title: Ca(2+) transients and Ca(2+) waves in purkinje cells : role in action potential initiation.
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 degrees 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 (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(1)) along the edges of the cells. Fluorescence then progressed toward the Purkinje cell core (velocity of propagation 180 to 313 microm/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 microm/s (n=13). Spontaneous Ca(2+) waves (L(2)*) propagated like L(2) (164+/-10 microm/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.
authors.affiliation: Department of Pharmacology, Columbia University, New York, NY, USA.
authors.forename: P A
authors.initials: PA
authors.lastname: Boyden
authors.forename: J
authors.initials: J
authors.lastname: Pu
authors.forename: J
authors.initials: J
authors.lastname: Pinto
authors.forename: H E
authors.initials: HE
authors.lastname: Keurs
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D002449:Cell Aggregation / Q000502:physiology; D004285:Dogs; D004558:Electric Stimulation; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D009206:Myocardium / Q000166:cytology; D011690:Purkinje Fibers / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D012016:Reference Values; D019284:Thapsigargin / Q000494:pharmacology
chemical_list: D004791:Enzyme Inhibitors; D019284:Thapsigargin; D002118:Calcium