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 isoprotereno l-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 macroscopi c current suggested the following selectivity sequence: NO3- > Br- gre ater-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 ro le 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)-) (4 0 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 presenc e of asymmetrical concentrations of permeant ions on either side of th e 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 concentr ations of Cl- inside and out (40 Cl(i)-/40 Cl(o)-), outward rectificat ion did not disappear. Reducing Cl(i)- by equimolar replacement with g lutamate caused a concentration dependent increase in the degree of re ctification. However, when Cl(i)- was replaced with more permeant anio ns (NO3- and Br-), rectification was not observed. These results can b e 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.