MINERALOCORTICOID MODULATION OF APICAL AND BASOLATERAL MEMBRANE H+/OH-/HCO3- TRANSPORT PROCESSES IN THE RABBIT INNER STRIPE OF OUTER MEDULLARY COLLECTING DUCT

To examine the mechanism by which mineralocorticoids regulate HCO3- absorption in the rabbit inner stripe of the outer medullary collecting duct, we microfluorometrically measured intracellular pH (pH(i)) in in vitro perfused tubules using 2',7'-bis (carboxyethyl)-5(6)-carboxyfluorescein (BCECF) assaying the apical and basolateral membrane H+/OH-/HCO3- transport processes in three groups of animals: those receiving chronic in vivo DOCA treatment (5 mg/kg per d X 2 wk); those with surgical adrenalectomy (ADX, [chronic X 2 wk]) on glucocorticoid replacement; and controls. Baseline pH(i) was not different in the three groups. Cellular volume (vol/mm) was increased 38% in DOCA tubules versus controls, but unchanged in ADX tubules versus controls. Buffer capacities (B(T)) were not different in the three groups. Apical membrane H+ pump activity, assayed as the Na+-independent pH(i) recovery from an acid load (NH3/NH4+ prepulse) and expressed as J(H) (dpH(i)/dt . vol/mm . B(T)) was increased 76% in DOCA tubules versus controls, and decreased 56% in ADX tubules versus controls. Basolateral membrane Cl-/HCO3- exchange activity assayed as the pH(i) response to basolateral Cl- addition was increased 73% in DOCA tubules versus controls, and decreased 44% in ADX tubules versus controls. When examined as a function of varying [CI-], the Vmax of Cl-/HCO3- exchange activity was significantly increased in DOCA tubules (control, 72.7+/-15.7 pmol . mm-1 . min-1 vs DOCA, 132.3+/-22.5 pmol . mm-1 . min-1, P < 0.02), while the K1/2 for Cl- was unchanged. Basolateral membrane Na+/H+ antiporter activity assayed as the Na+-dependent pH(i) recovery from an acid load was not changed in chronic DOCA tubules versus controls. In conclusion, the apical membrane H+ pump and basolateral membrane Cl-/HCO3- exchanger of the rabbit OMCD(i) are regulated in parallel without chronic alterations in pH(i) under the conditions of mineralocorticoid excess and deficiency. The parallel changes in these transporters accounts for the alterations in OMCD(i) HCO3- absorption seen under these conditions.