© 2000 American Heart Association, Inc.
Cellular Biology |
Ca2+ Transients and Ca2+ Waves in Purkinje Cells
Role in Action Potential Initiation
From the Department of Pharmacology (P.A.B., J. Pu, J. Pinto), Columbia University, New York, NY; and Department of Medicine (H.E.D.J.K.), Physiology and Biophysics, University of Calgary, Calgary, Alberta, Canada.
Correspondence to Dr Penelope A. Boyden, Department of Pharmacology, Columbia College of Physicians and Surgeons, 630 West 168th St, New York, NY 10032.
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 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°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 (L0) compared with resting levels. This was followed by a further increase in fluorescence (L1) 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 L0 were followed by focally arising Ca2+ waves (L2), which propagated at 158±14 µm/s (n=13). Spontaneous Ca2+ waves (L2*) propagated like L2 (164±10 µm/s) occurred between stimuli and caused slow membrane depolarization; 28% of L2* 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 ICaL and release from a subsarcolemmal compartment (L0). Ca2+ release during L0 either induced further Ca2+ release, which propagated toward the cell core (L1), or initiated Ca2+ release from small regions and caused L2 Ca2+ waves, which propagated throughout the aggregate. Spontaneous Ca2+ waves (L2*) induce action potentials.
Key Words: Purkinje fibers • action potentials • Ca2+ • electrophysiology • automaticity
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