Current Concepts in Intestinal Peptide Absorption

Today there is considerable interest in oral peptide delivery. However, oral administration of peptides is limited by a low bioavailability and a high variability in plasma levels. A review is given of the literature describing the major barriers in peptide absorption, the basic mechanisms of intestinal peptide transport, the experimental models and the pharmaceutical approaches currently used in the investigation of peptide and protein absorption processes.     

Absorption of Phosphate and Potassium Ions in Poplar Stems

Phosphate and potassium ions were drawn through segments ofpoplar stem using a vacuum. Analysis of the activity-time curvesfor selected points along the stem were consistent with a mathematicalmodel of mass flow combined with irreversible lateral flow alongthe conducting vessels. However, the phosphate ion could beflushed out of the stems with water, so that it was necessaryto consider this as a limiting case of a reversible-flow model.Comparison is made with the activity-time curves for ion absorptioninto plant tissue, which are considered similar in shape. Avalue of 50 µm min^–1 was obtained for the lateralvelocity, and the ratio of tracer lost laterally to tracer movingon along the pipe was calculated to be about 0.7     

Hypophosphataemia and Hyperphosphataemia in a Hospital Population

One hundred cases of hypophosphataemia (≤ 2·0 mg/100 ml) and 84 cases of hyperphosphataemia (≥ 5·0 mg/100 ml) occurring in a hospital population were studied in order to determine the cause of the abnormality. Examples of hyperphosphataemia due to renal failure were excluded from the study.A low serum phosphorus concentration was most frequently due to intravenous administration of carbohydrate, usually glucose, which accounted for 40% of cases. The next commonest cause was vomiting (12%). No obvious explanation could be found in 26% of cases, but in most of these factors were present which are known to affect phosphorus metabolism.No one cause of hyperphosphataemia was outstanding in frequency and in over 50% of cases no definite explanation for the abnormality could be found.     

Isolation and Characterization of the Enterobacter cloacae cheR Mutant Defective in Phosphate Taxis

A chemotaxis-defective mutant of Enterobacter cloacae IFO3320, designated EC1, was isolated after N-methyl-N'-nitro-N-nitrosoguanidine (NTG) mutagenesis. Computer-assisted capillary assays showed that EC1 failed to show chemotactic responses to peptone and inorganic phosphate (P_i). Cloning and sequence analysis showed that EC1 is a cheR mutant, suggesting that P_i taxis by E. cloacae is dependent on a methyl-accepting chemotaxis protein(s)(MCP). EC1 was further mutagenized with NTG to construct cheR pstS and cheR pstA double mutants. A recombinant plasmid pECT01.2, which contained the E. cloacae cheR gene, restored the ability of these double mutants to show chemotaxis toward peptone but not P_i. These results suggest that the phosphate-specific transport (Pst) system, together with a MCP(s), is required for detecting P_i in E. cloacae.     

Intestinal Scavenger Receptors Are Involved in Vitamin K1 Absorption

Vitamin K₁ (phylloquinone) intestinal absorption is thought to be mediated by a carrier protein that still remains to be identified. Apical transport of vitamin K₁ was examined using Caco-2 TC-7 cell monolayers as a model of human intestinal epithelium and in transfected HEK cells. Phylloquinone uptake was then measured ex vivo using mouse intestinal explants. Finally, vitamin K₁ absorption was compared between wild-type mice and mice overexpressing scavenger receptor class B type I (SR-BI) in the intestine and mice deficient in cluster determinant 36 (CD36). Phylloquinone uptake by Caco-2 cells was saturable and was significantly impaired by co-incubation with α-tocopherol (and vice versa). Anti-human SR-BI antibodies and BLT1 (a chemical inhibitor of lipid transport via SR-BI) blocked up to 85% of vitamin K₁ uptake. BLT1 also decreased phylloquinone apical efflux by ∼80%. Transfection of HEK cells with SR-BI and CD36 significantly enhanced vitamin K₁ uptake, which was subsequently decreased by the addition of BLT1 or sulfo-N-succinimidyl oleate (CD36 inhibitor), respectively. Similar results were obtained in mouse intestinal explants. In vivo, the phylloquinone postprandial response was significantly higher, and the proximal intestine mucosa phylloquinone content 4 h after gavage was increased in mice overexpressing SR-BI compared with controls. Phylloquinone postprandial response was also significantly increased in CD36-deficient mice compared with wild-type mice, but their vitamin K₁ intestinal content remained unchanged. Overall, the present data demonstrate for the first time that intestinal scavenger receptors participate in the absorption of dietary phylloquinone.     

Hypophosphataemia after parathyroidectomy in chronic renal failure.

