Ammonia absorption from different parts of chicken intestine and its quantitative evaluation in situ

Ammonia was more rapidly absorbed from the ileum than the jejunum, and the lower the small intestine was, the more rapid was the absorption of lumen fluid. About 95% of ammonia-15N introduced into the intestinal sac having Meckel's diverticulum in the middle part disappeared in 30 min, of which 90% was recovered as non-protein-N in the blood of the mesenteric vein draining the sac and in the mucosa of the tested portion in 30 min. About 57% of ammonia which disappeared from the intestinal lumen was found as ammonia-N and 26% as other non-protein-N in the mesenteric blood. Maximal rates of appearance of total non-protein-15N and ammonia-15N in the mesenteric blood were found within 10 min and returned to the levels at 0-5 min in 30 min.     

Transmucosal passage of polyalkylcyanoacrylate nanocapsules as a new drug carrier in the small intestine

The enteral absorption of particles has been investigated in the dog using a colloidal drug carrier, polyalkylcyanoacrylate nanocapsules loaded with an iodized oil (Lipiodol), as a tracer for X-ray microprobe analysis in a scanning electron microscope. Nanocapsules are spherical capsules, 100 to 200 nm in diameter, with a continuous polymeric wall surrounding a cavity which encapsulates the drug. Administered in the jejunal lumen, Lipiodol nanocapsules improved the absorption of the tracer as indicated by increased concentration of iodine in the plasma of mesenteric blood. In order to follow nanocapsules at the cellular level, all tissue compartments were preserved in a life-like state by cryofixation and freeze-drying of intestinal biopsies. Nanocapsules appeared in the intestinal lumen close to the mucus, then in intercellular spaces and defects of the mucosa and finally in the lamina propria and blood capillaries; in this latter compartment, the iodine content was four-fold higher than after intra-jejunal administration of Lipiodol emulsion. This complete phenomenon occurred only at the tip of the villi and happened within less than 60 min. We conclude that nanocapsules enhance the rate of absorption of Lipiodol and transport the drug from the intestinal lumen to the vascular compartment using a paracellular pathway. Thus they may be useful as drug carrier for oral administration of many chemicals.     

Colonic luminal ammonia and portal blood l-glutamine and l-arginine concentrations: a possible link between colon mucosa and liver ureagenesis

The highest ammonia concentration in the body is found in the colon lumen and although there is evidence that this metabolite can be absorbed through the colonic epithelium, there is little information on the capacity of the colonic mucosa to transfer and metabolize this compound. In the present study, we used a model of conscious pig with a canula implanted into the proximal colon to inject endoluminally increasing amounts of ammonium chloride and to measure during 5 h the kinetics of ammonia and amino acid concentration changes in the portal and arterial blood. By injecting as a single dose from 1 to 5 g ammonia into the colonic lumen, a dose-related increase in ammonia concentration in the portal blood was recorded. Ammonia concentration remained unchanged in the arterial blood except for the highest dose tested, i.e. 5 g which thus apparently exceeds the hepatic ureagenesis capacity. By calculating the apparent net ammonia absorption, it was determined that the pig colonic epithelium has the capacity to absorb 4 g ammonia. Ammonia absorption through the colonic epithelium was concomitant with increase of l-glutamine and l-arginine concentrations in the portal blood. This coincided with the expression of both glutamate dehydrogenase and glutamine synthetase in isolated colonic epithelial cells. Since l-glutamine and l-arginine are known to represent activators for liver ureagenesis, we propose that increased portal concentrations of these amino acids following increased ammonia colonic luminal concentration represent a metabolic link between colon mucosa and liver urea biosynthesis.     

Excretion of metals into the rat intestine

The acute excretion of metals across the intestinal wall and by bile was investigated in vivo within 2 h after iv administration in rats. Heavy metals of biological interest, such as copper and zinc, and of toxicological importance, such as cobalt, cadmium, mercury, lead, and bismuth, as well as rubidium and strontium as examples of the alkali and alkali-earth metals were chosen. Most of the metals were excreted along a concentration gradient from blood into the intestinal lumen. Rubidium is the only metal excreted against a concentration gradient from blood into the lumen of both the small and large intestines. For all metals investigated, excretion into the small intestine exceeds that into the large intestine. Metal excretion by bile also occurred mainly along a concentration gradient from liver to bile, e.g., cobalt, zinc, mercury, rubidium, and lead, which is chosen as example of this group. Copper and strontium are excreted against a considerable concentration gradient from blood into bile. This holds true also for cadmium and bismuth in low doses.     

