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Marine teleost fish continuously ingest seawater to prevent dehydration and their intestines absorb fluid by mechanisms linked to three separate driving forces: 1) cotransport of NaCl from the gut fluid; 2) bicarbonate (HC[O.sup.-.sub.3]) secretion and [Cl.sup.-] absorption via [Cl.sup.-]/HC[O.sup.-.sub.3] exchange fueled by metabolic C[O.sub.2]; and 3) alkaline precipitation of [Ca.sup.2+] as insoluble CaC[O.sub.3], which aids [H.sub.2]O absorption). The latter two processes involve high rates of epithelial HC[O.sup.-.sub.3] secretion stimulated by intestinal [Ca.sup.2+] and can drive a major portion of water absorption. At higher salinities and ambient [Ca.sup.2+] concentrations the osmoregulatory role of intestinal HC[O.sup.-.sub.3] secretion is amplified, but this has repercussions for other physiological processes, in particular, respiratory gas transport (as it is fueled by metabolic CO2) and acid-base regulation (as intestinal cells must export [H.sup.+] into the blood to balance apical HC[O.sup.-.sub.3] secretion). The flounder intestine was perfused in vivo with salines containing 10, 40, or 90 mM [Ca.sup.2+]. Increasing the luminal [Ca.sup.2+] concentration caused a large elevation in intestinal HC[O.sup.-.sub.3] production and excretion. Additionally, blood pH decreased (-0.13 pH units) and plasma partial pressure of C[O.sub.2] (PC[O.sub.2]) levels were elevated (+1.16 mmHg) at the highest Ca perfusate level after 3 days of perfusion. Increasing the perfusate [[Ca.sup.2+]] also produced proportional increases in net acid excretion via the gills. When the net intestinal flux of all ions across the intestine was calculated, there was a greater absorption of anions than cations. This missing cation flux was assumed to be protons, which vary with an almost 1 : 1 relationship with net acid excretion via the gill. This study illustrates the intimate link between intestinal HC[O.sup.-.sub.3] production and osmoregulation with acid-base balance and respiratory gas exchange and the specific controlling role of ingested [Ca.sup.2+] independent of any other ion or overall osmolality in marine teleost fish. carbon dioxide; oxygen; protons; gill; carbonic anhydrase doi: 10.1152/ajpregu.00513.2009.

The intestine of marine teleosts must effectively absorb fluid from ingested seawater to avoid dehydration. This fluid transport has been almost exclusively characterized as driven by NaCl absorption. However, an additional feature of the osmoregulatory role of the intestine is substantial net HC[O.sup.-.sub.3] secretion. This is suggested to drive additional fluid absorption directly (via [Cl.sup.-]/HC[O.sup.-.sub.3] exchange) and indirectly by precipitating ingested [Ca.sup.2+] as CaC[O.sub.3], thus creating the osmotic gradient for additional fluid absorption. The present study tested this hypothesis by perfusing the intestine of the European flounder in vivo with varying [[Ca.sup.2+]]: 10 (control), 40, and 90 mM. Fractional fluid absorption increased from 47% (control) to 73% (90 mM [Ca.sup.2+]), where almost all secreted HC[O.sup.-.sub.3] was excreted as CaC[O.sub.3]. This additional fluid absorption could not be explained by NaCl cotransport. Instead, a significant positive relationship between [Na.sup.+]-independent fluid absorption and total HC[O.sup.-.sub.3] secretion was consistent with the predicted roles for anion exchange and CaC[O.sub.3] precipitation. Further analysis suggested that [Na.sup.+]-independent fluid absorption could be accounted for by net C1 and [H.sup.+] absorption (from [Cl.sup.-]/HC[O.sup.-.sub.3] exchange and CO, hydration, respectively). There was no evidence to suggest that CaC[O.sub.3] alone was responsible for driving fluid absorption. However, by preventing the accumulation of luminal [Ca.sup.2+] it played a vital role by dynamically maintaining a favorable osmotic gradient all along the intestine, which permits substantially higher rates of solute-linked fluid absorption. To overcome the resulting hyperosmotic and highly acidic absorbate, it is proposed that plasma HC[O.sup.-.sub.3] buffers the absorbed [H.sup.+] (from HC[O.sup.-.sub.3] production), and consequently reduces the osmolarity of the absorbed fluid entering the body. osmoregulation; chloride/bicarbonate exchange; in vivo perfusion doi: 10.1152/ajpregu.00545.2009.

