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PubMed1989

Interrelated effects of dietary fiber and fat on lymphatic cholesterol and triglyceride absorption in rats.

Lymph cannulated rats were administered intragastrically a test emulsion containing 25 mg of [14C]cholesterol, 50 mg of either guar gum, cellulose or chitosan, and 200 mg of either safflower, high-oleic safflower or palm oil, and the absorption of labeled cholesterol and fatty acids was measured. The type of both dietary fiber (P less than 0.001) and fat (P less than 0.05) significantly influenced cholesterol absorption. A significant interaction of fiber and fat on cholesterol absorption (P less than 0.05) was also observed. Chitosan effectively lowered cholesterol absorption more than did guar gum or cellulose, and this effect was more significant when given with safflower or high-oleic safflower oil than with palm oil. When guar gum was the source of dietary fiber, dietary fats did not modify cholesterol absorption. Dietary fiber also significantly affected triglyceride absorption (P less than 0.05). Absorption tended to be low in the chitosan, high in the cellulose and intermediate in the guar gum group. Absorption of safflower and high-oleic safflower oils tended to be higher than that of palm oil when cellulose or guar gum was fed. Guar gum, as compared with the other fibers, altered the absorption pattern of both cholesterol and triglyceride. The results showed that the type of dietary fat significantly influenced the effect that dietary fiber exerted on lipid absorption.

Animals↗
PubMed1992

Inhibitory effect of soybean protein vs. casein on apparent absorption of magnesium in rats is due to greater excretion of endogenous magnesium.

Apparent magnesium absorption is depressed in rats fed diets containing soybean protein or enriched with sodium phytate or phosphate in comparison with casein, whereas it is raised in rats fed lactose. However, the possibility that changes in apparent absorption are caused by changes in fecal excretion of endogenous magnesium cannot be excluded. We studied the effects of casein, soybean protein, sodium phytate, lactose and phosphate on apparent and true absorption of magnesium. True magnesium absorption was measured with the use of oral and intraperitoneal administration of tracer doses of 28Mg. Fecal excretion of endogenous magnesium was calculated from apparent and true absorption. True magnesium absorption was not affected by either substitution of soybean protein for casein or by the addition of sodium phytate to a diet containing casein. Endogenous magnesium excretion in feces was significantly increased by soybean protein and sodium phytate. Thus the observed impairment of apparent magnesium absorption in rats fed soybean protein or sodium phytate is due to enhanced fecal excretion of endogenous magnesium. With the other dietary treatments, enhanced fecal excretion of endogenous magnesium was not associated with a discrepancy in the effects on apparent and true magnesium absorption. Dietary lactose vs. dextrose and supplemental phosphate both stimulated fecal excretion of endogenous magnesium, but lactose raised both true and apparent magnesium absorption, and phosphate depressed both true and apparent magnesium absorption.

Absorption↗
PubMed1998

Absorption of nutrients is only slightly reduced by supplementing enteral formulas with viscous fiber in miniature pigs.

Viscous polysaccharides reduce intestinal absorption of glucose and diminish postprandial hyperglycemia. However, it is unknown whether viscous fiber also inhibits absorption of nutrients under conditions of enteric feeding. Therefore, we measured the absorption rates of nutrients in miniature pigs by perfusing a 150-cm length of jejunum with 8.37 kJ/min of the three following enteral diets: an isoosmotic oligomeric diet (1670 kJ/L), a hyperosmotic oligomeric diet and an isoosmotic polymeric diet (both 3350 kJ/L). The diets were supplemented with guar gum from 0 to 4.4 g/L. With the three guar-free diets, the mean absorption rate of energy was 5.2 +/- 0.32 kJ/min, corresponding to 62% of the energy infused. Absorption rates of carbohydrate, protein, fat and energy linearly declined as concentrations of guar or the logarithm of chyme viscosity increased. Due to modulations in viscosity, the inhibitory effects of guar were significantly different among the three diets. With the isoosmotic and hyperosmotic oligomeric and the polymeric diets, the addition of 1 g guar/L diminished the absorption of energy by 9.7, 6. 6 and 3.7%, respectively. The strong inhibitory effect on nutrient absorption with the isoosmotic oligomeric diet was caused by an increase in chyme viscosity due to water absorption. With the hyperosmotic oligomeric and the polymeric diets, the chyme viscosity and thus inhibitory effects on absorption were diminished by water secretion and the concomitant infusion of pancreatic enzymes. Results indicate that the addition of small amounts of guar gum to enteral diets of high energy density exerts only small effects on absorption of nutrients.

