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Rabbit medullary collecting duct (MCD) acidification has been demonstrated to occur by means of a sodium-independent, aldosterone-stimulated mechanism. We have examined the anionic dependence of this process by means of the isolated perfused tubule technique. Total replacement of perfusate chloride with gluconate enhanced tubular bicarbonate reabsorption (JHCO3), from a basal rate of 10.7 +/- 1.0 pmol X mm-1 X min-1 to a rate of 15.01 +/- 1.0 pmol X mm-1 X min-1. Removal of bath chloride, with and without removal of perfusate chloride completely abolished acidification. Bath, but not luminal 4-acetamido-4' isothiocyano-2,2'-disulfonic stilbene provoked a marked decrease in JHCO3 from 10.1 +/- 1.2 pmol X mm-1 X min-1 to 2.3 +/- 0.3 pmol X mm-1 X min-1. Measurement of chloride reabsorptive rate (JCl) revealed colinearity between JHCO3 (9.18 +/- 0.9 pmol X mm-1 X min-1) and JCl (9.75 +/- 1.18 pmol X mm-1 X min-1). We propose a model of mammalian distal nephron acidification in which (a) cellular base exit is effected by means of a basolateral membrane Cl-base exchanger and (b) net electroneutrality of electrogenic proton secretion is maintained by the parallel movement of an anionic species, functionally chloride.

Rabbit medullary collecting duct (MCD) from inner stripe of outer medulla has been identified as a major distal nephron acidification site. The isolated, perfused tubule technique was used to examine the roles of mineralocorticoid and glucocorticoid in regulation of MCD acidification. Surgical adrenalectomy reduced bicarbonate reabsorptive rate (JHCO3, pmol X mm-1 X min-1) from the normal of 9.79 +/- 1.21 to 0.67 +/- 1.1. Chronic administration of deoxycorticosterone acetate (DOCA) increased JHCO3 of MCD significantly to 18.02 +/- 1.62 whereas chronic dexamethasone administration did not affect JHCO3. The direct effects of aldosterone and dexamethasone upon MCD acidification were examined by perfusing tubules harvested from adrenalectomized rabbits in the presence of aldosterone or dexamethasone. Aldosterone, at 5 X 10(-8) M, increased JHCO3 significantly from 1.27 +/- 0.28 to 3.09 +/- 0.34. At 10(-6) M, aldosterone produced a greater increase in JHCO3 from 0.67 +/- 1.1 to 9.39 +/- 1.59. In vitro dexamethasone treatment had no effect on JHCO3. Studies examining the sodium dependence of aldosterone-stimulated acidification demonstrated that JHCO3 in tubules harvested from normal and deoxycorticosterone acetate-treated animals was unaffected by total replacement of sodium with tetramethylammonium. Likewise, luminal amiloride (5 X 10(-5) M) had no effect on JHCO3 in tubules harvested from adrenalectomized and normal animals. Moreover, the acute, in vitro stimulatory effect of aldosterone was seen to occur in the presence of luminal amiloride. These studies define a mammalian distal nephron segment that possesses major acidifying capacity, which is modulated by mineralocorticoid but independent of luminal sodium.