1. Micropuncture studies of chloride and bicarbonate absorption in the proximal tubule under free flow conditions were performed during exposure to 12% CO2 in air, during recovery from CO2 on a low chloride diet with and without concomitant sodium restriction, and during re-exposure to 12% CO2 in air with continuing chloride restriction.

2. After 3 hr of exposure to CO2 there was a significant increase in the TF/P Cl (ratio of tubular fluid to plasma chloride) in the proximal tubule and a significant increase in urinary chloride excretion with a drop in the plasma chloride. Maximum plasma bicarbonate and TF/P Cl levels were reached at 24 hr with no further increase after 48 hr exposure to CO2. Bicarbonate absorption in the proximal tubule formed a reciprocal pattern to chloride absorption.

3. It is concluded, in conjunction with the accompanying balance experiments of Luke et al. (1970), that a rising TF/P Cl in the proximal tubule correlates with increasing proximal tubular absorption of bicarbonate, a rising plasma bicarbonate, and an increasing urinary acid excretion, but not necessarily with external chloride balance, which is regulated by a more distal site in the nephron. In respiratory acidosis and chloride depletion the TF/P Cl in the proximal tubule reflects the intratubular dynamics which influence bicarbonate absorption.

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