Differential expression and regulation of Na(+)/H(+) exchanger isoforms in rabbit parietal and mucous cells

Several Na(+)/H(+) exchanger (NHE) isoforms are expressed in the stomach, and NHE1 and NHE2 knockout mice display gastric mucosal atrophy. This study investigated the cellular distribution of the NHE isoforms NHE1, NHE2, NHE3, and NHE4 in rabbit gastric epithelial cells and their regulation by intracellular pH (pH(i)), hyperosmolarity, and an increase in cAMP. Semiquantitative RT-PCR and Northern blot experiments showed high NHE1 and NHE2 mRNA levels in mucous cells and high NHE4 mRNA levels in parietal and chief cells. Fluorescence optical measurements in cultured rabbit parietal and mucous cells using the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein and NHE isoform-specific inhibitors demonstrated that in both cell types, intracellular acidification activates NHE1 and NHE2, whereas hyperosmolarity activates NHE1 and NHE4. The relative contribution of the different isoforms to pH(i)- and hyperosmolarity-activated Na(+)/H(+) exchange in the different cell types paralleled their relative expression levels. cAMP elevation also stimulated NHE4, whereas an increase in osmolarity above a certain threshold further increased NHE1 and not NHE4 activity. We conclude that in rabbit gastric epithelium, NHE1 and NHE4 regulate cell volume and NHE1 and NHE2 regulate pH(i). The high NHE1 and NHE2 expression levels in mucous cells may reflect their special need for pH(i) regulation during high gastric acidity. NHE4 is likely involved in volume regulation during acid secretion.

Role of Na(+)HCO(3)(-) cotransporter NBC1, Na(+)/H(+) exchanger NHE1, and carbonic anhydrase in rabbit duodenal bicarbonate secretion

BACKGROUND & AIMS

HCO(3)(-) supply to the enterocyte is rate limiting for duodenal HCO(3)(-) secretion (J(HCO3-)). This study defines the molecular nature of the major HCO(3)(-) uptake pathways in rabbit duodenocytes and investigates their physiologic significance and regulation during basal and stimulated J(HCO3-).

METHODS & RESULTS

pH gradient-driven (22)Na(+) uptake into duodenal basolateral membrane vesicles was partly HCO(3)(-) dependent, stilbene sensitive, and therefore mediated by Na(+)HCO(3)(-) cotransport, and partly HCO(3)(-) independent, Hoechst 642 sensitive, and therefore mediated by the Na(+)/H(+) exchanger isoform NHE1. Semiquantitative polymerase chain reaction (PCR) revealed high duodenal expression levels for the NBC1 isoform of the Na(+)HCO(3)(-) cotransporter gene family and NHE1. Cloning and comparison of full-length rabbit with human gastrointestinal and kidney NBC1 subtype revealed a conserved protein kinase A consensus sequence in the cytoplasmic N-terminus of the gastrointestinal NBC1. Inhibition of either Na(+)HCO(3)(-) cotransport or carbonic anhydrase reduced ouabain-sensitive J(HCO3-) in in vitro rabbit duodenal mucosae by approximately 50%, but did not affect 8-Br-cAMP-induced DeltaJ(HCO3-), suggesting cAMP-mediated up-regulation of the alternative pathway. However, inhibition of both Na(+)HCO(3)(-) cotransport and either carbonic anhydrase or NHE1 strongly reduced DeltaJ(HCO3-).

CONCLUSIONS

NBC1 and NHE1 are the major base importers in rabbit duodenocytes. Na(+)HCO(3)(-) cotransport and CO(2) hydration/Na(+)/H(+) exchange are equally important pathways for duodenal HCO(3)(-) supply and are up-regulated during cAMP-mediated stimulation.

Na+/HCO3- cotransport and expression of NBC1 and NBC2 in rabbit gastric parietal and mucous cells

BACKGROUND & AIMS

The gastric epithelium protects itself against luminal acid by secreting HCO3--rich fluid into the mucous layer and by HCO3--dependent intracellular pH regulation, but the basolateral HCO3- uptake mechanisms are incompletely characterized. This study examined the expression and functional significance of the Na+/HCO3- cotransporters NBC1 and NBC2 in rabbit gastric epithelial cells.

METHODS

Rabbit NBC1 and NBC2 complementary DNA fragments were cloned and sequenced, and cellular expression levels were assessed by semiquantitative polymerase chain reaction. Na+/HCO3- cotransport activity was measured fluorometrically in cultured rabbit parietal and mucous cells.

RESULTS

NBC1 expression was 4.5-fold lower in the stomach than kidney cortex and 5.5-fold higher in mucous than parietal cells. NBC2 expression in the stomach was much lower than in the eye, approximately 4-fold lower than NBC1 expression in the stomach, and 2.5-fold higher in mucous than parietal cells. The Na+- and HCO3--dependent, dimethylamiloride-insensitive (which at 500 micromol/L completely inhibits all Na+/H+ exchanger isoforms) base influx rates were 4.6 +/- 0.02 and 16.2 +/- 0.04 mmol/L/min in acidified parietal and mucous cells, respectively, and were not significantly different in the absence of Cl-.

CONCLUSIONS

This study shows that NBC1 and NBC2 are expressed in rabbit stomach, with high levels in mucous cells where Na+/HCO3- cotransport is the major base-importing mechanism in the presence of CO2/HCO3-.

