Ca 2+ Transients and Ca 2+ Waves in Purkinje Cells

Abstract

Abstract —Purkinje cells contain sarcoplasmic reticulum (SR) directly under the surface membrane, are devoid of t-tubuli, and are packed with myofibrils surrounded by central SR. Several studies have reported that electrical excitation induces a biphasic Ca 2+ transient in Purkinje fiber bundles. We determined the nature of the biphasic Ca 2+ transient in aggregates of Purkinje cells. Aggregates (n=12) were dispersed from the subendocardial Purkinje fiber network of normal canine left ventricle, loaded with Fluo-3/AM , and studied in normal Tyrode’s solution (24°C). Membrane action potentials were recorded with fine-tipped microelectrodes, and spatial and temporal changes in [Ca 2+ ] i were obtained from fluorescent images with an epifluorescent microscope (×20; Nikon). Electrical stimulation elicited an action potential as well as a sudden increase in fluorescence (L 0 ) compared with resting levels. This was followed by a further increase in fluorescence (L 1 ) along the edges of the cells. Fluorescence then progressed toward the Purkinje cell core (velocity of propagation 180 to 313 μm/s). In 62% of the aggregates, initial fluorescent changes of L 0 were followed by focally arising Ca 2+ waves (L 2 ), which propagated at 158±14 μm/s (n=13). Spontaneous Ca 2+ waves (L 2 *) propagated like L 2 (164±10 μm/s) occurred between stimuli and caused slow membrane depolarization; 28% of L 2 * elicited action potentials. Both spontaneous Ca 2+ wave propagation and resulting membrane depolarization were thapsigargin sensitive. Early afterdepolarizations were not accompanied by Ca 2+ waves. Action potentials in Purkinje aggregates induced a rapid rise of Ca 2+ through I CaL and release from a subsarcolemmal compartment (L 0 ). Ca 2+ release during L 0 either induced further Ca 2+ release, which propagated toward the cell core (L 1 ), or initiated Ca 2+ release from small regions and caused L 2 Ca 2+ waves, which propagated throughout the aggregate. Spontaneous Ca 2+ waves (L 2 *) induce action potentials.