Animals↗
PubMed1999

Ingestion of guar gum hydrolysate, a soluble fiber, increases calcium absorption in totally gastrectomized rats.

Gastrectomy induces osteopenia. We examined the effects of feeding a diet containing soluble dietary fiber, guar gum hydrolysate (GGH, 50 g/kg diet), on intestinal calcium absorption and bone mineralization in totally gastrectomized (Roux-en-Y esophagojejunostomy) rats by comparing them with those in two control groups (laparotomized and bypassed rats). In the bypassed rats, chyme bypassed the duodenum and upper jejunum without gastrectomy. In a second separate experiment, we compared calcium absorption and bone mineralization in the gastrectomized rats fed diets containing soluble and insoluble calcium salts and in bypassed rats fed insoluble calcium. In Experiment 1, apparent absorption of calcium supplied as a water-insoluble salt was more than 50% lower in gastrectomized rats than in the intact (laparotomized) or bypassed rats 3 wk after the start of feeding the test diets (P < 0.05). Calcium absorption was higher (P < 0.05) in gastrectomized rats fed the GGH diet than in those rats fed the GGH-free diet. In Experiment 2, absorption of soluble calcium in the gastrectomized rats did not differ from the absorption of calcium from calcium carbonate by bypassed rats. The soluble calcium pool in the cecal contents was significantly lower in gastrectomized rats (Experiment 1) than in intact or bypassed control rats, and was higher (P < 0.05) in the GGH-fed gastrectomized rats than in those fed the GGH-free diet. However, calcium absorption correlated most closely (r = 0.787, P < 0.01) with cecal propionic acid concentration. The femur calcium content was significantly lower in gastrectomized rats fed insoluble calcium than in bypassed rats fed the same diet, but was partially restored in the rats fed soluble calcium (Experiment 2). Bone calcium was not increased by feeding GGH in gastrectomized rats (Experiment 1). We conclude that the severely diminished calcium absorption following total gastrectomy is totally due to a decrease in calcium solubilization, and feeding GGH partially restores calcium absorption. The decrease in bone calcium that occurs as a result of gastrectomy is mainly due to diminished intestinal calcium absorption.

Administration, Oral↗
PubMed2000

Dietary factors influencing zinc absorption.

Marginal zinc deficiency and suboptimal zinc status have been recognized in many groups of the population in both less developed and industrialized countries. Although the cause in some cases may be inadequate dietary intake of zinc, inhibitors of zinc absorption are most likely the most common causative factor. Phytate, which is present in staple foods like cereals, corn and rice, has a strong negative effect on zinc absorption from composite meals. Inositol hexaphosphates and pentaphosphates are the phytate forms that exert these negative effects, whereas the lower phosphates have no or little effect on zinc absorption. The removal or reduction of phytate by enzyme (phytase) treatment, precipitation methods, germination, fermentation or plant breeding/genetic engineering markedly improves zinc absorption. Iron can have a negative effect on zinc absorption, if given together in a supplement, whereas no effect is observed when the same amounts are present in a meal as fortificants. Cadmium, which is increasing in the environment, also inhibits zinc absorption. The amount of protein in a meal has a positive effect on zinc absorption, but individual proteins may act differently; e.g., casein has a modest inhibitory effect of zinc absorption compared with other protein sources. Amino acids, such as histidine and methionine, and other low-molecular-weight ions, such as EDTA and organic acids (e.g., citrate), are known to have a positive effect on zinc absorption and have been used for zinc supplements. Knowledge about dietary factors that inhibit zinc absorption and about ways to overcome or remove these factors is essential when designing strategies to improve the zinc nutrition of vulnerable groups.