Is glutamine essential for the maintenance of intestinal function? A study in the isolated perfused rat small intestine

Glutamine has received considerable interest as a gut-targeted nutrient due to its proposed key role in the maintenance of intestinal structure and function. We used a preparation of isolated vascularly perfused rat small intestine to investigate whether glutamine is essential for the maintenance of intestinal function. When glutamine was available, arterial glutamine was extracted at 15 +/- 2%, and net uptake was -89 +/- 5 nmol min-1 g-1. Nitrogenous metabolites ammonia, alanine, and citrulline (41 +/- 7, 41 +/- 4, and 11 +/- 2 nmol min-1 g-1, respectively) were released into the venous perfusate, but only ammonia was also excreted into the lumen (36 +/- 3 nmol min-1 g-1). In the absence of exogenous glutamine alanine release was halved and that of citrulline and ammonia nullified. Additional inhibition of glutamine synthetase yielded the same results. In all cases variables of tissue function were fully maintained also in the absence of exogenous and/or endogenous glutamine. The inhibition of glutaminase/amidotransferase reactions, however, was accompanied by a reduction in glutamine consumption and a graded deterioration in tissue function. In conclusion, glutamine seems to be dispensable as a metabolic fuel to be fully oxidized by the mucosa. However, the inhibition of major glutamine consuming pathways was associated with impaired tissue function and viability. Therefore the role of intestinal glutamine metabolism seems to be threefold: (a) providing affluent amounts of nitrogen precursors for mucosal anabolic pathways to maintain intestinal structure and function, (b) feeding the liver with an optimal substrate mix, and (c) providing citrulline and thereby arginine for the whole organism.

Effects of dexamethasone on glutamine metabolism in the isolated vascularly perfused rat small intestine

Post-stress metabolism is associated with a large glutamine (Gln) efflux from muscle and an increased Gln utilization by the small intestine. Both appear to be modulated by corticosteroids. The present investigation was performed to better characterize the mechanism of corticoid action on Gln metabolism in an isolated preparation of vascularly perfused rat small intestine. In all perfusions, a synthetic perfusate free from blood components was used with only 0.6 mM Gln and 10 mM glucose as substrates. Irrespective of dexamethasone concentrations in the vascular perfusate (none, 0.25 mg l-1, or 2.5 mg l-1, isolated intestines from normal rats exhibited unchanged extraction rates of Gln (-85 +/- 8, -89 +/- 10, and -87 +/- 16 nmol min-1 g-1) and unchanged production rates of alanine (43 +/- 9, 40 +/- 7, and 51 +/- 5 nmol min-1 g-1) and ammonia (49 +/- 15, 45 +/- 13, and 54 +/- 13 nmol min-1 g-1). Similarly, when intestines were vascularly perfused 2 or 9 days after dexamethasone injection (0.45 mg kg-1 BW), net Gln uptake also remained unchanged (-88 +/- 16 and -84 +/- 11 nmol min-1 g-1). There was, however, a shift in nitrogenous products of Gln metabolism from ammonia (-31% and -38%) to alanine (+16% and +64%). Thus, the failure of dexamethasone to increase Gln uptake in the isolated rat intestine may indicate that rather than acting directly on the mucosa, dexamethasone could regulate intestinal Gln consumption in vivo by indirect mechanisms possibly involving extramucosal tissues. Dexamethasone pretreatment may modulate the pattern of nitrogenous products in portal venous blood presented to the liver and thus support enhanced nitrogen loss through ureagenesis by metabolic cooperation between gut and liver.

Use of an artificial oxygen carrier in isolated rat liver perfusion: first demonstration of net glucose uptake at physiological portal glucose concentrations using a hemoglobin-free perfusate

A defect in isolated perfused rat-liver (IPRL) preparations has been proposed to explain discrepancies between in vivo and in vitro findings regarding hepatic glucose metabolism. The aim of the present study was to investigate whether a preparation of IPRL using a synthetic hemoglobin-free perfusate was capable of net glucose uptake and glycogen deposition at physiological portal substrate concentrations. Livers from fed anaesthetized rats were perfused in a recirculating system using a fluorocarbon emulsion as artificial oxygen carrier. Depending on the prevailing glucose concentration, livers exhibited net glucose uptake or release with a threshold value of 5.5-6.0 mM glucose. Net glucose uptake was associated with net glycogen deposition (+0.23 to +0.59 mumol C6 min-1 g-1). From 5.8 mM (n = 3) and 10.0 mM (n = 8), initial concentration glucose levels fell to 5.3 +/- 0.2 mM after 210 min (n = 3) and 6.3 +/- 0.9 mM after 120 min (n = 8), respectively. This was equivalent to a net glucose uptake of -0.16 and -0.45 mumol min-1 g-1. Anoxia reversibly switched hepatic glucose balance from net uptake (-0.42 mumol min-1 g-1) to release (+0.69 mumol min-1 g-1) followed by net uptake (-0.50 mumol min-1 g-1) after reinstitution of aerobic conditions. We conclude that the composition of perfusion media might play a pivotal role for studies of glucose metabolism in the isolated perfused rat liver. In our experimental model, using a hemoglobin-free synthetic medium, net glucose uptake was readily demonstrated at physiological portal substrate concentrations similar to the in vivo situation.

[Isolated perfusion of the small intestine of the rat with perfluorotributylamine as an artificial oxygen carrier]

A new model of isolated intestinal perfusion could be established with fluorocarbon (FC-43) as artificial oxygen carrier. Morphology, cell metabolism of xenobiotics (phase I and II), as well as the transport of a monosaccharide, were stable for at least 2 h. Without interference with the changing metabolism of erythrocytes in the perfusion medium, the proposed model allows one to investigate exclusively the metabolic function of the perfused organ.