Effects of barium on the membrane currents in the rabbit S-A node

In small preparations of rabbit sinoatrial node voltage clamp experiments with the two microelectrode technique were carried out. The effects of extracellular barium ions on the slow inward current and outward currents were studied and the following results were obtained: 1. Ba increased the amplitude of the slow inward current without a change in the time course of inactivation. In 10 mM Ba the steady-state inactivation curve (f infinity) shifted in the positive direction (3-4 mV), suggesting a neutralization of negative surface charges. A similar shift of the steady state activation curve (d infinity) could not be detected. 2. Ba reduced the amplitude of the time-dependent (IK, Ix) and time-independent (Ibg) potassium currents in a concentration-dependent manner. 3. The block of the time-dependent potassium current, IK, depended on the membrane potential. The block was stronger at negative than at positive potentials. The block could be relieved by depolarizing pulses, the degree of unblock increased with longer duration of the depolarizing pulse. 4. Ba blocked the slow outward current Ix in a voltage- and time-dependent manner. Like for IK, the block of Ix was stronger at negative than at positive potentials. A given concentration of Ba produced stronger block of Ix than of IK and the removal of block of Ix by depolarizing pulses was slower than the removal of IK block. 5. The effects of Ba on Ix suggest that this current is a potassium current.