Metabolism and gastric acid secretion. Substrate-dependency of aminopyrine accumulation in isolated rat parietal cells

High α-tocopherol dosing increases lipid metabolism by changing redox state in damaged rat gastric mucosa and liver after ethanol treatment

Regeneration of ethanol-injured rat gastric mucosa must undergo changes in major metabolic pathways to achieve DNA replication and cell proliferation. These events are highly dependent on glucose utilization and inhibited by vitamin E (VE) (α-tocopherol) administration. Therefore, the present study aimed at assessing lipid metabolism in the gastric mucosa and ethanol-induced gastric damage and the effect of α-tocopherol administration. For this, rates of fatty acid β-oxidation and lipogenesis were tested in gastric mucosa samples. Through histological analysis, we found loss of the mucosa's superficial epithelium, which became gradually normalized during the recovery period. Proliferation of gastric mucosa occurred with augmented formation of β-oxidation by-products, diminished synthesis of triacylglycerols (TGs), as well as of phospholipids, and a reduced cytoplasmic NAD/NADH ratio, whereas the mitochondrial redox NAD/NADH ratio was much less affected. In addition, α-tocopherol increased palmitic acid utilization in the gastric mucosa, which was accompanied by the induction of 'mirror image' effects on the cell redox state, reflected in an inhibited cell gastric mucosa proliferation by the vitamin administration. In conclusion, the present study shows, for the first time, the role of lipid metabolism in the adaptive cell gastric mucosa changes that drive proliferation after a chronic insult. Moreover, α-tocopherol increased gastric mucosa utilization of palmitic acid associated with energy production. These events could be associated with its antioxidant properties in co-ordination with regulation of genes and cell pathways, including changes in the cell NAD/NADH redox state.

Dietary pectin up-regulates monocaboxylate transporter 1 in the rat gastrointestinal tract

This work was undertaken to study the effect of pectin feeding on the expression level, cellular localization and functional activity of monocarboxylate transporter 1 (MCT1) in the gastrointestinal tract of rats. The results indicated that MCT1 protein level was significantly increased along the entire length of the gastrointestinal tract of pectin-fed rats in comparison with control animals. Immunohistochemical analysis revealed an increase in MCT1 in the stratified squamous epithelia of the forestomach as well as in the basolateral membranes of the cells lining the gastric pit of the glandular stomach of pectin-fed rats when compared with control animals. The parietal cells, which showed barely any or no detectable MCT1 in the control group, exhibited a strong intensity of MCT1 on the basolateral membranes in pectin-fed rats. In the small intestine of pectin-fed rats, strong immunopositivity for MCT1 was detected in the brush border and basolateral membranes of the absorptive enterocytes lining the entire villi, while in control rats, weak reactivity was detected on the brush border membrane in a few absorptive enterocytes in the villus tip. In the large intestine of control animals, MCT1 was detected on the basolateral membranes of the epithelia lining the caecum and colon. This staining intensity was markedly increased in pectin-fed rats, along with the appearance of strong reactivity for MCT1 on the apical membranes of the surface and crypt epithelia of caecum and colon. Our results also showed that MCT1 co-localizes with its chaperone, basigin (CD147), in the rat gastrointestinal tract, and that the pectin feeding increased the expression of CD147. In vivo functional studies revealed an enhanced acetate absorption in the colon of pectin-fed in comparison with control animals. We conclude that MCT1 is up-regulated along the gastrointestinal tract of pectin-fed rats, which might represent an adaptive response to the increased availability of its substrates.

An amino acid transporter involved in gastric acid secretion

Gastric acid secretion is regulated by a variety of stimuli, in particular histamine and acetyl choline. In addition, dietary factors such as the acute intake of a protein-rich diet and the subsequent increase in serum amino acids can stimulate gastric acid secretion only through partially characterized pathways. Recently, we described in mouse stomach parietal cells the expression of the system L heteromeric amino acid transporter comprised of the LAT2-4F2hc dimer. Here we address the potential role of the system L amino acid transporter in gastric acid secretion by parietal cells in freshly isolated rat gastric glands. RT-PCR, western blotting and immunohistochemistry confirmed the expression of 4F2-LAT2 amino acid transporters in rat parietal cells. In addition, mRNA was detected for the B(0)AT1, ASCT2, and ATB(0+) amino acid transporters. Intracellular pH measurements in parietal cells showed histamine-induced and omeprazole-sensitive H+-extrusion which was enhanced by about 50% in the presence of glutamine or cysteine (1 mM), two substrates of system L amino acid transporters. BCH, a non-metabolizable substrate and a competitive inhibitor of system L amino acid transport, abolished the stimulation of acid secretion by glutamine or cysteine suggesting that this stimulation required the uptake of amino acids by system L. In the absence of histamine glutamine also stimulated H+-extrusion, whereas glutamate did not. Also, phenylalanine was effective in stimulating H+/K+-ATPase activity. Glutamine did not increase intracellular Ca2+ levels indicating that it did not act via the recently described amino acid modulated Ca2+-sensing receptor. These data suggest a novel role for heterodimeric amino acid transporters and may elucidate a pathway by which protein-rich diets stimulate gastric acid secretion.

