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LIVER AND BILIARY TRACT

Calcium waves in intact guinea pig gallbladder smooth muscle cells

Departments of ¹Anatomy and Neurobiology and ²Pharmacology, University of Vermont, Burlington, Vermont

Submitted 20 January 2006 ; accepted in final form 21 April 2006

Intracellular Ca²⁺ waves and spontaneous transient depolarizations were investigated in gallbladder smooth muscle (GBSM) whole mount preparations with intact mucosal layer [full thickness (FT)] by laser confocal imaging of intracellular Ca²⁺ and voltage recordings with microelectrodes, respectively. Spontaneous Ca²⁺ waves arose most often near the center, but sometimes from the extremities, of GBSM cells. They propagated regeneratively by Ca²⁺-induced Ca²⁺ release involving inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] receptors and were not affected by TTX and atropine (ATS). Spontaneous Ca²⁺ waves and spontaneous transient depolarizations were more prevalent in FT than in isolated muscularis layer preparations and occurred with similar pattern in GBSM bundles. Ca²⁺ waves were abolished by the Ins(1,4,5)P₃ receptor inhibitors 2-aminoethoxydiphenyl borate and xestospongin C and by caffeine and cyclopiazonic acid. These events were reduced by voltage-dependent calcium channels (VDCCs) inhibitors diltiazem and nifedipine, by PLC inhibitor U-73122, and by thapsigargin and ryanodine. ACh, CCK, and carbachol augmented Ca²⁺ waves and induced Ca²⁺ flashes. The actions of these agonists were inhibited by U-73122. These results indicate that in GBSM, discharge and propagation of Ca²⁺ waves depend on sarco(endo)plasmic reticulum (SR) Ca²⁺ release via Ins(1,4,5)P₃ receptors, PLC activity, Ca²⁺ influx via VDCCs, and SR Ca²⁺ concentration. Neurohormonal enhancement of GBSM excitability involves PLC-dependent augmentation and synchronization of SR Ca²⁺ release via Ins(1,4,5)P₃ receptors. Ca²⁺ waves likely reflect the activity of a fundamental unit of spontaneous activity and play an important role in the excitability of GBSM.

motility; sarcoplasmic reticulum; calcium transients; transient depolarizations; pacemaker; slow waves; action potentials

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