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Rabbit cortical collecting tubules were perfused in vitro to investigate the control of bicarbonate transport. Bicarbonate was measured by microcalorimetry as total CO2. The perfusate and bath were identical solutions containing 25 mM bicarbonate at pH 7.4. The mean pH of the urine in the bladders of untreated rabbits at the time they were killed was 7.4. Their individual tubules, studied in vitro, either absorbed or secreted bicarbonate, and, combining the results, there was on the average no significant net transport. When the rabbits were treated with NH4Cl the day before the experiment, their urine was acidic and their tubules studied in vitro absorbed bicarbonate (i.e., there was net lumen-to-bath transport). In contrast, when the rabbits were treated with NaHCO3, their urine was significantly more alkaline, and their tubules studied in vitro generally secreted bicarbonate (i.e., net bath-to-lumen transport). Thus, the direction of bicarbonate transport by cortical collecting tubules studied under standard conditions in vitro correlated with the urine pH and was determined by the preceding treatment of the animals in vivo with acidifying or alkalinizing salts. These results demonstrate a previously unrecognized mechanism which contributes to the control of urinary bicarbonate excretion.

Bicarbonate transport by rabbit cortical collecting tubules. Effect of acid and alkali loads in vivo on transport in vitro.