Virtual sources associated with linear and curved strands of cardiac cells.

Transmembrane potential (V(m)) responses in cardiac strands with different curvature were characterized during uniform electric-field stimulation with the use of modeling and experimental approaches. Linear and U-shaped strands (width 100-150 micrometer) were stained with voltage-sensitive dye. V(m) was measured by optical mapping across the width and at sites of beginning curvature. Field pulses were applied transverse to the strands during the action-potential plateau. For linear strands, V(m) contained 1) a rapid passive component (V(m)(ar)) nearly linear and symmetric across the width, 2) a slower hyperpolarizing component (V(m)(as)) greater and faster on the anodal side, and 3) at high field strengths a delayed depolarizing component (V(m)(ad)) greater on the anodal side. For U-shaped strands, V(m) at sites of beginning curvature also contained rapid and slow components (V(m)(br) and V(m)(bs), respectively) that included contributions from the linear strand response and from the fiber curvature. V(m)(ar), V(m)(br), and part of V(m)(bs) could be attributed to passive behavior that was modeled, and V(m)(as), V(m)(ad), and part of V(m)(bs) could be attributed to active membrane currents. Thus curved strands exhibit field responses separable into components with characteristic amplitude, spatial, and temporal signatures.