Phase dependency of electrotonic spread of hyperpolarizing current pulses in the rabbit sinoatrial node

Electrotonic current spread in the SA node of the rabbit was measured by means of hyperpolarizing current pulses (1 to 10 μA, 60 ms), which were injected intracellularly through a K⁺-perfused suction electrode. The pulses were applied at the beginning, middle or end of the diastolic depolarization phase. The resulting membrane potential change of nodal fibers was measured with microelectrodes. Space constants were calculated by fitting single exponential curves to the data. The input resistance (R_in) of fibers at different sites in the SA node was measured by means of a double barrel microelectrode (current pulses 5.5 to 11 nA, 60 ms) to detect a change in the internal resistance during the diastolic depolarization phase. During diastole the average electrotonic potential increased by 30% (P < 0.001), the increase of the space constant ranged from 9 to 183% (P < 0.05). R_in however, did not change during diastole. It is concluded that the electrotonic spread increased phase dependently, due to an increase of membrane resistance; the internal resistance was not phase dependent.