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Titratable acid, PCO2, bicarbonate and ammonium ions along the rat proximal tubule.

The proximal tubule of cortical nephrons of male albino rats were studied. The intratubular pH was measured with antimony microelectrodes. The concentrations of bicarbonate ions, the ammonium ions and titratable acid were measured in the same free flow samples from single tubules, using a recently developed ultramicroanalytical system, which also permits the estimation of intratubular Pco2. The concentration of titratable acid, which is thought mainly to emanate from phosphate, was found to be constant, about 1 mM, along the proximal tubule. This is explained as a gradient limited transport process for phosphate as well as for hydrogen ions. The concentration of ammonium ions was found to be about 2 mM along the convoluted proximal tubule in spite of water reabsorption, indicating some reabsorptive mechanisms for ammonium or the base, ammonia. Furthermore the present investigation indicates a high Pco2 in the untreated proximal tubule which could explain the phenomenon of disequilibrium pH.