Effects of CO2 and acetazolamide on bicarbonate and fluid transport in rabbit proximal tubules.

Early superficial (SF) and juxtamedullary (JM) proximal convolutions of the rabbit kidney were perfused in vitro to determine the effects of carbonic anhydrase inhibition (10(-4) M acetazolamide) and acute changes in PCO2 (decreases to approximately equal to 15 and increases to approximately equal to 74 mmHg) on potential differences (PD in mV), volume reabsorption (Jv in nl x mm-1 x min-1), and bicarbonate reabsorption (JCO2 in pmol x mm-1 x min-1). At PCO2 37 mmHg early JM exhibited a more lumen-negative PD (-7.5 vs. -5.3), greater Jv (1.13 vs. 0.82), and greater JCO2 (86.7 vs. 44.4) than early Sf. Sf and JM had similar responses to acetazolamide: PD became more negative (-5.2 to -5.9 in SF; -8.8 to -10.1 in JM), Jv decreased (0.92 to 0.68 in SF; 1.11 to 0.76 in JM), and JCO2 decreased (35.7 to 7.7 in SF; 99.2 to 27.4 in JM). Increasing PCO2 to approximately equal to 74 mmHg decreased lumen-negative PD, increased Jv, and increased JCO2 in SF and JM (-5.5 to -4.8, 0.72 to 0.95, and 47.6 to 80.4 in SF; -6.6 to -5.7, 1.19 to 1.47, and 78.0 to 111.3 in JM). Decreasing PCO2 to approximately equal to 15 mmHg increased lumen-negative PD, decreased JCO2, but had no effect on Jv in both segments (-4.9 to -5.8, 51.3 to 6.3, and 0.80 to 0.79 in SF; -7.0 to -7.9, 75.3 to 19.6, and 1.34 to 1.41 in JM). It is concluded that 1) early SF and JM display quantitative heterogeneity, 2) PCO2 changes within the physiologic range produce large changes in HCO3 absorption in early proximal tubules and 3) large changes in HCO3- reabsorption are dissociated from changes in volume reabsorption during hypocapnia.