Bursting calcium rotors in cultured cardiac myocyte monolayers

Abstract

Rotating waves (rotors) of cellular activity were observed in nonconf luent cultures of embryonic chick heart cells by using a macroscopic imaging system that detected f luorescence from intracellular Ca 2؉ . Unlike previous observations of rotors or spiral waves in other systems, the rotors did not persist but exhibited a repetitive pattern of spontaneous onset and offset leading to a bursting rhythm. Similar dynamics were observed in a cellular automaton model of excitable media that incorporates spontaneous initiation of activity, and a decrease of excitability as a consequence of rapid activity (fatigue). These results provide a mechanism for bursting dynamics in normal and pathological biological processes.Since early studies demonstrated that bursting rhythms could originate from single cells (1), theoretical analyses of bursting have focused on cellular ionic mechanisms (2-5). It has been difficult to assess what contribution spatial patterns of activation within groups of participating cells have on the dynamics of a burst. This work was initiated to determine the spatial patterns of activation occuring in dispersed cultures of heart cells. Previous studies of excised slices of cardiac tissue demonstrated sustained rotors initiated by electrical stimulation and proposed that rotors might be a mechanism for abnormally rapid cardiac arrhythmias (tachycardias) (6, 7). Although tachycardias in people can be initiated by electrical stimulation, they usually start spontaneously. In most instances, the rhythms also terminate spontaneously. On occasion there is a repeated spontaneous paroxysmal initiation and termination of tachycardias (8,9). In this paper, we demonstrate repeated spontaneous initiation and termination of rotors in cultured cardiac myocytes.These phenomena are modeled by using a cellular automaton. In a cellular automaton, there is a lattice of sites with discrete states and a synchronous updating rule that is identical for all sites. Cellular automata have been used as simplified models for a large range of phenomena, including neural networks (10), cardiac arrhythmias (11), oscillating chemical reactions (12), and forest fires (13). However, none of these previous formulations display the repeated onset and offset of rotors observed here. METHODSCell Culture. Ventricular cells from 7-day embryonic chick ventricular myocytes were isolated as described (14). Cells were plated at just under confluent densities of 7.0-14.0 ϫ 10 3 cells per cm 2 within 12-mm diameter glass retaining rings on lysine-coated plastic cell culture dishes. Cells were kept in maintenance medium 818a at 5% CO 2 at 36°C for 1-2 days before experiments. Monolayers were loaded with calciumsensitive dye fluo-3 (5 M, Molecular Probes) (15) in Hanks' balanced salt solution (HBSS, 1.3 mM K) for 25-30 min, washed with fresh HBSS to remove excess dye, and then transferred to the imaging setup. Experiments were conducted at 36°Ϯ 1°C.Fluorescent Imaging. A macroscope was constructed (16) to allow low light level measurements...

Complex-periodic spiral waves in confluent cardiac cell cultures induced by localized inhomogeneities

“…The existence of cardiac spiral waves is now well confirmed through several different in vitro (22)(23)(24)(25)(26) as well as in situ (7,15,18) preparations. Stable two-armed cardiac spirals, each arm being evenly separated from the other, were also recently reported (27).…”

“…Subsequently, revealing and understanding any intermediate states relating the two extremes have become important. Some examples along this line of investigation are bursting rhythms (23,28) and alternans (29)(30)(31)(32).…”

Contraction-induced cluster formation in cardiac cell culture

“…This is because, compared to experiments using an intact heart, a variety of methods can be employed with relative ease to visualize and modify the dynamical activity of cells. Taken together, these advantages enabled us to explore the nature of the complex dynamics that arise in cell cultures [1,2,3,4,5,6].…”

Excito-oscillatory dynamics as a mechanism of ventricular fibrillation

“…Unfortunately, the existing cardiac monolayers are comprised of neonatal cells that exhibit pacemaker activity, [43] and large spatial heterogeneities are required to establish a coexistence of target patterns and spiral waves. [44,45] In addition, discerning target patterns resulting from pacemaker activity and those arising from the unstable equilibrium point at elevated V m levels from fluorescence recordings (which are not calibrated to absolute V m levels) remains a formidable challenge. Third, although excitable media do not exhibit target patterns in 2-D, target patterns on the heart surface can result from intramural excitable scroll waves in 3-D.…”