FIG. 16.

Role of Ca²⁺ inward current, I_Ca,L, in discontinuous propagation. Panel (a) depicts in the top part anterograde propagation from a small strand (light photosensitive diodes) into a bulk area (dark diodes) and in the bottom part retrograde propagation from the bull into the small strand. Signals are obtained from the fluorescence change of a voltage-sensitive dye in patterned neonatal rat myocytes. Panel (b) shows propagation during control. During anterograde propagation, the source-to-load mismatch produces a conduction delay at the transition of about 2 ms. During retrograde propagation, this delay is absent, because the cells in the strand receive excitatory current from a large bulk. Instead, the cells in the strand are excited almost simultaneously. Panel (c) shows the effect of superfusion of the small strand with Nifedipine, a blocker of I_Ca,L. Propagation from the small strand into the bulk becomes totally blocked. This demonstrates that I_Ca,L is needed to drive propagation across the geometrical expansion. The fact that Nifedipine has no effect on propagation in reverse direction proves that the source-to-load mismatch, a structural component, requires flow of I_Ca,L. Reproduced with permission from S. Rohr et al., Biophys. J. 72, 2 (1997). Copyright 1997 Elsevier.⁸⁶