Non‐selective cation current of guinea‐pig endocardial endothelial cells.

1. Endocardial endothelial (EE) cells, isolated by the enzymatic treatment of guinea‐pig heart, were used for whole‐cell voltage clamp experiments. 2. The inward rectifier K+ current was observed in about half of the experiments. The contribution of Ca(2+)‐dependent K+ current to the resting membrane conductance was also suggested. 3. After the K+ conductances were suppressed, removal of external Na+ revealed an inward cation current (1.2 pA pF‐1, at ‐45 mV), whose slope conductance was a saturable function of external Na+ concentration. When Na+ was totally replaced by various monovalent cations, the order of the membrane conductances was K+ > Rb+ > Cs+ > Na+ > Li+. 4. This basal non‐selective cation current was blocked by either Gd3+ or La3+, and showed slight outward rectification. 5. Addition of 20 mM Ca2+ or Ba2+, but not Mg2+ or Mn2+, to the Na(+)‐free solution, induced an inward current, indicating that this current possesses a significant Ca2+ permeability. 6. In approximately 15% of the experiments, ATP and histamine induced another type of non‐selective cation current, which showed different ion selectivity (Na+ > K+, Cs+) and rectification (inward). 7. The basal non‐selective cation current is responsible for both the low resting potential and the leak Ca2+ influx of EE cells.