Adult↗
PubMed2007

Iron absorption is more closely related to iron status than to daily iron intake in 12- to 48-mo-old children.

Few studies have evaluated iron absorption in small children after the first year of life. Our objectives were to examine the relations among iron intake, iron absorption, and iron status in a group of healthy children. We studied 28 children, ages 12 to 48 mo, after a 7-d home adaptation to a diet representative of their usual daily mineral intake. A multi-tracer stable isotope study was performed to assess iron absorption both from a meal ((58)Fe) and from a reference iron dose ((57)Fe) given with ascorbic acid without a meal. Iron intake was 6.9 +/- 2.4 mg, approximately the 35th percentile of typical U.S. intakes. Absorption of (58)Fe was related to serum ferritin (r(2) = 0.319, P = 0.0018) and more so to reference dose iron absorption (r(2) = 0.653, P < 0.0001). Iron absorption was negatively correlated with zinc intake (r(2) = 0.090, P = 0.0049) but was not correlated with iron intake (P = 0.20). However, zinc intake was not correlated with measures of iron status, including reference dose iron absorption and serum ferritin (r(2) < 0.1, P > 0.25). Total absorbed iron was similar to needs estimated by the Institute of Medicine. We conclude that iron absorption in young children is more closely related to iron status than to iron intake. Reference dose iron absorption may be superior to serum ferritin as a surrogate measure for iron status in this age group. Although zinc intake may affect iron absorption from a meal, it does not appear to have a detectable effect on overall iron status in otherwise well-nourished children.

Administration, Oral↗
PubMed2006

Effect of manganese source on manganese absorption by the intestine of broilers.

Two experiments were conducted to investigate the effect of Mn source on Mn absorption by the intestine of broilers. In Experiment 1, the effect of Mn source, including MnSO(4), 2 Mn-amino acid chelates (Mn-Gly and Mn-Met) synthesized in our laboratory, 3 Mn-amino acid complexes with different complex strengths (Mn-Met E, Mn-AA A, and Mn-AA B), and 2 mixtures of MnSO(4) with Gly or Met, on Mn absorption was assessed with ligated loops of different small intestinal segments of broilers. In Experiment 2, the absorption of Mn from MnSO(4), Mn-AA A, and Mn-AA B was compared with intact broilers fed ad libitum. The criterion used for comparison was the Mn content of hepatic portal vein plasma. The absorption of Mn was higher (P < 0.0002) by ligated ileal loops than by duodenal and jejunal ones. Met supplementation increased (P < 0.03) the absorption of Mn as MnSO(4). The absorption of Mn as Mn-AA A and Mn-AA B with moderate and strong complex strengths, respectively, were higher (P < 0.05) than those of Mn as MnSO(4) and Mn-Met E with weak complex strength. On d 7 and 9 of Experiment 2, the Mn content of portal vein plasma was higher (P < 0.03) for Mn-AA B with strong complex strength than for MnSO(4). On d 9, Mn content in plasma was higher (P < 0.01) for Mn-AA B with strong complex strength than for Mn-AA A with a moderate one. The results from this study confirm that the ileum was the main site of Mn absorption for broilers, and Met was more effective in facilitating Mn absorption than Gly as a ligand. Organic Mn was more efficiently absorbed than inorganic Mn (MnSO(4)); the absorption of organic Mn with moderate and strong complex strengths was greater than that of the organic Mn, which was weak, and the absorption of organic Mn with strong complex strength was greater than that of the organic Mn with a moderate strength.

Analysis of Variance↗
PubMed1996

Facilitating effect of amino acids on fructose and sorbitol absorption in children.

