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Toxicological information

Basic toxicokinetics

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Administrative data

Endpoint:
basic toxicokinetics in vivo
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
3 (not reliable)
Rationale for reliability incl. deficiencies:
other: Documentation insufficient for assessment

Data source

Reference
Reference Type:
publication
Title:
Renal damage following the ingestion of a diet containing an excess of inorganic phosphate.
Author:
MacKay EM, Oliver J
Year:
1935
Bibliographic source:
J Exp Med. 61(3):319-334, PMID: 19870361

Materials and methods

Objective of study:
distribution
Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
Female albino rats were fed different diets containing inorganic phosphate. After 44 days, the animals were killed and anatomical measurements were conducted. Kidneys, heart and liver were preserved and subjected to histopathological examination.
GLP compliance:
no

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
- Name of test material (as cited in study report): Na2HPO4 (diet 4)
Radiolabelling:
no

Test animals

Species:
rat
Strain:
other:
Sex:
female
Details on test animals and environmental conditions:
TEST ANIMALS
- Age at study initiation: 26 days
- Individual cages: no (4-6 rats/cage)
- Diet (e.g. ad libitum): specified in Table 1

Administration / exposure

Route of administration:
oral: feed
Vehicle:
unchanged (no vehicle)
Details on exposure:
PREPARATION OF DOSING SOLUTIONS:
Inorganic phosphates were mixed in the feed
Duration and frequency of treatment / exposure:
44 days, no data on food amount/day was provided
No. of animals per sex per dose:
18-25 rats/diet
Control animals:
yes, concurrent vehicle

Results and discussion

Preliminary studies:
not performed

Toxicokinetic / pharmacokinetic studies

Details on absorption:
not examined
Details on distribution in tissues:
The mean results of the anatomical measurements made at death comprise Table 3. From these it is evident that the addition of an excess of inorganic phosphate in any form to the diet leads to a tremendous increase in the weight of the kidneys. They are likewise much larger in size than the controls. The variation in the phosphate intake had no constant effect (Table 3) upon the weight of the heart or the liver.
From the results summarized in Table 4 it is seen that there is no constant relationship between the phosphate intake and the increase in renal weight. Neither is there a consistent difference in the effect of various types of phosphate. It is evident, however, that there is a general tendency for the increase in the weight of the kidneys to keep pace with the increasing phosphate intake.

The gross appearance of all of the phosphate kidneys was unusual. Their capsules always stripped readily and the color was then ordinarily more of a greyish white than the normal red-brown, particularly after the blood had been drained from the organ. In some of the experiments most of the kidneys had a smooth but mottled surface while in others the majority were coarsely granular or even pebbled in appearance. The cortical surfaces were most granular in Experiment 4 but also very noticeably so in Experiment 3, and less so in No. 5. The kidneys from all of the phosphate-fed animals felt very much firmer than normal and on section the cortex and medulla stood out clearly, particularly the outer stripe of the outer zone of the medulla.

Microscopical examination showed essentially the same lesion in all the kidneys, irrespective of the nature of the phosphate used in the experiment. No abnormalities were found in sections of the heart or liver. The kidneys of the control animals of all the experiments were entirely normal. The lesion consists of a complete disorganization of the outer stripe of the outer zone of the medulla.
In the kidneys of the phosphate-fed animals this section is transformed into a poorly outlined zone of distorted tubules that have lost all their normal characteristics. They are lined by a regenerated epithelium of markedly atypical appearance in which hyperplasia of the component cells is so extreme that giant cell-like masses may occlude the lumen. If an irregular lumen persists it is commonly filled with debris and masses of calcareous material. Other tubules are greatly dilated and at least in part are packed with calcium deposits. About the distorted cross-sections of the tubules may be seen round cell infiltration and some fibrosis.
The cortex is also involved to a greater or less degree in all the kidneys. The lesion in this part of the kidney appears to be an extension upward from the more severely involved outer stripe of the medulla and consists of either cystic dilatation of tubules or collapse of them associated with more or less round cell infiltration in the interstitial tissue. If such areas reach the free surface of the organ a retracted scar results.
Details on excretion:
not examined

Metabolite characterisation studies

Metabolites identified:
not measured
Details on metabolites:
not measured

Any other information on results incl. tables

Table 3: Group means of anatomical measurements made at dead (day 44 of treatment)

Experiment No.

