Anisotropy of electrotonus in the sinoatrial node of the rabbit heart

In fibers of the sinoatrial node of isolated right atria of rabbits the decay of the electrotonic potential caused by intracellular current injection was measured in two directions: parallel to the crista terminalis and perpendicular to it. A K⁺-perfused extracellular suction electrode was used to apply current pulses (10^-5 A, 60 ms) to fibers located in the primary center of the SA node every fourth cardiac cycle at a fixed moment during diastole. The decay of the electrotonic spread was measured in a series of impalements on a straight line from the current source. Space constants were calculated by fitting single exponential curves to the data. Considerable regional differences in space constant values were found in either direction. Parallel to the crista terminalis the mean value was 529 ± 446 μm (s.d., n = 7), perpendicular to it 306 ± 295 μm (n = 12); the difference was not significant (P < 0.2). However, a significant anisitropy (P < 0.05) of the electrotonic spread was found when measurements were taken from small areas of the node. Large abrupt changes in the electrotonic potential within 200 μm were observed in the center of the node. These data indicate a non-uniformity of electrotonic spread in this part of the SA node.