Mechanisms of bicarbonate transport by cultured rabbit inner medullary collecting duct cells

Abstract

The inner medullary collecting duct (IMCD) is the final portion of the mammalian renal tubule that is able to significantly regulate systemic acid-base balance. Although the H+ transporters of this segment are relatively well studied, little is known regarding the mechanisms of HCO3- transport. The mechanisms of HCO3- transport in primary cultures of rabbit IMCD were studied using the pH-sensitive dye, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, in CO2/HCO3(-)-containing solutions at 37 degrees C. Removal of Cl- from the extracellular solution caused reversible intracellular alkalinization, demonstrating the presence of Cl-/HCO3- exchange. Alkalinization with Cl- removal was independent of changes in membrane potential, did not require the presence of extracellular Na+, and was inhibited by the disulfonic stilbene, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 10(-4) M). Half-maximal intracellular pH (pHi) recovery with readdition of Cl- to the extracellular solution occurred at a Cl- concentration of 37.4 +/- 5.7 mM. When rabbit IMCD were cultured on permeable support membranes, Cl-/HCO3- exchange activity was found only on the basolateral membrane. However, there was no evidence of band 3 protein immunoreactivity. In contrast, no evidence for Na(+)-(HCO3-)n > 1 cotransport activity was found. Depolarization of IMCD cells by acute increases in extracellular K+ did not alter pHi, nor was Na(+)-dependent, 5-(N-ethyl-N-isopropyl)amiloride-insensitive pHi recovery from an acid load inhibited by DIDS (10(-4) M). Finally, recovery from intracellular alkalosis induced by incubation in 0 mM Cl-, 50 mM HCO3- extracellular solution required Cl- and was independent of Na+. These studies indicate that the major mechanism of HCO3- transport in primary cultures of the rabbit IMCD is via a band 3 protein-negative, Na(+)-independent, basolateral, Cl-/HCO3- exchanger.

Immunolocalization of the electrogenic Na+- HCO 3 − cotransporter in mammalian and amphibian kidney

“…9). The absence of NBC immunoreactivity in the thin limbs of Henle's loop and all segments distal of the macula densa also is in keeping with the functional data (33) and with our knowledge of the particular mechanisms of HCO 3…”

Characterization of basolateral chloride/bicarbonate exchange in macula densa cells

“…Changes in pH i with basolateral chloride removal and readdition can result from either Cl Ϫ /HCO 3 Ϫ exchange or could be due to parallel, membrane potential-coupled Cl Ϫ and HCO 3 Ϫ conductances (31). There are basolateral Cl Ϫ -conductive pathways in macula densa cells, one of which is responsible for basolateral membrane depolarization in response to increased apical NaCl entry and elevations in [Cl Ϫ ] i (17).…”