K+ and Cl- contribute to resting membrane conductance of cultured porcine endocardial endothelial cells

The conventional whole cell patch-clamp technique was used to measure the resting membrane conductance and membrane currents of single, nonstimulated, cultured endocardial endothelial cells of the porcine right ventricle in different ionic conditions. All cells displayed the barium-sensitive, inwardly rectifying potassium (K+) current (I-Ki) In 65% of the cells, I-Ki was the predominant membrane current. The mean zero-current potential (V-o) was -61.0 +/- 12.5 mV (+/- SD, n = 45). In 35% of the cells, I-Ki was superposed on an outwardly rectifying (OR) current. V-o of these cells was more depolarized (-33.5 +/- 22.0 mV, n = 26). High intracellular Cl- (122 instead of 52 mmol/l) activated or increased the OR current and shifted V-o in the direction of the equilibrium potential for Cl-. In cells displaying the OR current, V-o was dependent on extracellular Cl-, indicating the contribution of an OR Cl- current in setting V-o. At low intracellular Cl- (6 instead of 52 mmol/l), the OR current was decreased and V-o shifted in the direction of the equilibrium potential for K+. In cells not displaying the OR current, V-o was dependent on extracellular K+ but not on Cl-, indicating major permeability to K+ in these conditions. Block of the OR current by the Cl--channel blockers anthracene-9-carboxylic acid (1 mmol/l), flufenamic acid (100-500 pmol/l), and Zn2+ (100-200 mu mol/l) provided further evidence for the anionic nature of the OR current. After inhibition of I-Ki and the OR Cl- current, a third current component was observed in 50% of the cells. The pharmacology and voltage dependence of this current suggested the presence of Ca2+ activated K+ channels in endocardial endothelial cells. We concluded that the resting membrane conductance of nonstimulated endocardial endothelial cells is mainly determined by the combined activity of inwardly rectifying K+, OR Cl-, and Ca2+-activated K+ channels.