Suppression of acidification along inner medullary collecting duct

The purpose of these experiments was to evaluate the effect of acute respiratory alkalosis (ARA) and chronic bicarbonate drinking (CBD) on inner medullary collecting duct (IMCD) acidification. Microcatheterization was used to measure pH and PCO2, and samples were simultaneously obtained for measurement of bicarbonate, titratable acid (TA), and ammonium. In ten ARA rats (arterial pH was 7.54 +/- 0.02; PCO2 was 20 +/- 1 mmHg), IMCD equilibrium pH was not different (deep pH was 5.65 +/- 0.06 and PCO2 was 20 +/- 1 mmHg; tip pH was 5.54 +/- 0.07 and PCO2 was 22 +/- 1 mmHg). Delivery of bicarbonate, TA, and ammonium also did not differ between collection sites. Thus net acidification along the IMCD was negligible. Nine rats drank NaHCO3 for 5-8 days (pH = 7.48 +/- 0.02) but did not receive NaHCO3 during the experiment so that arterial pH fell to 7.40 +/- 0.01. IMCD equilibrium pH was different at deep (pH was 5.68 +/- 0.06; PCO2 was 32 +/- 1 mmHg) and tip (pH was 5.57 +/- 0.04; PCO2 was 27 +/- 1 mmHg; P less than or equal to 0.05) collection sites. However, only minimal changes in the delivery of bicarbonate, TA, and ammonium were noted, and net acidification along the IMCD was negligible. In ten control rats, net acidification was 219 nmol/min between collection sites (P less than 0.001). We conclude that ARA and CBD abolish acidification along the IMCD. In addition, CBD produces an intrinsic modification along the IMCD, which suppresses acid secretion and persists after acute recovery from alkalemia.