[215][216][217][218] 2000). Of the four NBC4 variants currently present in GenBank, our own cloning efforts yielded only variant c. We expressed NBC4c (GenBank accession no. AF293337) in Xenopus laevis oocytes and assayed membrane potential (V m) and pH regulatory function with microelectrodes. Exposing an NBC4c-expressing oocyte to a solution containing 5% CO2 and 33 mM HCO 3 Ϫ elicited a large hyperpolarization, indicating that the transporter is electrogenic. The initial CO2-induced decrease in intracellular pH (pH i) was followed by a slow recovery that was reversed by removing external Na ϩ . Two-electrode voltage clamp of NBC4c-expressing oocytes revealed large HCO 3 Ϫ -and Na ϩ -dependent currents. When we voltage clamped Vm far from NBC4c's estimated reversal potential (Erev), the pHi recovery rate increased substantially. Both the currents and pH i recovery were blocked by 200 M 4,4Ј-diisothiocyanostilbene-2,2Ј-disulfonic acid (DIDS). We estimated the transporter's HCO Romero et al. (36) led to the cloning of many other electrogenic and electroneutral Na ϩ -driven HCO 3 Ϫ transporters. The original electrogenic NBC in mammals (NBCe1) is currently represented by three splice variants: NBCe1-A, which is found predominantly in kidney (8, 35); NBCe1-B, which is found in pancreas, heart, and many other tissues (1, 12); and NBCe1-C, which is found predominantly in brain (4).The electroneutral Na ϩ -driven HCO 3 Ϫ transporters in mammals share ϳ70% sequence identity on the amino acid level and appear to fall into two functional groups. The first group mediates Cl Ϫ -independent Na ϩ -HCO 3 Ϫ cotransport. The first member of this group has been cloned under various names, including NBC2 from human retina (22), NBC3 from human skeletal muscle (31), and NBCn1 from rat smooth muscle (11). However, the NH 2 -terminal 90 amino acids of NBC2 are missing, and the next 28 represent a cloning artifact.The second group of electroneutral Na ϩ -driven HCO 3 Ϫ transporters mediates the Cl Ϫ -dependent transport of Na ϩ and HCO 3 Ϫ , functionally described as Na ϩ -driven Cl Ϫ /HCO 3 Ϫ exchange. In mammals, this transporter is represented by a clone from human brain, NDCBE1 (18). A partial clone had been named NBC3 (3), representing degeneracy in the nomenclature. A related Drosophila clone (NDAE) encodes a Na ϩ -driven anion exchanger (37). NBC4, originally cloned by Pushkin et al. (32), is a new member of the HCO 3 Ϫ transporter superfamily. It is ϳ60% identical at the amino acid level to the three electrogenic NBCe1 splice variants and 40-50% identical to the electroneutral Na ϩ -driven HCO 3 Ϫ transporters. A Northern blot (32) and a similar mirror image (33) showed high levels of NBC4 mRNA in liver, spleen, and testes and moderate levels in heart, kidney, placenta, and stomach.We mapped the NBC4 sequence onto a recent topology model developed for the anion exchanger AE1 (Fig. 1A), which is in the same superfamily as the Na ϩ -coupled HCO 3 Ϫ transporters. Pushkin et al. (32, 33) described two variants of NBC4, NBC4...
Protons are powerful modulators of cardiac function. Their intracellular concentration is regulated by sarcolemmal ion transporters that export or import H+-ions (or their ionic equivalent: HCO3-, OH-). One such transporter, which imports H+-equivalents, is a putative Cl-/OH- exchanger (CHE). A strong candidate for CHE is SLC26A6 protein, a product of the SLC26A gene family of anion transporters, which has been detected in murine heart. SLC26A6 protein is suggested to be an electrogenic 1Cl-/2OH-(2HCO3-) exchanger. Unfortunately, there is insufficient characterization of cardiac CHE against which the properties of heterologously expressed SLC26A6 can be matched. We therefore investigated the proton, Cl-, and voltage dependence of CHE activity in guinea-pig ventricular myocytes, using voltage-clamp, intracellular pH fluorescence, and mathematical modeling techniques. We find that CHE activity is tightly regulated by intracellular and extracellular pH, is voltage-insensitive over a wide range (+/-80 mV), and displays substrate dependence suggestive of electroneutral 1Cl-/1OH- exchange. These properties exclude electrogenic SLC26A6 as sole contributor to CHE. Either the SLC26A6 product in heart is electroneutral, or CHE comprises at least two transporters with oppositely balanced voltage sensitivity. Alternatively, CHE may comprise an H+-Cl- coinflux system, which cannot be distinguished kinetically from an exchanger. Irrespective of ionic mechanism, CHE's pH sensitivity helps to define resting intracellular pH, and hence basal function in the heart.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.