Micropuncture studies of chloride and bicarbonate absorption in the proximal renal tubule of the rat in respiratory acidosis and in chloride depletion.

1. Micropuncture studies of chloride and bicarbonate absorption in the proximal tubule under free flow conditions were performed during exposure to 12% COz in air, during recovery from COz on a low chloride diet with and without concomitant sodium restriction, and during re-exposure to 12% COz in air with continuing chloride restriction. 2. After 3 hr of exposure to COz there was a significant increase in the TF/P C1 (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 C1 levels were reached at 24 hr with no further increase after 48 hr exposure to COz. 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 C1 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 C1 in the proximal tubule reflects the intratubular dynamics which influence bicarbonate absorption.