The facilitating effect of glucose on free fructose absorption has been suggested to be due to a sucrase-related transport mechanism. In contrast, the conditions influencing the absorption of sorbitol have hardly been investigated. As amino acids promote transcellular water flow, we investigated their effects on the absorption of fructose and sorbitol. We studied 15 healthy children using breath hydrogen tests following the ingestion of fructose and sorbitol, alone and in combination with glucose or amino acids. Similarly, the effect of acarbose pretreatment on sucrose and fructose-glucose absorption was investigated. The inhibition of sucrase isomaltase by acarbose impedes the absorption of sucrose but not of the fructose-glucose mixture. Fructose absorption is enhanced by glucose and by the amino acids L-alanine, L-glutamine, L-phenylalanine, and L-proline. Similarly, the absorption of sorbitol is facilitated by glucose and L-alanine. These results are not in concordance with a sucrase-related fructose-transport system and suggest another mechanism for glucose-induced enhancement of fructose (and sorbitol) absorption. We hypothesize that the absorption of fructose and sorbitol may be stimulated by the increased water flux induced by active absorption of glucose as well as amino acids.

Adolescent↗
PubMed1986

Local control of jejunal absorption by the renin-angiotensin system.

The purpose of this study was to determine if generation of angiotensin II (ANG II) locally within the intestine can influence ion and water absorption. Addition of ANG II to the serosal surface of isolated segments of rat jejunum significantly increased sodium and water absorption. Serosal addition of angiotensin III (ANG III) also increased jejunal absorption. Jejunum is the site of considerable angiotensinase activity. ANG III may thus mediate, at least in part, increased absorption in response to ANG II. Angiotensin I (ANG I) increased jejunal absorption. However, the increase in jejunal absorption following addition of ANG I could be prevented by inhibition of jejunal-converting enzyme. Thus, ANG I is without inherent activity to stimulate jejunal absorption. Intraperitoneal injection of a saline extract of rat kidney significantly stimulated jejunal absorption. Plasma and jejunal renin activity also increased in response to injection of kidney extract. The increase in jejunal absorption induced by kidney extract was not prevented by blockade of converting enzyme. Thus, ANG II may be generated by other enzyme pathways present in kidney homogenates. Alternatively, kidney may produce substances other than renin that are capable of influencing jejunal absorption.

Angiotensin I↗
PubMed1997

Colonic absorption of antiepileptic agents.

PURPOSE: To evaluate a canine intestinal accessport model to study colonic absorption of drugs. The antiepileptic drugs phenytoin and gabapentin were chosen to study absorption of a lipophilic and hydrophilic compound, respectively. METHODS: Drug plasma level-time plots were generated subsequent to small intestinal and colonic drug administration of both drugs. The poorly water-soluble phenytoin was administered in two doses to evaluate the impact of dissolution rate limits on colonic absorption. Maximal plasma concentration (Cmax) and area under the plasma level-time curve (AUC) were used to assess the relative contribution of colonic absorption to plasma levels. RESULTS: Whereas colonic gabapentin AUC and Cmax were only 0.25 and 0.15 of those seen after small intestinal administration, colonic phenytoin AUC and Cmax were one half and equivalent to, respectively, those observed for small intestinal administration. Furthermore, colonic administration of a higher phenytoin dose showed secondary maxima and continued increases in drug plasma levels with time. CONCLUSIONS: Colonic gabapentin absorption is poor compared with upper intestinal absorption, consistent with membrane transport rate limits to the absorption of this hydrophilic AED. Peak phenytoin plasma levels from colonic and small intestinal administration are comparable, indicating membrane transport does not limit absorption of this lipophilic agent. Continued plasma-level increases from higher phenytoin doses are consistent with dissolution-rate control of drug absorption in the colon. We suggest that colonic absorption provides a greater potential for toxicity from phenytoin overdose as a function of continued drug dissolution than for gabapentin overdose.

Acetates↗
PubMed1980

Effects of pentagastrin on intestinal absorption and blood flow in the anaesthetized dog.