Regulatory mechanisms in the luminal and portal release of vasoactive intestinal polypeptide during vagal nerve stimulation in the cat

The effect of vagal stimulation in chloralose-anesthetized cats on release of vasoactive intestinal polypeptide into the jejunal lumen and portal venous blood was tested simultaneously, and the effect of atropine and hexamethonium was investigated to elucidate the regulatory mechanisms involved in the release. Vagal stimulation caused a significant increase in vasoactive intestinal polypeptide concentrations in the luminal perfusates. A significant concomitant increase was seen in portal plasma. Gel filtration chromatography of luminal and portal samples demonstrated that the vasoactive intestinal polypeptide coeluted with synthetic porcine vasoactive intestinal polypeptide. Vasoactive intestinal polypeptide infusion at 80 and 160 pmol/kg.min produced portal plasma levels of at least 3000 pM but did not increase vasoactive intestinal polypeptide concentrations in the luminal perfusates. Thus, luminal vasoactive intestinal polypeptide originates from gastrointestinal tissue rather than by transduction from the circulation. Vagally induced release of vasoactive intestinal polypeptide into the lumen and portal plasma was not abolished by atropine but was totally suppressed by hexamethonium. The regulatory mechanisms controlling the parallel release of vasoactive intestinal polypeptide into both the jejunal lumen and portal circulation are identical and involve a non-muscarinic process which is under cholinoceptive, nicotinic control.     

Characteristic transport of lactoferrin from the intestinal lumen into the bile via the blood in piglets

Lactoferrin is a major iron-binding protein in milk from several species, such as humans, monkeys, mice and sows. Using neonatal and weaner piglets, the characteristic transfer of lactoferrin from intestinal lumen into bile via the circulation was investigated. Bovine lactoferrin (1 or 3 g/kg body weight) was infused into the stomach through a polyethylene tube or into the duodenum through a duodenal catheter over 5 min. Peripheral blood and bile samples were collected after the infusion. Lactoferrin absorbed into plasma and bile were assayed quantitatively by double-antibody enzyme-linked immunosorbent assay, and homogeneity of bovine lactoferrin in plasma and bile was identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting methods. Morphological investigation was carried out according to the peroxidase anti-peroxidase method. Following oral administration in neonatal pigs, bovine lactoferrin appeared in the blood circulation and reached a peak level after 2 h. It was confirmed immunohistochemically that lactoferrin was transported by endocytosis via the epithelial cells. Lactoferrin absorbed into the blood was also detected in the bile and reached a peak value 12 h after oral administration. Transportation of lactoferrin from the intestinal lumen into the bile via the bloodstream was also observed in weaner piglets. Lactoferrin transported into plasma and bile was confirmed to be the same substance as administrated lactoferrin by electrophoresis and immunoblotting methods. Lactoferrin transported into bile was re-absorbed into the blood in neonatal pigs. These results demonstrate that lactoferrin contained in milk is transported into the circulation from the intestinal lumen and excreted into the bile, suggesting the possibility of entero-hepatic circulation of lactoferrin in neonatal pigs.     

Electron microscopic characteristics of plant peroxidase transport pathways in the microcirculatory bed of the rabbit peritoneum

The dynamics of exogenic peroxidase transfer from blood into the roots of the rabbit mesenteric lymphatic system have been studied by means of electron microscopic methods in combination with the trasser technique. Light optic identification of the vascular segments and selection of samples for electron microscopic analysis make it possible to reveal certain differences in the pathways of protein transport via the walls of the blood capillaries and venules. The vesicular transport is the only means for peroxidase to be transferred via the walls of the mesenteric blood capillaries. The time for transendothelial transfer of the marker is more than 10 min. In the venules the vesicular transport of protein does not differ from that in the capillaries, however, the predominant leakage of peroxidase from blood into the interstitium is performed through open interendothelial contacts. The hemato-interstitial transport via the intercellular clefts takes less than 3 min. For transferring protein from the interstitium into the lumen of the lymphatic capillaries and postcapillaries, the vesicular mechanism is used, and to a less extent--the open intercellular contacts. A suggestion is made that the term "open contact" should be understood in functional meaning and this means should be considered as an intercellular pathway for transporting molecules of a definite size.     

