Published online before print February 17, 2005, doi: 10.1161/01.RES.0000159387.00749.3c
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© 2005 American Heart Association, Inc.
Cellular Biology |
Cellular Mechanism of Calcium-Mediated Triggered Activity in the Heart
From The Heart and Vascular Research Center, MetroHealth Campus, and the Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio.
Correspondence to Kenneth R. Laurita, PhD, MetroHealth Campus, Case Western Reserve University, 2500 MetroHealth Dr, Rammelkamp, 6th fl, Cleveland, OH 44109-1998. E-mail klaurita{at}metrohealth.org
Calcium overload due to enhanced calcium entry is a mechanism for spontaneous calcium release (SCR) from the sarcoplasmic reticulum, delayed-afterdepolarizations (DAD), and triggered activity. However, the exact mechanistic relationship between elevated intracellular calcium levels and triggered activity originating from a specific location remains unclear. We hypothesize that under conditions of enhanced calcium entry, elevation of intracellular calcium will result in multiple calcium release events of which only one is more likely to initiate a triggered beat. We used optical mapping of action potentials and ratiometric calcium transients in an electromechanically-uncoupled canine wedge model of enhanced calcium entry, using IKs blockade with ß-adrenergic stimulation. Under conditions of enhanced calcium entry, the rate of calcium uptake was faster compared with control conditions; however, during rapid pacing, cytoplasmic calcium elevation at the endocardium was significantly increased (15±4%) compared with control (10±3, P<0.04). Rapid pacing induced multiple simultaneous SCR events with largest amplitude and earliest onset near the endocardium compared with the epicardium. Furthermore, SCR events with largest amplitude and earliest onset served as a focus for DAD-mediated triggered activity. Interestingly, polymorphic VT occurred in some experiments when multiple SCR events occurred. In conclusion, multiple, simultaneous SCR events occur over a broad region of relatively slower calcium uptake and elevated diastolic calcium levels. However, SCR events closer to the endocardium have the largest amplitude and earliest onset and are, thereby, more likely to initiate DAD-mediated triggered activity. Finally, multiple SCR events may be a mechanism of polymorphic VT under calcium overload conditions.
Key Words: intracellular calcium • delayed afterdepolarization • triggered activity • optical mapping • arrhythmias
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