1. Pentagastrin (1, 10 micrograms/min) was infused I.V. into fed and fasted anaesthetized dogs and the intestinal absorption of NaCl and H2O and blood flow were determined. The influence of pentagastrin-induced cardiovascular changes on absorption was investigated. 2. 22Na and 3H2O were used to determine the unidirectional Na and H2O fluxes from saline perfused through the ileal lumen and the clearances of 3H2O were used to calculate total and absorptive site blood flow. 3. Ileal absorption of Na and H2O was reduced by 10 micrograms/min pentagastrin due primarily to significant increases in the secretory flux of Na and decreases in the absorptive flux of H2O in both fed and fasted animals. 4. Neither total intestinal blood flow, arterial nor mesenteric vein pressure were changed by pentagastrin but absorptive site blood flow was decreased in fasted but not in fed dogs. 5. Pretreatment with atropine reduced the effects of pentagastrin but pretreatment with guanethidine potentiated the effects of pentagastrin. 6. Absorptive site blood flow was positively linearly correlated with the absorptive fluxes of both Na and H2O. The relationships between the secretory fluxes of Na and H2O and estimated capillary pressure were changed from a positive relationship in control periods to a less positive or negative relationship following pentagastrin. 7. It was concluded that pentagastrin reduces intestinal absorption through both a cardiovascular effect and an effect on the intestinal epithelium. Also, there is a strong autonomic component in the effects of pentagastrin on intestinal absorption.

Animals↗
PubMed2004

A novel strategy for the enhancement of drug absorption using a claudin modulator.

Claudin, a tight junction integral membrane protein and a family of proteins, forms the actual sealing element of the tight junction. There are more than 20 members of the claudin family with different tissue-specific expression and barrier functions. Thus, a family of claudin may be a target for modifying the absorption of drugs. Here, we examined whether modulation of claudin could be used to enhance drug absorption. In the current studies, we used a C-terminal fragment of Clostridium perfringens enterotoxin (C-CPE) as a modulator of claudin-4. The absorption of dextran was assessed in an in situ loop assay in rats to evaluate the absorption-enhancing effects of C-CPE. Treatment with C-CPE dose-dependently enhanced the absorption of dextran (mol. wt. 4000). These effects were not accompanied by injury of the intestinal mucosa as assessed by leakage of lactose dehydrogenase and histological observation. C-CPE was over 400-fold more potent at enhancing dextran absorption than capric acid, a clinically used enhancer of absorption. C-CPE interacted directly with claudin-4, and C-CPE lacking a part the C terminus neither bound claudin-4 nor enhanced absorption in the rat jejunum. These results suggest that C-CPE enhances the absorption of dextran in rat jejunum, apparently through interactions with claudin-4, and this effect may represent an effective novel strategy for enhancing the absorption of drugs.

Animals↗
PubMed1977

Significance of Ca-soap formation for calcium absorption in the rat.

The significance of calcium soap formation in the inhibition of calcium absorption has been studied in rats. 47Ca labelled soaps of fatty acids were introduced into the duodenum and the absorption of calcium measured after four hours in a whole body counter. The absorption of calcium was inversely correlated with the chain length of the fatty acid varying from 1% for Ca-stearate to 60% for Ca-hexanoate. Increasing the degree of unsaturation of the fatty acid was accompanied by increased calcium absorption. The availability of calcium for absorption from the soaps was correlated with their solubility in 1% aqueous Na-tauroglycocholate. The percentages of calcium as soap in the small intestine and the faeces after intragastric administration of calcium and fats were similar, which suggests that the faecal content of calcium soaps is an index of intestinal soap formation. Soap formation was negligible when CaCl2 was given with tristearate, triolaeate, or tridecanoate and no depression of calcium absorption was observed. Calcium absorption was markedly impaired by the addition of phosphates at a Ca/P ratio of 1:1 irrespective of the presence of neutral fats. Stearic acid resulted in significant soap formation and reduced calcium absorption. The degree of Ca-soap formation and the inhibition of calcium absorption were well correlated. The results suggest that, although calcium soap formation may markedly depress calcium absorption in the rat, no significant soap formation takes place when fats are given in the form of triglycerides.

Animals↗
PubMed1987

Jejunal water and electrolyte absorption from two proprietary enteral feeds in man: importance of sodium content.