Stimulation of uric acid production and depression of increase in plasma glutamine concentration by insulin in chickens infused with ammonia

1. A single intraportal injection of insulin remarkably stimulated uric acid production in the chicken infused with ammonium acetate and significantly depressed the increase in plasma glutamine concentration by the infusion of ammonia (P less than 0.01 at 20 min, P less than 0.05 at 40 min after the start of infusion). 2. The increases in ammonia concentration in the blood and kidney during the infusion of ammonia were not affected by the insulin pretreatment. 3. The depressive effect of insulin on the increase in plasma glutamine concentration by ammonia may be due to the acceleration by insulin of incorporation of glutamine into biosynthetic pathway of uric acid in the chicken.     

Analysis of intestinal flora of a patient with congenital absence of the portal vein

Abstract A 14-year-old female patient, admitted for a closer examination of liver tumour (hepatocellular adenoma), was diagnosed as having a congenital absence of the portal vein. The blood ammonia level (approximately 120 μg dl⁻¹) in the superior mesenteric vein was markedly low compared to the normal value of 300–350 μg dl⁻¹ in the portal vein. The decreased ammonia concentration and urease activity of the patient's faeces were demonstrated. The dominant intestinal flora in the faeces of the patient, before operation, was Bifidobacterium sp., Bifidobacterium breve, Bifidobacterium lonqum, Lactobacillus plantarum , and after the operation Bacteroides vulgatus, Veillonella parvula, Peptococcus magnus Bifidobacterium longum . In contrast, Bifidobacterium bifidum, Bacteroides ureolyticus, Bacteroides ovatus and Bacteroides distasonis, B. ovatus, Bifidobacterium adolescentis were dominant flora in the faeces of two healthy volunteers, respectively. Among microorganisms isolated from the patient, Morganella morganii, Candida sp., Eubacterium aerofacience and Eubacterium rectale were strongly positive in urease activity in vitro; Streptococcus mitior, Staphylococcus intermedius, Micrococcus kristinae, Selenomonas ruminantum, Bacteroides ureolyticus and Lactobacillus casei ss. pseudoplantarum from the healthy volunteers. These results imply the homeostatic regulation system of faecal ammonia concentration by urease-producing microorganisms in the patient.     

Histological alterations of intestinal villi in chickens fed dried Bacillus subtilis var. natto

Two experiments were conducted. In experiment 1, chickens were fed dried Bacillus subtilis var. natto for 3 or 28 days. Growth performance and internal organs were not different from controls, but feed efficiency tended to be improved in the 28-day feeding. In these birds, blood ammonia concentration was decreased (P<0.05). Blood glucose concentration, and amylase and lipase activity in the intestinal content were not significantly different among dietary groups. These results suggest that the B. subtilis natto depressed ammonia concentration. In experiment 2, chickens were fed dietary B. subtilis natto for 28 days. These birds had a tendency to display greater growth performance and intestinal histologies, such as villus height, cell area and cell mitosis, than the controls. Flat cell outline on the duodenal villus surface in controls developed large, protruded cell clusters and cell protuberances after feeding of dietary B. subtilis natto. These results indicate that intestinal function was activated by the depressed blood ammonia concentration in the body of the chicken. The present results may suggest that the B. subtilis natto has the potential to be a beneficial microorganism in chickens.     

Calcium-dependent translocation of calbindin-D28k from intestine to blood

Calbindin-D (vitamin D-induced calcium-binding protein; CaBP) is known to be present in blood at concentrations which vary directly with levels in the intestinal mucosa. Employing a sensitive radioimmunoassay and sampling mesentery venous blood, the present experiments demonstrated a direct relationship between intestinal calcium absorption and serum CaBP. Solutions containing 150 mM NaCl and 45Ca-labeled calcium chloride (5 or 20 mM) were placed in the lumen of ligated duodenal preparations in situ and mesentery venous blood sampled with time. The concentration of absorbed 45Ca in serum was maximal at 5 min, followed by a significant increase in mesentery CaBP maximizing at 15-20 min. Elevation of serum CaBP was not observed when calcium in the dosing solution was omitted or replaced by either glucose or glycine. The possible transfer of absorbed calcium from the enterocyte to the circulation as a CaBP complex was ruled out by calculations revealing that considerably more calcium was transferred than could be accounted for by the low and high affinity binding sites on the protein. It is proposed that vitamin D-dependent enhanced transcellular calcium transport constitutes a stimulus for the increased release of intestinal CaBP into the circulation.     

