Site of potential difference measurements in single renal proximal tubules of Necturus

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Abstract
The electrical potential differences from cells and lumina of the proximal tubules of the kidney of Necturus maculosus were measured with micropipette-electrodes, in vivo, and the site of measurement was permanently marked by deposition of carmine in the tissue by iontophoresis from the tip, during recording of each potential difference. The carmine deposits were subsequently localized using serial histological sections. Two negative electrical potential steps were measured, with reference to the interstitial fluid, and the tip position unequivocally marked. a) The potential difference across the peritubular face of the cell, with the microelectrode tip in the cell, averaged –70 mv (cell negative); b) the transtubular potential difference with the tip in the tubular lumen, averaged –21 mv (lumen negative).