Hyperpolarization induced by sodium removal in rabbit sinoatrial node cells: Possible role of electrogenic sodium-calcium exchange

Abstract

Spontaneously active rabbit sinoatrial node (SAN) cells were bathed in K-free solution or in K-free ouabain (20 μM)-containing solution to depress the electrogenic Na + K + pump activity. In SAN cells exposed to K-free solution, the automatic action potentials ceased with gradual depolarization, followed by an eventual steady-state membrane potential of −32 ± 1 mV. Under conditions where the Na + K + pump was blocked, removal of external Na + produced a large and rapid hyperpolarization in the membrane potential and the membrane was hyperpolarized by 23 ± 0.5 mV. When the external Na + was lowered, Na + was replaced by Li +. The Na-free hyperpolarization was not affected by applications of verapamil (4 μM), lidocaine (1 mM), and quinidine (50 μM), but was inhibited by either quinacrine (50 μM) or Cd 2+ (10 mM), which are blockers of Na + Ca 2+ exchange. In the absence of external K +, replacement of external NaCl by sucrose produced a hyperpolarization similar to that seen in the replacement of external Na + by Li +. In the K-free ouabain (20 μM)-containing solution, removal of external Na + also produced a hyperpolarization, and the membrane potential dropped from −29 ± 1 to −48 ± 1 mV. The intracellular acidification due to NH 4Cl removal after exposure to NH 4Cl (20 mM) produced a decrease in Na-free hyperpolarization, which, in the presence of ouabain was inhibited by the application of Cd 2+ (10 mM). Removal of external Ca 2+ nearly completely blocked Na-free hyperpolarization. It can be concluded that Na-free hyperpolarizations are related to the functioning of an electrogenic Na + Ca 2+ exchange mechanism.