Grain boundaries (GBs) in high-temperature superconductors represent the major factor limiting high current applications. Via magneto-optical imaging and direct transport measurements, we observed an increase of the intergranular Jc in 4° and 8°  low-angle grain boundaries via substitution of Y by Ca in YBa2Cu3O7−x by up to 100%. Since Ca substitution leads to hole overdoping, these results suggest that charge depletion and band bending may be responsible for the suppression of superconductivity in GBs. By using electron holography, we directly observe an electrostatic potential at the dislocation cores. The radius as well as the value of the potential are reduced in Ca-doped grain boundaries, directly reflecting the improved transport properties.