Singla A, Dwivedi A, Saksena S, Gill RK, Alrefai WA, Ramaswamy K, Dudeja PK. Mechanisms of lysophosphatidic acid (LPA) mediated stimulation of intestinal apical [C1.sup.-]/[OH.sup.-] exchange. Am J Physiol Gastrointest Liver Physio1298: G 182-G189, 2010. First published November 12, 2009; doi: 10.1152/ajpgi.00345.2009.--Lysophosphatidic acid (LPA), a potent bioactive phospholipid, is a natural component of food products like soy and egg yolk. LPA modulates a number of epithelial functions and has been shown to inhibit cholera toxin-induced diarrhea. Antidiarrheal effects of LPA are known to be mediated by inhibiting chloride secretion. However, the effects of LPA on chloride absorption in the mammalian intestine are not known. The present studies examined the effects of LPA on apical [Cl.sup.-]/[OH.sup.-] exchangers known to be involved in chloride absorption in intestinal epithelial cells. Caco-2 cells were treated with LPA, and [C1.sup.-]/[OH.sup.-] exchange activity was measured as DIDS-sensitive [sup.36][Cl.sup.-] uptake. Cell surface biotinylation studies were performed to evaluate the effect of LPA on cell surface levels of apical [Cl.sup.-]/[OH.sup.-] exchangers, downregulated in adenoma (DRA) (SLC26A3), and putative anion transporter-1 (SLC26A6). Treatment of Caco-2 cells with LPA (100 [micro]M) significantly stimulated [Cl.sup.-]/[OH.sup.-] exchange activity. Specific agonist for LPA2 receptor mimicked the effects of LPA. LPA-mediated stimulation of [C1.sup.-]/[OH.sup.-] exchange activity was dependent on activation of phosphatidylinositol 3-kinase/Akt signaling pathway. Consistent with the functional activity, LPA treatment resulted in increased levels of DRA on the apical membrane. Our results demonstrate that LPA stimulates apical [Cl.sup.-]/[OH.sup.-] exchange activity and surface levels of DRA in intestinal epithelial cells. This increase in [Cl.sup.-]/[OH.sup.-] exchange may contribute to the antidiarrheal effects of LPA. downregulated in adenoma; chloride absorption; human intestine; LPA receptor 2; phosphatidylinositol 3-kinase/Akt doi: 10.1152/ajpgi.00345.2009

Fouillet H, Juillet B, Gaudichon C, Mariotti F, Tome D, Bos C. Absorption kinetics are a key factor regulating postprandial protein metabolism in response to qualitative and quantitative variations in protein intake. Am J Physiol Regul lntegr Comp Physiol 297: R1691-R1705, 2009. First published October 7, 2009; doi: 10.1152/ajpregu.00281.2009.--We have previously demonstrated that increasing the habitual protein intake widened the gap in nutritional quality between proteins through mechanisms that arc not yet fully understood. We hypothesized that the differences in gastrointestinal kinetics between dietary proteins were an important factor affecting their differential response to an increased protein intake. To test this hypothesis, we built a 13-compartment model providing integrative insight into the sequential dynamics of meal nitrogen (Nm) absorption, splanchnic uptake, and metabolism, and subsequent peripheral transfer and deposition. The model was developed from data on postprandial Nm kinetics in certain accessible pools, obtained from subjects having ingested a [sup.5]N-labeled milk or soy protein meal, after adaptation to normal (NP) or high (HP) protein diets. The faster absorption of Nm after soy vs. milk caused its earlier and stronger splanchnic delivery, which favored its local catabolic utilization (up to +30%) and limited its peripheral accretion (down to -20%). Nm absorption was also accelerated after HP vs. NP adaptation, and this kinetic effect accounted for most of the HP-induced increase (up to +20%) in splanchnic Nm catabolic use, and the decrease (down to -25%) in peripheral Nm anabolic utilization. The HP-induced acceleration in Nm absorption was more pronounced with soy than with milk, as were the HP effects on Nm regional metabolism. Our integrative approach identified Nm absorption kinetics, which exert a direct and lasting impact on Nm splanchnic catabolic use and peripheral delivery, as being critical in adaptation to both qualitative and quantitative changes in protein intake. interorgan amino acid metabolism; milk protein; soy protein; highprotein diets; compartmental model doi: 10.1152/ajpregu.00281.2009

