Rapid Communications Anisotropic Conduction in Monolayers of Neonatal Rat Heart Cells Cultured

Anisotropic impulse conduction was studied in neonatal rat heart cell monolayers produced by culturing cells on a growth-directing substrate of collagen. Monolayers consisting of parallel-oriented cells without visible intercellular clefts were selected for experiments; cell lengths and widths were 65.8±+12.5 and 12.2+3.2 gm (n=49), respectively. Action potential upstrokes were measured by using 12 photodiodes selected within a lOx10 diode array and a voltage-sensitive dye (RH-237). The size of the area sensed by a single diode was 14x 14 gim. High-density multiple recordings (resolution, up to 15 gim) demonstrated the variability of local activation delays and of the maximal rate of rise of the action potential upstroke (Vm.), which are presumably related to the microscopic cellular architecture. Mean macroscopic conduction A nisotropy of cardiac muscle plays an important role in the propagation of excitation waves and in the initiation and maintenance of cardiac arrhythmias. Sano et all presented the first evidence that tissue anisotropy caused directional differences in impulse spread with fast velocity in a direction parallel to fiber orientation and slow velocity in a direction perpendicular to the fiber axis. Later, Clerc2 demonstrated that intracellular resistance in ventricular trabecula was 10 times larger in a direction transverse to fiber orientation than in a longitudinal direction. In accordance with continuous cable theory, conduction velocity was three times larger in the longitudinal than in the transverse direction in the same experiments. More recently Spach et a13 presented results on anisotropic propagation that could not be explained by continuous cable theory. They found that the maximal rate of rise of the action potential upstroke (Vm:) was larger in the transverse than in the longitudinal direction. Moreover, premature impulses propagating in the longitudinal direction were blocked, while they continued to propagate in the transverse direction. Both the higher transverse Vma and the dependence of block on fiber orientation were interpreted as reflecting a higher safety factor for transverse than for longitudinal propagation. The subject of directional differences in Vm,, and the occurrence of unidirectional conduction block was addressed in a large number of studies.4-12 Received March 30, 1994; accepted June 10, 1994. From the Department of Physiology, University of Berne (Switzerland). Correspondence to V.G. Fast, PhD, Department of Physiology, University of Berne, Buihlplatz 5, CH-3012, Berne, Switzerland. ©3 1994 American Heart Association, Inc. velocities measured over distances of 135 gm were 34.6+4.5 and 19.0+4.3 cm/s (mean+SD, n=13, P<.0001) in longitudinal and transverse directions, respectively. The anisotropic velocity ratio was 1.89+0.38 (n=13). Mean Vmna was not significantly different in two directions (122.0+17.4 V/s longitudinally versus 125.2±15.6 V/s transversely, n= 13, P=NS). In conclusion, we developed an anisotropic cell culture model suitable for studying impulse conduction with cellular resolution. The anisotropic velocity ratio was close to values measured in vivo. By contrast, Vmax was not dependent on the direction of propagation. (Circ Res. 1994;75:591-595.)