Jejunostomy losses of Na+ and water during enteral nutrition after massive intestinal resection may be severe. We have attempted to analyse this practical problem by using an in vivo perfusion technique in healthy volunteers to study Na+, water and nutrient absorption from a short (25 cm) segment of jejunum during perfusion of an isotonic solution of the elemental diet Vivonex. Further solutions made from the amino acid and carbohydrate components of Vivonex were also perfused in part I of the study in order to determine the causes of the marked Na+ and water secretion seen during Vivonex perfusion. Low initial Na+ concentration was found to be the major determinant of net Na+ secretion, initial Na+ concentration correlating significantly with Na+ absorption (r = 0.95, n = 7 p less than 0.001). Water absorption correlated with net absorption of NaCl (r = 0.82, n = 7 p less than 0.01). There was, however, a better correlation with total absorption of NaCl plus amino acids (r = 0.99, n = 7, p less than 0.01). In part II of the study separate isotonic solutions of NaCl, glucose, and the polymeric diet, Ensure were also studied. Net sodium secretion occurred during glucose and Ensure perfusion, as predicted from their low Na+ concentration. Owing to rapid sucrose absorption from Ensure there was substantial luminal disappearance of osmotically active particles and hence marked water absorption, which was accurately predicted using the regression equation for water absorption derived in part I, substituting sucrose absorption for amino acid absorption. We conclude that the marked Na+ and water secretion observed during Vivonex perfusion is not a unique property of this amino acid based diet but is due to its low Na+ content.

Amino Acids↗
PubMed1982

Comparison of vitamin D and 25-hydroxyvitamin D absorption in the rat.

We have studied the intestinal absorption of physiological amounts of vitamin D and 25-hydroxyvitamin D [25(OH)D3] in vivo from jejunal sacs in rats with thoracic and bile duct cannulas. Under all test conditions, absorption of 25(OH)D was greater than absorption of vitamin D. The majority of absorbed vitamin D and 25(OH)D was transported from the intestine in portal blood rather than lymph. When the luminal fluid contained 2.5 mM oleic acid and monoolein, the presence of taurocholate did not affect total intestinal absorption of vitamin D or 25(OH)D but increased recovery of vitamin in lymph. When luminal fat content was increased to 10 mM oleic acid and monoolein, total absorption of both vitamin D and 25(OH)D was enhanced by taurocholate. No significant metabolism of vitamin D or 25(OH)D occurred during absorption and transport in lymph. Fifty-three percent of lymph vitamin D was found in the chylomicron fraction, compared with only 13% of 25(OH)D. Inhibition of chylomicron synthesis by cycloheximide decreased vitamin D absorption by 46% but diminished 25(OH)D absorption by only 30%. These differences in behavior of vitamin D and 25(OH)D during absorption may explain the superior absorption of 25(OH)D in patients with malabsorption.

25-Hydroxyvitamin D 2↗
PubMed1985

Regulation of jejunal blood flow and oxygenation during glucose and oleic acid absorption.

To differentiate the mechanisms whereby actively absorbed glucose and passively absorbed oleic acid increase blood flow and oxygen uptake during their absorption, the effects of these two nutrients on jejunal blood flow, arteriovenous oxygen difference [(a-v)O2], O2 uptake, absorption, rubidium extraction, and capillary permeability-surface area product (PS) were compared in anesthetized dogs. Oleic acid (37 mM) produced significantly greater hyperemia (+28.2%) than glucose (270 mM) did (+12.5%). As estimated by (a-v)O2, tissue oxygen extraction was decreased by oleic acid (-12%) but increased by glucose (+6.5%); the increases in O2 uptake by these two nutrients did not differ significantly. Glucose absorption was accompanied by an increase in rubidium extraction and capillary PS (+11.3%), whereas oleic acid absorption was not. Unlike glucose, intra-arterial infusion of oleic acid decreased vascular resistance and increased blood flow equally to the mucosa and muscularis layers. A significant relation existed between oleic acid absorption and blood flow but not between glucose absorption and blood flow. The enhancement of glucose-induced hyperemia by bile was not related to glucose absorption. Unmasking of oleic acid-induced hyperemia by bile is unrelated to oleic acid absorption but is related to solubility of oleic acid in aqueous solution. The above findings suggest that glucose absorption affects both resistance and exchange vessels, whereas oleic acid absorption affects primarily resistance vessels.

