Table 1.
Techniques used to assess intestinal phosphorus uptake/absorption
| Technique | Type | Brief Description | Advantages | Disadvantages | Example of Use |
|---|---|---|---|---|---|
| Oral gavage |
in vivo Measures absorption |
A transport solution with 33P* is injected via a small tube directly through the esophagus and into the stomach or small intestine via a syringe. Absorption of 33P into blood is measured. | Measures absorption with all tissues intact. Does not need to be a terminal procedure. | Cannot completely isolate paracellular absorption component. Measurement may be affected by tissue sequestration of circulating phosphorus. | (35) |
| Ligated loop |
in vivo/in situ Measures absorption |
Ligatures are placed to close off an intestinal segment (~5cm). A transport buffer with 33P is injected into the lumen and tied off. Absorption of 33P is measured into blood and from the loop. | Keeps circulation and nervous system intact. Can test individual intestinal segments. | Cannot completely isolate paracellular absorption component. Anesthesia may affect absorption. Measurement may be affected by tissue sequestration of circulating phosphorus. Terminal procedure. | (10) |
| Everted gut sac |
ex vivo/in vitro Measures transport |
Excised intestinal segment (~5cm) is everted around a glass rod to form a sac that is filled with a buffer. Both ends are sutured closed and sac is incubated in buffer with 33P. Transport of 33P into the sac is determined. | Can test individual intestinal segments and isolate transcellular and paracellular components. Measures transport through both apical and basolateral membranes. | Tissue is detached from nervous system and circulation. Variability in measurement may occur if sac volumes differ. |
(8) |
| Everted sleeve |
ex vivo/in vitro Measures uptake |
Excised intestinal segment (~2–4 cm) is everted around a glass rod and incubated in buffer with 33P followed by an uptake termination step. Tissue is digested, and 33P uptake into the tissue is measured. | Can test individual intestinal segments and isolate transcellular and paracellular components. | Tissue is detached from nervous system and circulation. Measures uptake into the tissue, not transport through both membranes. | (15) |
| Ussing chamber |
ex vivo/in vitro Measures transport |
A segment of epithelial membrane is placed between two chambers and ion movement across the membrane is measured by the change in the potential difference upon crossing the epithelium. | Can tightly control temperature, pH, and buffer concentration. Can measure both active and passive components. | Tissue is detached from nervous system and circulation. | (12) |
| BBMV Rapid filtration |
in vitro Measures uptake |
The mucosa is scraped from an intestinal segment and the brush border membrane vesicles isolated. The vesicles are incubated in uptake buffer with 33P. The reaction mixture is transferred to a filter and 33P is counted. | Can test individual intestinal segments and isolate transcellular and paracellular components. | Measures uptake into vesicles and not absorption per se. | (36) |
| 24h urine |
in vivo/clinical Absorption biomarker |
Urine is collected from subjects over a timed 24h period and phosphorus is measured. | Noninvasive, easy to collect and measure. | Prone to timing errors/incomplete collections. May not reflect absorption, could also be affected by renal excretion rat and bone turnover. | (37) |
| Metabolic balance |
in vivo/clinical Measures absorption |
Animals are placed in individual metabolic cages. Phosphorus is measured from collected urine and feces, as well as food consumed over the time period. Net absorption can be calculated from intake and fecal excretion. In humans, balance measurements are made from controlled study diets and complete urine and fecal collections. | Does not require radioactive substances and can be performed in addition to other absorption methods in animals. | Net absorption measurement is not as precise and reflects both transcellular and paracellular pathways. |
(10) |
| Fractional Absorption with Radiotracer |
in vivo/clinical Measures absorption |
33P is administered orally and by IV. Fecal, urine, and blood collection allow for 33P measurement and kinetic modeling. | Allows for direct absorption measurement in vivo. | Burdensome to participants, requires radioactivity dosing. | (11) |
*
32P may also be used with all techniques. 33P has a lower beta emission energy so it is safer for use in humans.
Transport = movement across both the apical and basolateral membranes.
Uptake = movement across the apical membrane into the cell.
Absorption = movement across both the apical and basolateral membranes and into circulation.