1

3

4

5

Form of added phosphate

- (control)

NaH2PO4

Na2HPO4

Na3PO4

No. of rats in experiment

25

24

18

22

Body weight (gross) [g]

135

132

111

130

Body surface [cm3]

290

284

261

285

Heart weight [mg]

501

516

446

493

Heart weight/100 cm³ body surface [mg]

173

182

178

173

Liver weight [g]

5.02

6.10

5.64

5.06

Liver weight/100 cm³ body surface [g]

2.11

2.23

2.26

1.77

Kidney weight (average) [mg]

449

848

858

794

Kidney weight/100g body weight [mg]

337

678

834

631

Kidney weight/100 cm³ body surface [mg]

154

291

342

278

 

Table 4: Food intake and kidney weight, group averages

No.

Form of added phosphate

Intake per 100 cm³ body surface/day

Kidney per 100 cm³ body surface

Phosphate above control

Food [g]

Phosphate [m.-eq.]

Total [mg]

Above control [mg]

1

- (control)

3.06

0.61

154

 

 

3

NaH2PO4

3.54

5.60

291

135

5.01

4

Na2HPO4

2.83

4.47

342

186

3.88

5

Na3PO4

3.17

3.49

278

122

2.90

Applicant's summary and conclusion

Conclusions:
Interpretation of results (migrated information): bioaccumulation potential cannot be judged based on study results
The addition of an excess of inorganic phosphate in any form to the diet leads to a tremendous increase in the weight of the kidneys, which are much larger in size than the controls, but not of liver or heart. Furthermore, excess phosphate uptake leads to necrosis of the terminal portion of the proximal convolution in the nephrons and to deposition of calcium salts in the necrotic debris that fills the involved tubules.
Executive summary:

Female rats were fed with different sodium phosphate salts (including phosphate, hydrogenorthophosphate and dihydrogenorthophosphate) for 44 days. Subsequently, the animals were killed, anatomical measurements were performed and kidneys, heart and liver were preserved and subjected to histopathological examination. For the whole methodological procedure, the authors referred to another publication which is not available to date (MacKay LL, MacKay EM, Am. J. Physiol., 1927, 83: 179). The amount of phosphate in the diet was only given in the unusual unit of milli-equivalents which (without knowledge of the food consumption of the animals) cannot be converted to daily doses. Nearly all anatomical parameters are normalized on the mean body surface of the animals which is not an accurate measure as the skin of the animals is flexible to a certain degree.

The results make evident that the addition of an excess of inorganic phosphate in any form to the diet leads to a tremendous increase in the weight of the kidneys, which are much larger in size than the controls. The variation in the phosphate intake had no effect upon the weight of the heart or the liver.

The gross appearance of the kidneys of the rats treated with phosphate was markedly different from the control animals’ kidneys.

Microscopical examination showed essentially the same lesion in all the kidneys, irrespective of the nature of the phosphate used in the experiment. No abnormalities were found in sections of the heart or liver. The kidneys of the control animals of all the experiments were entirely normal. The lesion consists of a complete disorganization of the outer stripe of the outer zone of the medulla and in part also of the cortex.

Taken together, the repeated uptake of phosphate salts in any form leads to increase of the kidneys accompanied with marked changes in macroscopic and microscopic appearance which is due to definite necrosis of the terminal portion of the proximal convolution, just at the point where the broad proximal tubulus abruptly decreases in diameter to form the narrow limb of Henle’s loop and to deposition of calcium salts in the necrotic debris that fills the involved tubules.