Single proximal tubules of Necturus kidney. V: Unidirectional sodium movement

¹ Biophysical Laboratory, Harvard Medical School, Boston, Massachusetts

Proximal tubules of Necturus kidneys have been perfused with isotonic solutions of various NaCl concentrations, using the stopped-flow microperfusion technique. At the close of the perfusion period, the tubular fluid was collected for measurement of: Na concentration, Na²⁴ concentration, and freezing point depression. Net H₂O movement was measured with C¹⁴ inulin. When the tubule is filled with 100 mm NaCl, net Na efflux is 62 ± 13 pmoles/cm² sec. Since this net Na efflux is directed against an electrochemical potential gradient, it provides direct experimental evidence for active Na transport. Apparent unidirectional Na fluxes have also been calculated from the disappearance of Na²⁴ from the perfused tubule under various experimental conditions.