CHARACTERIZATION OF IMPULSE PROPAGATION AT THE MICROSCOPIC LEVEL ACROSS GEOMETRICALLY DEFINED EXPANSIONS OF EXCITABLE TISSUE - MULTIPLE-SITE OPTICAL-RECORDING OF TRANSMEMBRANE VOLTAGE (MSORTV) IN PATTERNED GROWTH HEART CELL-CULTURES

Impulse propagation across sudden expansions of excitable tissue has b een shown to exhibit various forms of conduction disturbance on a macr oscopic scale, ranging from small delays to unidirectional or complete conduction block. With the present study, we attempted to characteriz e systematically the dependence of impulse propagation on the geometry of the underlying excitable tissue on a microscopic scale by investig ating the spatio-temporal pattern of transmembrane voltage changes ass ociated with impulse propagation from a narrow cell strand to a large cell area using multiple site optical recording of transmembrane volta ge (MSORTV) in conjunction with patterned growth of neonatal rat heart cells in culture. While action potential propagation was smooth in th e case of funneled expansions, delays of variable size occurred during propagation into rectangular or incised expansions. Close to the abru pt expansion, which functionally represented an increased electrical l oad to the narrow cell strand, the delays were accompanied by marked d istortions of the action potential upstroke, exhibiting, in extreme ca ses, an initial depolarization to 50% followed by a delayed secondary depolarization to 100% of the full-signal amplitude. These distortions , which were based on bidirectional electrotonic interactions across t he transition, were maximal immediately downstream from the expansion. The maximal slowing of impulse conduction across abrupt expansions wa s, in agreement with recently published results obtained from two-dime nsional computer simulations, always situated in the expanded region. At high stimulation rates, the delays sometimes turned into intermitte nt unidirectional blocks, as revealed by reverse stimulation. These bl ocks were always characterized by a marked abbreviation of the action potentials upstream From the region causing the block which might, in an appropriate network, facilitate reentry because of the associated s hortening of the refractory period. Because the patterns were composed of cells having identical membrane propel ties, the results show that the local action potential shape can be modulated profoundly by the t wo-dimensional architecture of the underlying cell ensemble alone.