Absorption of elastase through the jejunal mucosa of the rat. An immunocytochemical study

In order to reveal the absorption process of elastase from the intestine, hog pancreatic elastase was injected into the ligated jejunum lumen of the rat, and the tissues were cytochemically observed at various times after injection. The peroxidase anti-peroxidase (PAP) method using anti-hog-elastase rabbit antibody was used for light microscopy, and the anti-elastase Fab'-peroxidase conjugate was used for electron microscopy. The tissues stained by the PAP method exhibited a dense deposition of reaction products on the luminal surface of epithelial cells and a moderate deposition in the blood and lymph capillaries of the intestinal villi. Immunoelectron microscopy revealed that the reaction product was deposited on the surface of the microvilli and in their pocketing; some was found in the pinocytotic vesicles in the terminal-web area and on the inner surface of the enlarged smooth endoplasmic reticulum. Round droplets which gave a positive reaction were found in the widened intercellular cleft and the thick basement membrane lining the blood capillaries and lymphatics. The jejunum retained its normal ultrastructure. The results indicate that the elastase molecules, which were introduced into the rat jejunum lumen, were absorbed without being decomposed through healthy intestinal epithelial cells by pinocytosis and translocated into blood and lymph capillaries.     

Effect of endothelin-1 and vasoactive intestinal contractor on blood flow and output of vasoactive intestinal polypeptide in the feline colon

Injection of the structurally related peptides, endothelin-1 and vasoactive intestinal contractor (VIC), into a branch of the superior mesenteric artery in anesthetized cats caused dose-dependent reductions in blood flow in the portal vein and inferior mesenteric artery. The maximum effect occurred after 1 minute and was more prolonged in the portal vein. The effects of the two peptides were not significantly different. The colonic output of vasoactive intestinal polypeptide (VIP) into portal venous blood was decreased significantly by endothelin-1 and VIC, returning to baseline more rapidly than blood flow. When norepinephrine was injected to produce comparable reductions in blood flow, the output of VIP into portal venous blood was not altered significantly. These results suggest that inhibition of output of the vasodilator VIP contributes to the vasoconstrictor effects of endothelin-1 and VIC in the feline colonic vascular bed.     

Internalization and transcytosis of pancreatic enzymes by the intestinal mucosa.

As early as the beginning of the twentieth century some data indicated that macromolecules are able to cross the intestinal mucosa to reach the blood. Further evidence was added over the years; however, pathways for this transport still remain to be established. We report here the transfer of two pancreatic enzymes, amylase and lipase, from the intestinal lumen to the blood. Both are present in higher concentrations in the intestinal mucosa and in blood of fed rats. Upon cholinergic stimulation of pancreatic secretion, there was not only an increase in blood enzyme concentrations, but evidence for internalization by duodenal enterocytes was obtained. Following insertion of fluorochrome-tagged amylase and lipase into the duodenal lumen of fasting rats, blood and intestinal tissues were sampled at different time points. Serum activities for both enzymes clearly increased with time. Light microscopy established internalization of both proteins by duodenal enterocytes, and immunogold outlined the pathway taken by both proteins across the enterocytes. From the intestinal lumen, enzymes are channeled through the endosomal compartment to the Golgi apparatus and to the basolateral membrane reaching the interstitial space and blood circulation. Transcytosis through the intestinal mucosa thereby represents an access route for pancreatic enzymes to reach blood circulation.     

Quinidine,but Not Eicosanoid Antagonists or Dexamethasone,Protect the Gut from Platelet Activating Factor-Induced Vasoconstriction,Edema and Paralysis