Animals↗
PubMed2004

Transepithelial HCO3- absorption is defective in renal thick ascending limbs from Na+/H+ exchanger NHE1 null mutant mice.

In the medullary thick ascending limb (MTAL) of rat kidney, inhibiting basolateral Na(+)/H(+) exchange with either amiloride or nerve growth factor (NGF) results secondarily in inhibition of apical Na(+)/H(+) exchange, thereby decreasing transepithelial HCO(3)(-) absorption. To assess the possible role of the Na(+)/H(+) exchanger NHE1 in this regulatory process, MTALs from wild-type and NHE1 knockout (NHE1(-/-)) mice were studied using in vitro microperfusion. The rate of HCO(3)(-) absorption was decreased 60% in NHE1(-/-) MTALs (15.4 +/- 0.5 pmol.min(-1).mm(-1) wild-type vs. 6.0 +/- 0.5 pmol.min(-1).mm(-1) NHE1(-/-)). Transepithelial voltage, an index of the NaCl absorption rate, did not differ in wild-type and NHE1(-/-) MTALs. Basolateral addition of 10 microM amiloride or 0.7 nM NGF decreased HCO(3)(-) absorption by 45-49% in wild-type MTALs but had no effect on HCO(3)(-) absorption in NHE1(-/-) MTALs. Inhibition of HCO(3)(-) absorption by vasopressin and stimulation by hyposmolality, both of which regulate MTAL HCO(3)(-) absorption through primary effects on apical Na(+)/H(+) exchange, were similar in wild-type and NHE1(-/-) MTALs. Thus the regulatory defect in NHE1(-/-) MTALs is specific for factors (bath amiloride and NGF) shown previously to inhibit HCO(3)(-) absorption through primary effects on basolateral Na(+)/H(+) exchange. These findings demonstrate a novel role for NHE1 in transepithelial HCO(3)(-) absorption in the MTAL, in which basolateral NHE1 controls the activity of apical NHE3. Paradoxically, a reduction in NHE1-mediated H(+) extrusion across the basolateral membrane leads to a decrease in apical Na(+)/H(+) exchange activity that reduces HCO(3)(-) absorption.

Absorption↗
PubMed1990

Thiazide-sensitive NaCl absorption in rat cortical collecting duct.

The mechanism of transepithelial NaCl transport was investigated in isolated perfused cortical collecting ducts from the kidneys of deoxycorticosterone-treated rats. In the presence of vasopressin, hydrochlorothiazide (0.1 mM) markedly reduced the net rate of Na absorption, Cl absorption, and active fluid absorption but did not significantly change the transepithelial voltage. Similarly, in the absence of vasopressin, hydrochlorothiazide decreased the rate of sodium absorption by 50% without affecting transepithelial voltage. Amiloride (30 microM) completely eliminated the lumen-negative voltage but decreased net sodium absorption only by approximately 50%. In the presence of amiloride, chloride absorption occurred against an electrochemical gradient for chloride, indicating that there was active chloride absorption. Bumetanide (0.1 mM) did not affect chloride absorption or spontaneous fluid absorption in the presence of vasopressin. The combination of amiloride and hydrochlorothiazide inhibited net sodium absorption by a greater extent than did either agent alone. These results demonstrate the presence of the following two parallel sodium transport pathways in cortical collecting ducts from mineralocorticoid-replete rats: 1) an electrogenic pathway blocked by amiloride, which presumably involves an apical sodium channel, and 2) a thiazide-inhibitable electroneutral pathway, which presumably utilizes apical Na-Cl cotransport and mediates secondary active transport of chloride.

Absorption↗