Enhanced intracellular calcium promotes metabolic and secretory disturbances in rat gastric mucosa during ethanol-induced gastritis

Changes in the Ca(2+) homeostasis have been implicated in cell injury and death. However, Ca(2+) participation in ethanol-induced chronic gastric mucosal injury has not been elucidated. We have developed a model of ethanol-induced chronic gastric injury in rats, characterized by marked alterations in plasma membranes from gastric mucosa and a compensatory cell proliferation, which follows ethanol withdrawal. Therefore, the present study explored the possible role of intracellular Ca(2+) in the oxidative metabolism and in acid secretion in this experimental model. Glucose oxidation was greatly enhanced in the injured mucosa, as evaluated by CO(2) production by isolated mucosal preparations incubated with (14)C-radiolabeled glucose in different carbons. Oxygen consumption and acid secretion (aminopyrine accumulation) were also stimulated. A predominating secretory status was morphologically identified by electron microscopy in oxyntic cells of gastric mucosa from ethanol-treated rats. A coupling between secretory and metabolic effects induced by ethanol (demonstrated by an inhibitory effect of omeprazole in both parameters) was found. These ethanol-induced effects were also inhibited by addition of Ca(2+) chelators to isolated gastric mucosa samples. Lanthanum, a Ca(2+) channel blocker, inhibited ethanol-promoted increase of oxidative metabolism. In addition, a stimulated Ca(2+) uptake by mucosal minces and increased in vivo Ca(2+) levels in cytosolic and mitochondrial fractions, were also noticed. Enhanced glucose and oxygen consumptions were associated with higher ATP and NADP+ availability, whereas cytosolic NAD/NADH ratio (assessed by mucosal levels of lactate and pyruvate) was not significantly modified by the chronic ethanol administration. In conclusion, changes in Ca(2+) homeostasis, probably mainly due to increased extracellular Ca(2+) uptake, could mediate secretory and metabolic alterations found in the gastric mucosa from rats chronically treated with ethanol.

Characterization of oxyntic glands isolated from the rat gastric mucosa

A simple and reproducible method for isolating oxyntic glands from the rat gastric mucosa was developed. The mucosa was incubated with pronase and EGTA, and then treated mechanically to release glands that were separated from single cells by sedimentation. Parietal cells were identified by immunostaining using a monoclonal antibody against H,K-ATPase. The glandular cells appeared morphologically intact. By careful control of the conditions of gland isolation, long glandular structures comprising hundreds of cells surrounding the lumen were obtained. Intraperitoneal injection of Br-deoxyuridine in the rat 1.5 h before the isolation procedure resulted in glands with a labeling of cells in their neck region. The glands were viable, as demonstrated by their ability to respond to various hormones. Histamine dose-dependently stimulated the acid formation which was measured as the accumulation of [14C]aminopyrine. At 100 microM histamine the accumulation was increased 5-10-fold. At 100 nM, pentagastrin potentiated the histamine stimulated accumulation by approximately 40% but pentagastrin alone did not stimulate. The oxyntic glands obtained by the present procedure appear useful for studies on cell physiology, including regulation of acid secretion, cellular interactions, and possibly also differentiation and proliferation mechanisms since long glandular fragments that contained the proliferative zone could be isolated.

Inhibition of gastric acid secretion by epidermal growth factor. Effects on cyclic AMP and on prostaglandin production in rat isolated parietal cells

Histamine (0.5 mM) stimulated the cyclic AMP content of cell suspensions containing greater than 80% parietal cells. Epidermal growth factor (EGF) inhibited this stimulatory effect of histamine, but had no effect on basal cyclic AMP content. The half-maximally effective concentration of EGF for inhibition of histamine-stimulated cyclic AMP was 3.9 nM. The equivalent measurement for the inhibition of histamine-stimulated aminopyrine accumulation was 3.0 nM. Aminopyrine accumulation was measured because it provides an index of the secretory activity of the cell. The cyclic AMP phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) prevented the inhibitory effect of EGF on cyclic AMP content. This effect of IBMX was not caused by its ability to raise cellular cyclic AMP content in the presence of histamine. Prevention by IBMX of the inhibitory action of EGF on histamine-stimulated aminopyrine accumulation had been shown previously [Shaw, Hatt, Anderson & Hanson (1987) Biochem. J. 244, 699-704]. EGF stimulated prostaglandin E2 (PGE2) production in the cell fraction containing greater than 80% parietal cells, with the half-maximally effective concentration being 7.5 nM. EGF was ineffective in stimulating PGE2 production if the cell fraction was depleted of parietal cells (12%), or if 0.5 mM-histamine was added to the enriched parietal-cell fraction. In conclusion, EGF may inhibit histamine-stimulated acid secretion by decreasing the cyclic AMP content of parietal cells. This effect could be mediated by an increase in cyclic AMP phosphodiesterase activity, but it is unlikely to involve an effect of EGF on parietal-cell prostaglandin production.