Marine teleost fish precipitate divalent cations as carbonate deposits in the intestine to minimize the potential for excessive [Ca.sup.2+] entry and to stimulate water absorption by reducing luminal osmotic pressure. This carbonate deposit formation, therefore, helps maintain osmoregulation in the seawater (SW) environment and requires controlled secretion of HC[O.sup.-.sub.3] to match the amount of [Ca.sup.2+] entering the intestinal lumen. Despite its physiological importance, the process of HC[O.sup.-.sub.3] secretion has not been characterized at the molecular level. We analyzed the expression of two families of HC[O.sup.-.sub.3] transporters, Slc4 and Slc26, in fresh-waterand SW-acclimated euryhaline pufferfish, mefugu (Takifugu obscurus), and obtained the following candidate clones: NBCel (an [Na.sup.+]-HC[O.sup.-.sub.3] cotransporter) and Slc26a6A and Slc26a6B (putative [Cl.sup-]/n HC[O.sup.-.sub.3] exchangers). Heterologous expression in Xenopus oocytes showed that Slc26a6A and Slc26a6B have potent HC[O.sup.-.sub.3]-transporting activity as electrogenic [Cl.sup.-]/nHC[O.sup.-.sub.3] exchangers, whereas mefugu NBCel functions as an electrogenic [Na.sup.+]-nHC[O.sup.-.sub.3] cotransporter. Expression of NBCel and Slc26a6A was highly induced in the intestine in SW and expression of Slc26a6B was high in the intestine in SW and fresh water, suggesting their involvement in HC[O.sup.-.sub.3] secretion and carbonate precipitate formation. Immunohistochemistry showed staining on the apical (Slc26a6A and Slc26a6B) and basolateral (NBCel) membranes of the intestinal epithelial cells in SW. We therefore propose a mechanism for HC[O.sup.-.sub.3] transport across the intestinal epithelial cells of marine fish that includes basolateral HC[O.sup.-.sub.3] uptake (NBCel) and apical HC[O.sup.-.sub.3] secretion (Slc26a6A and Slc26a6B). osmoregulation; NBC; Slc26; intestine; calcium carbonate

Before cholesterol and fatty acid molecules in the small intestinal lumen can interact with their possible transporters for uptake and absorption, they must pass through a diffusion barrier, which may modify the kinetics of nutrient assimilation. This barrier includes an unstirred water layer and a surface mucous coat, which is located at the intestinal lumen-membrane interface. In the present study, we investigated whether disruption of the mucin gene (Muc)1 may influence intestinal uptake and absorption of cholesterol and fatty acid in male Muc[1.sup.-/-] mice. The wild-type mice displayed relatively high levels of Muc1, Muc2, Muc3, and Muc4 mRNAs and relatively low levels of Muc5ac and Muc5b mRNAs in the small intestine. The absence of Muc1 mRNA and protein in the small intestines of Muc[1.sup.-/-] mice confirmed complete knockout of the Muc1 gene, but the mRNA expression for other mucin genes remained unchanged. Intestinal uptake and absorption of cholesterol but not palmitic acid were significantly reduced in Muc[1.sup.-/-] mice compared with the wild-type mice. However, knockout of the Muc1 gene did not impair either expression levels of the genes that encode intestinal sterol efflux transporters Abcg5 and Abcg8 and fatty acid transporter Fatp4 or small intestinal transit rates. We conclude that physiological levels of the epithelial mucin produced by the Muc1 gene are necessary for normal intestinal uptake and absorption of cholesterol in mice. Our study implies that because cholesterol absorption efficiency is reduced by ~50% in Muc1-deficient mice, there may be one or more additional pathways for cholesterol absorption. ATP-binding cassette (transporter); intestinal uptake; surface mucous coat; mucin gel; micelle; nutrition

NHE3, the apical isoform of the [Na.sup.+]/[H.sup.+] exchanger, is central to the absorption of salt and water across the intestinal epithelium. We report that treatment of epithelial cells with toxin B of Clostridium difficile, a diarrheal pathogen, causes a pronounced inhibition of NHE3 activity, with little effect on the basolateral NHE1 isoform. Depression of NHE3 activity is accompanied by the translocation of apical exchangers to a subapical endomembrane compartment. Treatment of cells with toxin B increased the fraction of exchangers that were solubilized by nonionic detergents and induced dephosphorylation and extensive redistribution of ezrin. The Rho-kinase inhibitor, Y-27632, also altered the distribution and activity of NHE3. We suggest that inactivation of Rho-family GTPases by clostridial toxin B alters the interaction between NHE3 and the microvillar cytoskeleton, possibly by impairing the ability of ezrin to bridge the exchangers to filamentous actin. Detachment of NHE3 from the actin skeleton would facilitate its internalization, resulting in net disappearance from the apical surface. The consequent inhibition of transport is likely to contribute to the diarrheal effects of C. difficile. KEY WORDS: Na/H exchange * antiport * pH regulation * Rho * ezrin