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Subcellular Ca²⁺ alternans represents a novel mechanism for the generation of arrhythmogenic Ca²⁺ waves in cat atrial myocytes

Jens Kockskämper and Lothar A. Blatter

Ca²⁺ alternans is a potentially arrhythmogenic beat-to-beat alternation of the amplitude of the action potential-induced [Ca²⁺]_i transient in cardiac myocytes. Despite its pathophysiological significance the cellular mechanisms underlying Ca²⁺ alternans are poorly understood. Recent evidence, however, points to the modulation of Ca²⁺-induced Ca²⁺ release (CICR) from the sarcoplasmic reticulum (SR) by localized alterations in energy metabolism as an important determinant of Ca²⁺ alternans. We therefore studied the subcellular properties of Ca²⁺ alternans in field-stimulated cat atrial myocytes employing fast two-dimensional fluorescence confocal microscopy. Ca²⁺ alternans was elicited by an increase in stimulation frequency or by metabolic interventions targeting glycolysis. Marked subcellular variations in the time of onset, the magnitude, and the phase of alternans were observed. Longitudinal and transverse gradients of Ca²⁺ alternans were found as well as neighbouring subcellular regions alternating out-of-phase. Moreover, focal inhibition of glycolysis resulted in spatially restricted Ca²⁺ alternans. When two adjacent regions within a myocyte alternated out-of-phase, steep [Ca²⁺]_i gradients developed at their border giving rise to delayed propagating Ca²⁺ waves. The results demonstrate that Ca²⁺ alternans is a subcellular phenomenon caused by modulation of SR Ca²⁺ release, which is mediated, at least in part, by local inhibition of energy metabolism. The generation of arrhythmogenic Ca²⁺ waves by subcellular variations in the phase of Ca²⁺ alternans represents a novel mechanism for the development of atrial disrhythmias.

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