Is resting state HCO3- secretion in frog gastric fundus mucosa mediated by apical Cl(-)-HCO3- exchange?

Targeting the bicarbonate transporter SLC4A4 overcomes immunosuppression and immunotherapy resistance in pancreatic cancer

Solid tumors are generally characterized by an acidic tumor microenvironment (TME) that favors cancer progression, therapy resistance and immune evasion. By single-cell RNA-sequencing analysis in individuals with pancreatic ductal adenocarcinoma (PDAC), we reveal solute carrier family 4 member 4 (SLC4A4) as the most abundant bicarbonate transporter, predominantly expressed by epithelial ductal cells. Functionally, SLC4A4 inhibition in PDAC cancer cells mitigates the acidosis of the TME due to bicarbonate accumulation in the extracellular space and a decrease in lactate production by cancer cells as the result of reduced glycolysis. In PDAC-bearing mice, genetic or pharmacological SLC4A4 targeting improves T cell-mediated immune response and breaches macrophage-mediated immunosuppression, thus inhibiting tumor growth and metastases. In addition, Slc4a4 targeting in combination with immune checkpoint blockade is able to overcome immunotherapy resistance and prolong survival. Overall, our data propose SLC4A4 as a therapeutic target to unleash an antitumor immune response in PDAC.

Recent advances in the molecular and functional characterization of acid/base and electrolyte transporters in the basolateral membranes of gastric and duodenal epithelial cells

All segments of the gastrointestinal tract are comprised of an elaborately folded epithelium that expresses a variety of cell types and performs multiple secretory and absorptive functions. While the apical membrane expresses the electrolyte transporters that secrete or absorb electrolytes and water, basolateral transporters regulate the secretory or absorptive rates. During gastric acid formation, Cl⁻/HCO₃⁻ and Na(+) /H(+) exchange and other transporters secure Cl⁻ re-supply as well as pH and volume regulation. Gastric surface cells utilize ion transporters to secrete HCO₃⁻, maintain pH(i) during a luminal acid load and repair damaged surface areas during the process of epithelial restitution. Na(+)/H(+) exchange and Na(+)/HCO₃⁻ cotransport serve basolateral acid/base import for gastroduodenal HCO₃⁻ secretion. The gastric and duodenal epithelium also absorbs salt and water. Recent molecular information on novel ion transporters expressed in the gastric and duodenal epithelium has exploded; however, a function has not been found yet for all transporters. The purpose of this review is to summarize current knowledge on the molecular identity and cellular function of basolateral ion transporters in the gastric and duodenal epithelium.

Review article: duodenal bicarbonate - mucosal protection, luminal chemosensing and acid-base balance

The duodenum serves as a buffer zone between the stomach and the jejunum. Over a length of only 25 cm, large volumes of strong acid secreted by the stomach must be converted to the neutral-alkaline chyme of the hindgut lumen, generating large volumes of CO(2). The duodenal mucosa consists of epithelial cells connected by low-resistance tight junctions, forming a leaky epithelial barrier. Despite this permeability, the epithelial cells, under intense stress from luminal mineral acid and highly elevated Pco(2), maintain normal functioning. Bicarbonate ion uniquely protects the duodenal epithelial cells from acid-related injury. The specific protective mechanisms likely involve luminal bicarbonate secretion, intracellular pH buffering and interstitial buffering. Furthermore, the duodenum plays an active role in foregut acid-base homeostasis, absorbing large amounts of H(+) and CO(2). We have studied mucosal protection and acid-base balance using live-animal fluorescence ratio microimaging and by performing H(+) and CO(2) balance studies on duodenal perfusates. On the basis of these data, we have formulated novel hypotheses with regard to mucosal protection.

Extracellular calcium acts as a "third messenger" to regulate enzyme and alkaline secretion

It is generally assumed that the functional consequences of stimulation with Ca²⁺-mobilizing agonists are derived exclusively from the second messenger action of intracellular Ca²⁺, acting on targets inside the cells. However, during Ca²⁺ signaling events, Ca²⁺ moves in and out of the cell, causing changes not only in intracellular Ca²⁺, but also in local extracellular Ca²⁺. The fact that numerous cell types possess an extracellular Ca²⁺ “sensor” raises the question of whether these dynamic changes in external [Ca²⁺] may serve some sort of messenger function. We found that in intact gastric mucosa, the changes in extracellular [Ca²⁺] secondary to carbachol-induced increases in intracellular [Ca²⁺] were sufficient and necessary to elicit alkaline secretion and pepsinogen secretion, independent of intracellular [Ca²⁺] changes. These findings suggest that extracellular Ca²⁺ can act as a “third messenger” via Ca²⁺ sensor(s) to regulate specific subsets of tissue function previously assumed to be under the direct control of intracellular Ca²⁺.

