The recovery of resting potential and input resistance in sheep heart injured by knife or laser.

1. A lesion 100 mu in diameter with well-defined boundaries was made with a laser in Purkinje fibres from sheep hearts. The membrane potential and the input resistance recorded in the intact tissue at about 0.5 mm from the edge of the lesion were found to drop at the instant of injury. The corresponding decrease of input resistance fits quantitatively the transmission line theory applied to a cable terminated by a short circuit at the lesion.2. The input resistance and the membrane potential were found to rise simultaneously during healing-over. The membrane potential returned to its original level within 1 min. By then, the input resistance had settled either to the same value as before injury, or to a higher value matching quantitatively the theoretical resistance of a cable terminated by an infinite resistance at the lesion.3. Neither membrane potential nor input resistance recovered in calcium-free solutions. But healing-over rapidly occurred when calcium was added to solutions that might otherwise differ widely in composition.4. The transmission line model fits all observations if it is assumed that the injury causes a leak or short circuit in the ;cable' that is soon closed, or rendered open circuit, by the development of a new diffusion barrier in the presence of calcium ions.