels) by localized release of Ca2+ from internal stores close to the plasma membrane. Perforated-patch whole cell recordings were used to determine whether Ca2+-induced Ca2+ release (CICR) is involved in SMOC generation. We confirmed that BK channels are involved by showing that SMOCs are inhibited by 100 nM iberiotoxin or 500 µM tetraethylammonium (TEA), but not by 100 nM apamin. SMOC frequency is decreased in solutions that contain 0 Ca2+/3.6 mM Mg2+, and also in the presence of 1 µM nifedipine and 3 µM omega -conotoxin GVIA, suggesting that SMOC activation is dependent on calcium influx. However, Ca2+ influx alone is not sufficient; SMOC activation is also dependent on Ca2+ release from the caffeine- and ryanodine-sensitive Ca2+ store, because exposure to 2 mM caffeine consistently caused an increase in SMOC frequency, and 10-100 µM ryanodine altered the configuration of SMOCs and eventually inhibited SMOC activity. Depletion of intracellular Ca2+ stores by the Ca-ATPase inhibitor cyclopiazonic acid (10 µM) inhibited SMOC activity, even when Ca2+ influx was not compromised. We also tested the effects of the membrane-permeable Ca2+ chelators, bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid-AM (BAPTA-AM) and EGTA-AM. EGTA-AM (10 µM) caused no inhibition of SMOC activation, whereas 10 µM BAPTA-AM consistently inhibited SMOCs. After SMOCs were completely inhibited by BAPTA, 3 mM caffeine caused SMOC activity to resume. This effect was reversible on removal of caffeine and suggests that the source of Ca2+ that triggers the internal Ca2+ release channel is different from the source of Ca2+ that activates clusters of BK channels. We propose that influx of Ca2+ through voltage-dependent Ca2+ channels is required for SMOC generation, but that the influx of Ca2+ triggers CICR from intracellular stores, which then activates the BK channels responsible for SMOC generation.




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