Helicobacter pylori cytotoxin VacA increases alkaline secretion in gastric epithelial cells

Human infection by the bacterium Helicobacter pylori (Hp) may lead to severe gastric diseases by an ill-understood process involving several virulence factors. Among these, the cytotoxin VacA is associated with higher tissue damage. In this study, the isolated frog stomach model was used to characterize the acute effects of VacA on the gastric epithelium. Our results show that VacA partially inhibits gastric acid output by increasing HCO(3)(-) efflux. Experiments conducted with double-barrelled pH or Cl(-)-selective microelectrodes on surface epithelial gastric cells (SECs) and single gastric glands show that VacA does not impair the activity of the oxyntic cells but renders the apical membrane of SECs more permeable to HCO(3)(-) and Cl(-). Inhibition of this permeation by 5-nitro-2-(3-phenylpropylamino) benzoic acid indicates that this may be due to the formation of anion-selective pores by the toxin. We suggest that VacA-dependent HCO(3)(-) efflux from SECs improves the environmental conditions (pH, CO(2) concentration) of the niche parasitized by Hp, that is the gastric surface. This may favor Hp persistence in the tissue and the secondary development of a chronic inflammation.

Alkaline secretion by frog gastric glands measured with pH microelectrodes in the gland lumen

1. In the present work we have measured the pH of the secreted fluid within the gland lumen of isolated but intact gastric mucosa of Rana esculenta. Tissues were mounted in a double chamber allowing continuous perfusion of the mucosal and serosal compartment, and the measurements were made with double-barrelled pH glass microelectrodes inserted into the glands from the serosal surface under microscopic inspection. 2. During inhibition of H+ secretion by cimetidine (100 microM) the luminal gland pH (pHgl) averaged 7.60 +/- 0.05 pH units (mean +/- s.e.m.; n = 35), a value significantly higher than bath solution pH (7.45 +/- 0.02; P < 0.001) and also higher than intracellular pH of oxyntopeptic cells (pHi), which averaged 7.53 +/- 0.06 (n = 18). 3. Stimulation of acid secretion with histamine (500 microM) reversibly decreased pHgl to values which could be as low as 2.5. Together with electrophysiological criteria this response was routinely used to verify the proper location of the microelectrode tip within the gland lumen. 4. Stimulation with carbachol (100 microM) or pentagastrin (50 microM) in the presence of cimetidine rapidly and reversibly increased pHgl by 0.10 +/- 0.01 pH units (n = 24; P < 0.001) and 0.09 +/- 0.02 pH units (n = 6; P < 0.05), respectively. 5. The observation that gastric gland fluid is more alkaline than the bath solutions and that carbachol or pentagastrin further alkalinize it strongly suggests that oxyntopeptic cells participate in gastric alkaline secretion at least under cholinergic stimulation.

Electrogenic bicarbonate secretion in mouse gallbladder

Mouse gallbladders (4 mm2) were investigated using the short-circuit current (Isc) technique. Responses of 50 microA/cm2 were obtained in response to forskolin and agents that stimulated the adenylate cyclase system (IBMX and dibutyryl-cAMP). The calcium ionophore ionomycin increased Isc to 30% of the forskolin-stimulated increase. The forskolin-dependent current was inhibited 40% by acetazolamide but was insensitive to furosemide. Forskolin responses were dependent on the presence of bicarbonate ions; removal from both sides of the membrane or the basolateral side alone caused a significant reduction in responses. Removal of chloride ions from the basolateral side had no effect, while removal from the apical side caused a significant reduction in the forskolin responses, but only by 30%. It is argued that the remaining current (70%) cannot result from a parallel arrangement of a chloride channel and a chloride-bicarbonate exchanger and that bicarbonate is secreted through the apical membrane by a predominantly conductive mechanism. Apparently, forskolin converts a near electrically silent epithelium to an electrogenically secreting tissue.