Purkinje cells contain sarcoplasmic reticulum (SR) directly under the surfa
ce membrane, are devoid of t-tubuli, and are packed with myofibrils surroun
ded 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 ne
twork of normal canine left ventricle, loaded with Fluo-3/AM, and studied i
n 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 potentia
l 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 c
ell core (velocity of propagation 180 to 313 mu m/s). In 62% of the aggrega
tes, initial fluorescent changes of L-0 were followed by focally arising Ca
2+ 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 stimu
li and caused slow membrane depolarization; 28% of L-2* elicited action pot
entials. Both spontaneous Ca2+ wave propagation and resulting membrane depo
larization were thapsigargin sensitive. Early afterdepolarizations were not
accompanied by Ca2+ waves. Action potentials in Purkinje aggregates induce
d a rapid rise of Ca2+ through I-CaL and release from a subsarcolemmal comp
artment (L-1), Ca2+ release during L-0 either induced further Ca2+ release,
which propagated toward the cell core (L-1), or initiated Ca2+ release fro
m small regions and caused L-2 Ca2+ waves, which propagated throughout the
aggregate, Spontaneous Ca2+ waves (L-2*) induce action potentials.