Segmental electrical uncoupling and conduction blocks after calcium removal and replacement in a mammalian auricle

The cell-to-cell electrical conduction has been investigated in control conditions, during calcium depletion and after calcium repletion. When rat auricular strips are bathed in a Ca2+-free, EGTA-containing (5 mM) solution, the resting membrane potential slowly decreases to about -35 mV within 20 min. The electrotonic spread of intracellular current pulses remains similar to that observed in control conditions, with length constants of about 215 microns in the fibre direction and 52 microns perpendicular to it. Restoration of calcium ions to the bathing fluid at 37 degrees C induces an irreversible loss of the all-or-none electrical conduction of the action potential, and the auricular fibres become split up into aggregates of electrically coupled cells delimited by border zones where the electrical coupling and the conduction of action potentials are interrupted. Inside each of those islets the resting membrane potential is uniform, but it may vary abruptly (between about -10 and -80 mV) across the border of two islets. Islets with sufficient levels of membrane potential (less than -60 mV) can generate action potentials that do not propagate to adjacent islets. This fragmentation of the cardiac tissue into electrically independent subunits explains the irreversible loss of the propagated electro-mechanical activity (calcium paradox) that is observed after calcium repletion.