Ca2+ transients and Ca2+ waves in Purkinje cells -: Role in action potential initiation

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 Ca2+ transient in Purkinje fiber bundles. We determined the nature of the biphasic Ca2+ 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 degrees C). Membrane action potentials were recorded with fine-tipped microelectrodes, and spatial and temporal changes in [Ca2+](i) were obtained from fluorescent images with an epifluorescent microscope (x20; 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-i) along the edges of the cells. Fluorescence then progressed toward the Purkinje cell core (velocity of propagation 180 to 313 mu m/s). In 62% of the aggregates, initial fluorescent changes of L-0 were followed by focally arising Ca2+ waves (L-2), which propagated at 158+/-14 mu m/s (n=13). Spontaneous Ca2+ waves (L-2*) propagated Like L-2 (164+/-10 mu m/s) occurred between stimuli and caused slow membrane depolarization; 28% of L-2* elicited action potentials. Both spontaneous Ca2+ wave propagation and resulting membrane depolarization were thapsigargin sensitive. Early afterdepolarizations were not accompanied by Ca2+ waves. Action potentials in Purkinje aggregates induced a rapid rise of Ca2+ through I-CaL and release from a subsarcolemmal compartment (L-1), Ca2+ release during L-0 either induced further Ca2+ release, which propagated toward the cell core (L-1), or initiated Ca2+ release from small regions and caused L-2 Ca2+ waves, which propagated throughout the aggregate, Spontaneous Ca2+ waves (L-2*) induce action potentials.