Intestinal circulatory disturbances, atony, edema and swelling are of great clinical relevance, but the related mechanisms and possible therapeutic options are poorly characterized, in part because of the difficulties to comprehensively analyze these conditions. To overcome these limitations we have developed a model of the isolated perfused rat small intestine where all of these symptoms can be studied simultaneously. Here we used this model to study the role of eicosanoids, steroids and quinidine in platelet-activating factor (PAF)-induced intestinal disorders. A vascular bolus of PAF (0.5 nmol) triggered release of thromboxane and peptidoleukotrienes into the vascular bed (peak concentration 35 nM and 0.8 nM) and reproduced all symptoms of intestinal failure: mesenteric vasoconstriction, translocation of fluid and macromolecules from the vasculature to the lumen and lymphatics, intestinal edema formation, loss of intestinal peristalsis and decreased galactose uptake. All effects of PAF were abolished by the PAF-receptor antagonist ABT491 (2.5 μM). The COX and LOX inhibitors ASA and AA861 (500 μM, 10 μM) did not exhibit barrier-protective effects and the eicosanoid antagonists SQ29548 and MK571 (10 μM, each) only moderately attenuated the loss of vascular fluid, the redistribution to the lumen and the transfer of FITC dextran to the lumen. The steroid dexamethasone (10 μM) showed no barrier-protective properties and failed to prevent edema formation. Quinidine (100 μM) inhibited the increase in arterial pressure, stabilized all the intestinal barriers, and reduced lymph production and the transfer of FITC dextran to the lymph. While quinidine by itself reduced peristalsis, it also obviated paralysis, preserved intestinal functions and prevented edema formation. We conclude that quinidine exerts multiple protective effects against vasoconstriction, edema formation and paralysis in the intestine. The therapeutic use of quinidine for intestinal ailments deserves further study.     

Pathophysiology of cestode infections: Effect of Hymenolepis diminuta on oxygen tensions,pH and gastrointestinal function

Studies on the normal and parasitized rat intestine were used to investigate the effect of the tapeworm, Hymenolepis diminuta, on in vivo intestinal lumenal oxygen tensions, acid-base balance and mucosal absorption and accumulation of fluid and glucose.The lumenal bulk aqueous phase is considerable, well mixed and aerobic with an oxygen tension of 40–50 mm Hg. Neither the unstirred layers adjacent to the brush border membrane nor the area adjacent to the mucosa (“paramucosal lumen”) are significant barriers to the diffusion of oxygen from the blood to the intestinal lumen. In the uninfected distal ileum and colon anoxic conditions may occur in the central lumen, but, in the parasitized intestine fluid absorption is reduced and anoxic conditions do not occur. Increased H⁺ ion concentration in the parasitized intestine plays a role in increasing the availability of oxygen to intestinal helminths. Concomitant with the lower pH, the pCO₂ in the lumen of the parasitized intestine was twice as high as that found in normal animals. The total CO₂ in the parasitized intestine steadily decreased over a 3-h perfusion period, while in the normal intestine the total CO₂ content increased after an initial fall during the first 30 min of perfusion. When the worms were removed, the ability of the intestine to restore normal acid-base balance was restored. Glucose and fluid absorption in both the infected and uninfected intestine were reduced by an increase in H⁺ ion concentration; both parameters were lower in the parasitized intestine than in the normal animals. Low pH increased fluid and glucose transport by H. diminuta.While the dry weights of both the parasitized and uninfected total small intestine and of the intestinal mucosa were the same, the wet weights were considerably different, indicating defective fluid balance in the infected intestine. Accumulation of glucose by the parasitized mucosa was greater than in control animals and decreased with an increase in H⁺ ion concentration. The glucose transport system in the parasitized gut was therefore affected at two levels, one at the brush border, where transport into the mucosa was decreased by lowering the pH, and secondly at the level of the basal and lateral membranes, where transport out of the mucosal tissue into the circulatory system was also reduced.The above results are discussed in terms of current widely accepted but erroneous concepts relating to the intestinal ‘microcosm’.     

Influence of microbial species on small intestinal myoelectric activity and transit in germ-free rats

The effect of an intestinal microflora consisting of selected microbial species on myoelectric activity of small intestine was studied using germ-free rat models, with recording before and after specific intestinal colonization, in the unanesthetized state. Intestinal transit, neuropeptides in blood (RIA), and neuromessengers in the intestinal wall were determined. Clostridium tabificum vp 04 promoted regular spike burst activity, shown by a reduction of the migrating myoelectric complex (MMC) period from 30.5 +/- 3.9 min in the germ-free state to 21.2 +/- 0.14 min (P < 0.01). Lactobacillus acidophilus A10 and Bifidobacterium bifidum B11 reduced the MMC period from 27.9 +/- 4.5 to 21.5 +/- 2.1 min (P < 0.02) and accelerated small intestinal transit (P < 0.05). Micrococcus luteus showed an inhibitory effect, with an MMC period of 35.9 +/- 9.3 min compared with 27.7 +/- 6.3 min in germ-free rats (P < 0.01). Inhibition was indicated also for Escherichia coli X7 gnotobiotic rats. No consistent changes in slow wave frequency were observed. The concentration of neuropeptide Y in blood decreased after introduction of conventional intestinal microflora, suggesting reduced inhibitory control. Intestinal bacteria promote or suppress the initiation and aboral migration of the MMC depending on the species involved. Bacteria with primitive fermenting metabolism (anaerobes) emerge as important promoters of regular spike burst activity in small intestine.     