Effect of pertussis toxin on the inhibition of secretory activity by prostaglandin E2, somatostatin, epidermal growth factor and 12-O-tetradecanoylphorbol 13-acetate in parietal cells from rat stomach

Rat parietal cells were incubated for 2 h with pertussis toxin (100 ng/ml) which ADP-ribosylates and inactivates guanine nucleotide regulatory proteins (G proteins) of the 'Gi-like' family. The effect of this pretreatment on the action of inhibitors of parietal cell acid secretion was investigated by using the accumulation of the weak base aminopyrine as an index of secretory activity. The inhibitory actions of near maximally effective concentrations of prostaglandin E2 (PGE2), somatostatin and epidermal growth factor (EGF) on histamine-stimulated aminopyrine accumulation were reduced by 83%, 72% and 70%, respectively, by preincubation with pertussis toxin. By contrast, the inhibitory action of a near maximally effective concentration of 12-O-tetradecanoylphorbol 13-acetate on histamine-stimulated aminopyrine accumulation was reduced by only 12%. It is concluded that G-proteins are involved in the inhibitory actions of PGE2, somatostatin and EGF on parietal cells. However, since the inhibitory actions of PGE2 and EGF can be distinguished by the blockade of the action of EGF, but not that of PGE2, by 3-isobutyl-1-methylxanthine, it is possible that PGE2 and EGF either activate the same G-protein in different ways or work through different G-proteins.

Action of epidermal growth factor on acid secretion by rat isolated parietal cells

The site and mechanism of action of epidermal growth factor (EGF) on acid secretion by rat isolated parietal cells were investigated by using the intracellular accumulation of the weak base aminopyrine as an index of secretory activity. When parietal cells were stimulated with histamine (0.5 mM), the concentration of EGF required for half-maximal inhibition of acid secretion was 19 nM, with a maximally effective concentration of EGF producing 38% inhibition of secretory activity. EGF did not inhibit secretion stimulated by 0.1 mM-carbachol, or by 30 microM-, 56 microM-, 100 microM- or 1000 microM-dibutyryl cyclic AMP, low concentrations of which produced a secretory response comparable with that obtained with 0.5 mM-histamine. Addition of 0.1 mM-3-isobutyl-1-methylxanthine (IBMX) substantially increased aminopyrine accumulation in the presence of 0.5 mM-histamine. The inhibitory action of EGF on histamine-stimulated secretion was blocked by 0.1 mM-IBMX, even if low concentrations of histamine were used to generate aminopyrine accumulation ratios similar to those obtained with 0.5 mM-histamine alone. The cyclo-oxygenase inhibitor flurbiprofen (1-100 microM) and the cyclo-oxygenase and lipoxygenase inhibitor nordihydroguaiaretic acid (10-100 microM) did not affect the inhibitory action of EGF. The pattern of inhibition of secretion produced by the activator of Ca2+-sensitive phospholipid-dependent protein kinase, 12-O-tetradecanoylphorbol 13-acetate, was markedly different from that produced by EGF. In conclusion, a major site of the action of EGF on acid secretion in the intact stomach is probably a decrease in the stimulatory effect of histamine by a mechanism which does not involve Ca2+-sensitive phospholipid-dependent protein kinase or the production of prostaglandins, but which might involve enhancement of cyclic AMP phosphodiesterase activity.

Involvement of calcium-sensitive phospholipid-dependent protein kinase in control of acid secretion by isolated rat parietal cells

The relative potency with which phorbol esters inhibited histamine-stimulated aminopyrine accumulation (an index of acid secretion) paralleled that which has been established for the activation of purified protein kinase C. The inhibitory effect of 1-oleoyl-2-acetylglycerol on aminopyrine accumulation stimulated by various secretagogues was similar to that of 12-O-tetradecanoylphorbol 13-acetate. Protein kinase C activity was present in a parietal-cell-enriched fraction. In conclusion, protein kinase C could be involved in mechanisms regulating gastric acid secretion.