CFTR inhibition augments NHE3 activity during luminal high CO₂exposure in rat duodenal mucosa

Abstract

We hypothesized that the function of duodenocyte apical membrane acid-base transporters are essential for H ϩ absorption from the lumen. We thus examined the effect of inhibition of Na ϩ /H ϩ exchanger-3 (NHE3), cystic fibrosis transmembrane regulator (CFTR), or apical anion exchangers on transmucosal CO 2 diffusion and HCO 3 Ϫ secretion in rat duodenum. Duodena were perfused with a pH 6.4 high CO 2 solution or pH 2.2 low CO2 solution with the NHE3 inhibitor, S3226, the anion transport inhibitor, DIDS, or pretreatment with the potent CFTR inhibitor, CFTRinh-172, with simultaneous measurements of luminal and portal venous (PV) pH and carbon dioxide concentration ([CO2] Ϫ loss by NHE3 inhibition and reduced intracellular acidification during CFTR inhibition is consistent with activation or unmasking of NHE3 activity by CFTR inhibition, increasing cell surface H ϩ available to neutralize luminal HCO 3 Ϫ with consequent CO2 absorption. NHE3, by secreting H ϩ into the luminal microclimate, facilitates net transmucosal HCO 3 Ϫ absorption with a mechanism similar to proximal tubular HCO 3 Ϫ absorption.CO 2 absorption; bicarbonate secretion; portal venous PCO2 EPITHELIAL HCO 3 Ϫ SECRETION is considered to be the primary defense against duodenal injury from gastric acid. One of the unexplained mysteries of duodenal HCO 3 Ϫ secretion is why it is needed, given that secretion into the lumen from pancreas and liver is more than adequate to fully neutralize the bulk luminal content (13). Epithelial HCO 3 Ϫ secretion has thus been hypothesized to neutralize a distinct "preepithelial layer" adjacent to the apical plasma membrane of the mucosa (10, 24). Moreover, the upper intestine absorbs HCO 3 Ϫ by an electroneutral Na ϩ -dependent mechanism, hypothesized to be Na ϩ /H ϩ exchange (37). Since the epithelial Na ϩ /H ϩ exchanger (NHE) 3, which helps facilitate intestinal neutral NaCl absorption, is highly expressed in duodenum (31), we wondered whether duodenal NHE3 additionally functions as an H ϩ secretor, as it does in the proximal tubule (33, 38), facilitating HCO 3 Ϫ absorption. The duodenum must also absorb ϳ450 mmol of gastric H ϩ /24 h to decrease luminal hydrogen concentration ([H ϩ ]) by 6 log orders over its 15-cm length (12). HCl in the duodenal lumen is neutralized by HCO 3 Ϫ secreted by the pancreas and duodenal epithelium. This generates extremely high luminal CO 2 pressures (PCO 2 Ͼ 400 Torr), which are dissipated by the proximal jejunum (32). Gastric mucosal CO 2 and H ϩ permeability is low, since pyloric obstruction leads to severe metabolic alkalosis due to the inability of the normal stomach to absorb substantial quantities of H ϩ or CO 2 (16). Thus the duodenum is the major site for intestinal H ϩ and CO 2 absorption.We have demonstrated sequential transmucosal CO 2 and H ϩ movement from duodenal lumen into the portal vein (PV), facilitated by epithelial cytosolic and membrane-bound carbonic anhydrases (CAs) in the rat (27). Cellular acidification by luminal CO 2 /H ϩ rapidly induces protective responses su...

Recent Advances in Gut Nutrient Chemosensing

“…Of these, epithelial bicarbonate secretion, mucus secretion, cellular pH regulation, and mucosal blood flow, are all responsive to luminal acidification [81, 82]. Luminal pH sensing of the duodenal mucosa has been attributed to multiple interconversions of H + and CO 2 , facilitated by soluble and membrane-bound carbonic anhydrase, with transepithelial movement as CO 2 , akin to the Jacobs-Stewart cycle of red cells [83–85]. An ecto-purinergic mechanism was described wherein the pH-dependent hydrolytic activity of brush-border alkaline phosphatase serves as a pH sensor regulating surface layer pH and [ATP] [86–88].…”

Recent advances in small bowel diseases: Part I

“…NHE3 facilitates intestinal neutral NaCl absorption, and thereby increases the net transmucosal absorption of bicarbonate in rat duodenal mucosa. This mechanism is similar to the proximal renal tubular bicarbonate absorption [122] .…”

Modifier Gene Studies to Identify New Therapeutic Targets in Cystic Fibrosis

“…The levels of SLC9A3 protein at the plasma membrane are regulated by CFTR: reduction of CFTR in plasma membrane results in elevated levels of SLC9A3 [56]. Conversely, the over-expression of SLC9A3 causes a decline in CFTR protein in the plasma membrane [57]. NHERF-1 and NHERF proteins regulate the rates of SLC9A3 recycling at plasma membrane and directly interact with CFTR, thus controlling ion transport [55,57,58].…”

“…Conversely, the over-expression of SLC9A3 causes a decline in CFTR protein in the plasma membrane [57]. NHERF-1 and NHERF proteins regulate the rates of SLC9A3 recycling at plasma membrane and directly interact with CFTR, thus controlling ion transport [55,57,58]. The inhibition of SLC9A3 improves bicarbonate transport in epithelial cells [59].…”