双歧杆菌三联活菌胶囊对肝性脑病患者肠黏膜屏障功能及血氨的影响

目的 探讨双歧杆菌三联活菌胶囊对肝性脑病患者肠屏障功能及血氨的影响。方法 选取100例确诊为肝性脑病的患者,随机分为对照组和治疗组各50例。对照组患者给予肝性脑病常规综合治疗,治疗组在常规综合治疗的基础上同时口服双歧杆菌三联活菌胶囊进行治疗。治疗前及治疗两周后分别检测两组患者肠黏膜屏障功能及血氨水平并比较其变化。结果 治疗前两组患者肠屏障功能及血氨水平差异无统计学意义（P>0.05）;治疗2周后,治疗组患者的血氨水平、血清二胺氧化酶（DAO）水平、血清内毒素水平（LPS）及肿瘤坏死因子-α（TNF-α）水平较治疗前显著降低,且较对照组治疗后的水平亦有明显的降低（P＜0.05）。结论 肝性脑病患者在常规综合治疗的基础上,加用双歧杆菌三联活菌胶囊可以明显改善患者肠黏膜屏障功能,减少肠源性血氨的生成,临床疗效确切。     

The gut does not contribute to systemic ammonia release in humans without portosystemic shunting

The gut is classically seen as the main source of circulating ammonia. However, the contribution of the intestines to systemic ammonia production may be limited by hepatic extraction of portal-derived ammonia. Recent data suggest that the kidney may be more important than the gut for systemic ammonia production. The aim of this study was to quantify the role of the kidney, intestines, and liver in interorgan ammonia trafficking in humans with normal liver function. In addition, we studied changes in interorgan nitrogen metabolism caused by major hepatectomy. From 21 patients undergoing surgery, blood was sampled from the portal, hepatic, and renal veins to assess intestinal, hepatic, and renal ammonia metabolism. In seven cases, blood sampling was repeated after major hepatectomy. At steady state during surgery, intestinal ammonia release was equaled by hepatic ammonia uptake, precluding significant systemic release of intestinal-derived ammonia. In contrast, the kidneys released ammonia to the systemic circulation. Major hepatectomy led to increased concentrations of ammonia and amino acids in the systemic circulation. However, transsplanchnic concentration gradients after major hepatectomy were similar to baseline values, indicating the rapid institution of a new metabolic equilibrium. In conclusion, since hepatic ammonia uptake exactly equals intestinal ammonia release, the splanchnic area, and hence the gut, probably does not contribute significantly to systemic ammonia release. After major hepatectomy, hepatic ammonia clearance is well preserved, probably related to higher circulating ammonia concentrations.     

Early and direct effect of 1,25-dihydroxycholecalciferol on calcium uptake by duodena of rachitic chicks

Injection of 1,25 dihydroxycholecalciferol (1,25(OH)2D3, 10 micrograms) directly into the in situ ligated duodenal loop of rachitic chicks significantly elevated the tissue accumulation of 47Ca within 20-30 min. The transfer of 47Ca from lumen to blood, during the same time period, was not increased nor was there any measurable intestinal calcium-binding protein synthesized. Lesser amounts of 1,25(OH)2D3 (1 or 5 micrograms) did not result in any statistically significant elevation of 47Ca tissue accumulation, nor did they have any effect on 47Ca transfer from lumen to blood (transmural). Ten micrograms of 1,24R,25(OH)3D3 was similarly effective in elevating tissue accumulation, whereas 24R,25(OH)2D3 and 25(OH)D3 were not. These results provide additional evidence for an early and direct action of 1,25(OH)2D3 in altering intestinal epithelial membrane transport prior to the induction of synthesis of specific transport proteins.