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Departments of 1 Biomedical Engineering, 2 Medicine, and 3 Physiology, University of Alabama at Birmingham, Birmingham, Alabama 35294; and 4 Department of Physiology, University of Bern, 3012 Bern, Switzerland
Organization of cardiac tissue into cell strands and
layers has been implicated in changes of transmembrane potential
(
Vm) during defibrillation. To determine the
shock-induced Vm in such structures, cell
strands of variable width [strand width (SW) = 0.15-2
mm] were grown in culture. Uniform-field shocks with variable
strength [shock strength (SS) = 2-50 V/cm] were
applied across strands during the action potential (AP) plateau, and
Vm were measured optically. Three different
types of Vm were observed. Small
Vm [<40%AP amplitude (APA)] were
linearly dependent on SS and SW and were symmetrically distributed
about a strand centerline with maximal positive and negative
Vm on opposite strand sides being equal.
Intermediate Vm (<200%APA) were strongly
asymmetric with negative Vm > positive
Vm because of a negative time-dependent shift of
Vm at the depolarized side of the strands. For
large Vm (>200%APA), a second time-dependent
shift of Vm to more positive levels was observed in
the hyperpolarized portions of strands, causing reduction of the
Vm asymmetry. We conclude that during application of shocks to cell strands during the AP plateau, passive changes of Vm were followed by two voltage- and
time-dependent shifts of Vm, possibly reflecting
changes of ionic currents or membrane electroporation.
stimulation; optical mapping; voltage-sensitive dyes; cell cultures
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