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1 Department of Anatomy and Neurobiology, University of Vermont, Burlington, VT, USA
* To whom correspondence should be addressed. E-mail: gary.mawe{at}uvm.edu.
Spontaneous action potentials and Ca2+ transients were investigated in intact gallbladder preparations to determine how electrical events propagate and cellular mechanisms that modulate these events. Rhythmic phasic contractions were preceded by Ca2+ flashes that were either focal, being limited to one or a few bundles, multifocal, occurring asynchronously in several bundles, or global, with simultaneous flashes throughout the field. Ca2+ flashes and action potentials were abolished by inhibiting the sarcoplasmic reticulum (SR) Ca2+ release via IP3 channels with 2-APB and xestospongin C, or by inhibiting voltage dependent Ca2+ channels (VDCC) with nifedipine or diltiazem or nisoldipine. Inhibiting ryanodine channels with ryanodine caused multiple spikes superimposed upon plateaus of action potentials and extended quiescent periods. Depletion of SR Ca2+ stores with thapsigargin or cyclopiazonic acid increased the frequency and duration of Ca2+ flashes and action potentials. Acetylcholine, carbachol or cholecystokinin increased the frequency of and synchronized Ca2+ flashes and action potentials. The PLC inhibitor U-73122, did not affect Ca2+ flash or action potential activity, but inhibited the excitatory effects of acetylcholine on these events. These results indicate that Ca2+ flashes correspond to the action potentials, and that rhythmic excitation in the gallbladder is multifocal amongst GBSM bundles and can be synchronized by excitatory agonists. These events do not depend on PLC activation, but agonist stimulation involves activation of PLC. Generation of these events depends on Ca2+ entry via VDCC and on Ca2+ mobilization from SR via IP3 channels.
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