ON THE MECHANISM OF RECTIFICATION OF THE ISOPROTERENOL-ACTIVATED CHLORIDE CURRENT IN GUINEA-PIG VENTRICULAR MYOCYTES

The whole cell configuration of the patch clamp technique was used to investigate the mechanism underlying rectification of the isoproterenol-activated chloride (Cl-) current in isolated guinea pig ventricular myocytes. When extracellular Cl- was replaced with either bromide (Br-), glutamate (Glut), iodide (I-), isethionate (Iseth), or nitrate (NO3-), the magnitude of the shift in reversal potential of the macroscopic current suggested the following selectivity sequence: NO3- > Br- greater-than-or-equal-to Cl- greater-than-or-equal-to I- > Iseth greater-than-or-equal-to Glut. This information was used to investigate the role of permeant ions in rectification of this current. Consistent with previous observations, when the concentration of intracellular Cl- (Cl(i)-) was less than the concentration of extracellular Cl- (Cl(o)-) (40 mM Cl(i)-/150 mM Cl(o)-) the current exhibited outward rectification, but when Cl(i)- was increased to equal that outside (150 Cl(i)-/150 Cl(o)-), the current no longer rectified. Rectification in the presence of asymmetrical concentrations of permeant ions on either side of the membrane is predicted by constant field theory, as described by the Goldman-Hodgkin-Katz current equation. However, when the Cl- gradient was reversed (150 Cl(i)-/40 Cl(o)-) the current did not rectify in the opposite direction, and in the presence of lower symmetrical concentrations of Cl- inside and out (40 Cl(i)-/40 Cl(o)-), outward rectification did not disappear. Reducing Cl(i)- by equimolar replacement with glutamate caused a concentration dependent increase in the degree of rectification. However, when Cl(i)- was replaced with more permeant anions (NO3- and Br-), rectification was not observed. These results can be explained by a single binding site model based on Eyring rate theory, indicating that rectification is a function of the concentration and the permeability of the anions in the intracellular solution.