The kidney maintains systemic acid-base balance by reclaiming from the renal tubule lumen virtually all HCO filtered in glomeruli and by secreting additional H to titrate luminal buffers. For proximal tubules, which are responsible for about 80% of this activity, it is believed that HCO reclamation depends solely on H secretion, mediated by the apical Na/H exchanger NHE and the vacuolar proton pump. However, and the proton pump cannot account for all HCO reclamation. Here, we investigated the potential contribution of two variants of the electroneutral Na/HCO cotransporter NBCn2, the amino termini of which start with the amino acids MCDL (MCDL-NBCn2) and MEIK (MEIK-NBCn2). Western blot analysis and immunocytochemistry revealed that MEIK-NBCn2 predominantly localizes at the basolateral membrane of medullary thick ascending limbs in the rat kidney, whereas MCDL-NBCn2 localizes at the apical membrane of proximal tubules. Notably, NHCl-induced systemic metabolic acidosis or hypokalemic alkalosis downregulated the abundance of MCDL-NBCn2 and reciprocally upregulated NHE Conversely, NaHCO-induced metabolic alkalosis upregulated MCDL-NBCn2 and reciprocally downregulated NHE We propose that the apical membrane of the proximal tubules has two distinct strategies for HCO reclamation: the conventional indirect pathway, in which NHE and the proton pump secrete H to titrate luminal HCO, and the novel direct pathway, in which NBCn2 removes HCO from the lumen. The reciprocal regulation of NBCn2 and NHE under different physiologic conditions is consistent with our mathematical simulations, which suggest that HCO uptake and H secretion have reciprocal efficiencies for HCO reclamation versus titration of luminal buffers.

The SLC4 genes are all capable of producing multiple variants by alternative splicing or using alternative promoters. The physiological consequences of such diversity are of great interest to investigators. Here, we identified two novel variants of the electroneutral Na+/ cotransporter NBCn1, one full-length starting with “MIPL” and the other Nt-truncated starting with “MDEL”. Moreover, we identified a new promoter of Slc4a10 encoding NBCn2 and a novel type of Nt-truncated NBCn2 starting with “MHAN”. When heterologously expressed, the new NBCn1 variants were well localized to the plasma membrane and exhibited characteristic NBCn1 activity. However, MHAN-NBCn2 was poorly localized on the plasma membrane. By deletion mutations, we identified the Nt regions important for the surface localization of NBCn2. Interestingly, coexpressing the full-length NBCn2 greatly enhances the surface abundance of the Nt-truncated NBCn2. Co-immunoprecipitation and bimolecular fluorescence complementation studies showed that the full-length and Nt-truncated NBCn2 interact with each other to form heterodimers in neuro-2A cells. Finally, we showed that the isolated Nt domain interacts with and enhances the surface abundance of the Nt-truncated NBCn2. The present study expands our knowledge of the NBCn1 and NBCn2 transcriptome, and provides insights into how the Nt domain could affect transporter function by regulating its membrane trafficking.

Key points The roles of the Na+/HCO3– cotransporters NBCn1 and NBCn2 as well as their activators IRBIT and L‐IRBIT in the regulation of the mTAL transport of NH4+, HCO3–, and NaCl are investigated. Dietary challenges of NH4Cl, NaHCO3 or NaCl all increase the abundance of NBCn1 and NBCn2 in the outer medulla. The three challenges generally produce parallel increases in the abundance of IRBIT and L‐IRBIT in the outer medulla. Both IRBIT and L‐IRBIT powerfully stimulate the activities of the mTAL isoforms of NBCn1 and NBCn2 as expressed in Xenopus oocytes. Our findings support the hypothesis that NBCn1, NBCn2, IRBIT and L‐IRBIT appropriately promote NH4+ shunting but oppose HCO3– and NaCl reabsorption in the mTAL, and thus are at the nexus of the regulation pathways for multiple renal transport processes. Abstract The medullary thick ascending limb (mTAL) plays a key role in urinary acid and NaCl excretion. NBCn1 and NBCn2 are present in the basolateral mTAL, where NBCn1 promotes NH4+ shunting. IRBIT and L‐IRBIT (the IRBITs) are two powerful activators of certain acid‐base transporters. Here we use western blotting and immunofluorescence to examine the effects of multiple acid‐base and electrolyte disturbances on expression of NBCn1, NBCn2 and the IRBITs in rat kidney. We also use electrophysiology to examine the functional effects of IRBITs on NBCn1 and NBCn2 in Xenopus oocytes. NH4Cl‐induced metabolic acidosis (MAc) substantially increases protein expression of NBCn1 and NBCn2 in the outer medulla (OM) of rat kidney. Surprisingly, NaHCO3‐induced metabolic alkalosis (MAlk) and high‐salt diet (HSD) also increase expression of NBCn1 and NBCn2 (effect of NaHCO3 > HSD). Moreover, all three challenges generally increase OM expression of the IRBITs. In Xenopus oocytes, the IRBITs substantially increase the activities of NBCn1 and NBCn2. We propose that upregulation of basolateral NBCn1 and NBCn2 plus the IRBITs in the mTAL: (1) promotes NH4+ shunting by increasing basolateral HCO3– uptake to neutralize apical NH4+ uptake during MAc; (2) inhibits HCO3– reabsorption during MAlk by opposing HCO3− efflux via the basolateral anion exchanger AE2; and (3) inhibits NaCl reabsorption by mediating (with AE2) net NaCl backflux into the mTAL cell during HSD. Thus, NBCn1, NBCn2 and the IRBITs are at the nexus of the regulatory pathways for multiple renal transport processes.

The effects of forest management on carbon (C) sequestration are poorly understood, particularly in the Three Gorges Reservoir area. We aimed to identify the effects of forest management on C sequestration in Pinus massoniana plantations. An intact control forest (CK), a site undergoing regular shrub cutting with the simultaneous removal of residues (SC), a site under low-intensity thinning (LIT), and a site under high-intensity thinning (HIT) were compared for soil labile organic carbon (LOC), related enzyme activities, and soil characteristics. Soil organic carbon (SOC) significantly decreased in the HIT treatment as compared with that in the CK treatment. Soil EOC, DOC, MBC contents in treated plots were higher than those in the CK treatment; particularly, the HIT treatment significantly increased those values in 0–10 cm layer. Thinning resulted in a decrease in cellulase and amylase activities, but an increase in invertase activity. In addition, the SOC content was significantly correlated with four enzymes activities and LOC components, which suggested that the soil LOC components and enzymes activities were sensitive to the changes of SOC. Our results suggest that high-intensity thinning treatment in Pinus massoniana plantation could significantly decrease the SOC content and lead to an increase of LOC components.