Out of 24 patients receiving haemodialysis who were subjected to parathyroidectomy, 13 developed hypophosphataemia; this persisted for 3-52 weeks (mean 10.6 weeks). Before operation these 13 patients had had significantly higher plasma alkaline phosphatase activities (p less than 0.01) and significantly higher values in iliac crest bone biopsy samples for active resorption surface and active formation surface (p less than 0.05 in each case) than the group who remained normophosphataemic. Significantly more of the patients who remained normophosphataemic had shown periarticular calcification in preoperative skeletal radiographs (p less than 0.001). Hypophosphataemia may result from reduced mobilisation of phosphate from bone or its increased accretion into bone, and resorption of phosphate from periarticular mineral deposits may protect against development.     

Evaluation of Intestinal Phosphate Binding to Improve the Safety Profile of Oral Sodium Phosphate Bowel Cleansing

Prior to colonoscopy, bowel cleansing is performed for which frequently oral sodium phosphate (OSP) is used. OSP results in significant hyperphosphatemia and cases of acute kidney injury (AKI) referred to as acute phosphate nephropathy (APN; characterized by nephrocalcinosis) are reported after OSP use, which led to a US-FDA warning. To improve the safety profile of OSP, it was evaluated whether the side-effects of OSP could be prevented with intestinal phosphate binders. Hereto a Wistar rat model of APN was developed. OSP administration (2 times 1.2 g phosphate by gavage) with a 12h time interval induced bowel cleansing (severe diarrhea) and significant hyperphosphatemia (21.79 ± 5.07 mg/dl 6h after the second OSP dose versus 8.44 ± 0.97 mg/dl at baseline). Concomitantly, serum PTH levels increased fivefold and FGF-23 levels showed a threefold increase, while serum calcium levels significantly decreased from 11.29 ± 0.53 mg/dl at baseline to 8.68 ± 0.79 mg/dl after OSP. OSP administration induced weaker NaPi-2a staining along the apical proximal tubular membrane. APN was induced: serum creatinine increased (1.5 times baseline) and nephrocalcinosis developed (increased renal calcium and phosphate content and calcium phosphate deposits on Von Kossa stained kidney sections). Intestinal phosphate binding (lanthanum carbonate or aluminum hydroxide) was not able to attenuate the OSP induced side-effects. In conclusion, a clinically relevant rat model of APN was developed. Animals showed increased serum phosphate levels similar to those reported in humans and developed APN. No evidence was found for an improved safety profile of OSP by using intestinal phosphate binders.     

Phosphate Absorption, Fluxes, and Symplasmic Transport in Osmotically-Shocked Zea mays Roots

Osmotic shock with sequential 30 min treatments in ice-coldsaline solutions and distilled water inhibited both the subsequentuptake of orthophosphate (Pi) and its transport into the xylemof excised corn (Zea mays L.) roots. Measurements of Pi fluxeswith ³²P indicated that the decrease in net Pi uptake over a24 h period caused by osmotic shock was due primarily to delayedrecovery of Pi influx rather than to increasing efflux. Despitecomplete recovery of Pi absorption within 2–6 h aftershocking with 150–200 mM NaCl, transport to the xylemduring the subsequent 24 h only partially recovered. Leucineuptake and incorporation into protein was also markedly inhibitedby osmotic shock but both almost completely resumed controlrates within 24 h after shocking with up to 150 mM NaCl. Tetracyclineinhibited recovery of Pi uptake after NaCl treatment whereaspuromycin did not. These results with corn roots are consistentwith the hypothesis that recovery of Pi uptake activity aftermoderate osmotic shock requires de novo synthesis of membraneproteins. Incomplete recovery of Pi transport to the xylem suggeststhat osmotic shock may damage plasmodesmata. Key words: Corn, Ion uptake, Leucine uptake, NaCl, Puromycin, Tetracycline     

Importance of Environmental pH during Root Development on Phosphate Absorption

Wheat seedlings (Triticum aestivum L. cv Gamenya) were grown for 4 days in culture solutions of differing pH prior to studying their subsequent short-term absorption of ³²Pi from solutions of the same or different pH.

Increasing pH of the absorption solution from 5.5 to 7.0 or 8.0 depressed ³²Pi absorption from 1 and 10 micromolar Pi but had little effect at 100 and 1000 micromolar Pi. Increasing the pH of the culture solution from 4.5 to 6.5 doubled or trebled subsequent ³²Pi absorption from nearly all absorption solutions over a wide range of Pi concentrations, pH, and nutrient compositions.

When seedlings were transferred between culture pH treatments 4.5 and 6.5, their capacity for ³²Pi absorption remained unchanged for at least 5 hours and adjusted by 60 to 80% after 24 hours and completely after 48 hours. This suggests that the root's capacity to absorb Pi responds to pH through slow structural changes in its mechanism of Pi absorption. P content and concentration of wheat seedlings reflected the response of ³²Pi absorption to culture pH.

It is suggested that absorption pH affects an activity component of the process for Pi absorption and culture pH affects a capacity component. Failure to recognize the capacity component of the pH response explains why previously published results for short-term ³²Pi absorption conflict with those for long-term P accumulation in plants.