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Magnesium Toxicity: Bone Lesions
ENVIRONMENT AND HEALTH Magnesium Toxicity.- Bone Lesions S. R. LEE, W. M. BRITTON, 1 and G. N. ROWLAND2 Departments of Poultry Science and Veterinary Pathology, University of Georgia, Athens, Georgia 30602
ABSTRACT Broiler chicks fed corn-soy rations supplemented with toxic levels of magnesium from one day of age grew poorly, developed diarrhea, and exhibited characteristic skeletal abnormalities. Tibiae from magnesium intoxicated chicks were shortened, thickened, and bowed. Percent tibial ash was greatly reduced. Upon microscopic examination, the bone lesion appeared rachitic as evidenced by widened and lengthened growth plates, excessive osteoid seams on endochrondral bone, and osteoid or bone capped metaphyseal blood vessels with very few associated osteoblasts. Tibial calcium to phosphorus mass ratios decreased while tibial magnesium to phosphorus mass ratios increased concomitantly with increased dietary magnesium. Calcium and phosphorus homeostasis was possibly affected as evidenced by a general decrease in size and cellularity of parathyroid glands and a general increase in size and cellularity of ultimobranchial glands. (Key words: magnesium, intoxication, calcium, phosphorus, chick, rickets) 1980 Poultry Science 59:2403-2411 INTRODUCTION Excessive dietary magnesium (Mg) usually results in skeletal abnormalities when fed t o chicks. Buckner et al. ( 1 9 3 2 ) and later Milby ( 1 9 3 4 ) observed deleterious effects, especially bowing of t h e metatarsus, when t h e M g C 0 3 c o n t e n t of t h e diet was high (1.4%). However, Gardiner et al. ( 1 9 6 0 ) and Keene and C o m b s ( 1 9 6 2 ) r e p o r t e d n o undesirable effects when Mg level in practical diets was increased. Nugara and Edwards ( 1 9 6 3 ) reported t h a t chicks did perform well with u p t o 3 2 0 0 p p m Mg in a diet containing . 3 % p h o s p h o r u s (P) and . 6 % calcium (Ca), b u t feeding of diets with Mg levels above 3 2 0 0 p p m decreased p e r c e n t tibia ash. Chicco et al. ( 1 9 6 7 ) r e p o r t e d t h a t Mg s u p p l e m e n t a t i o n of levels u p t o .4% in diets low t o n o r m a l in Ca and n o r m a l t o high in P improved growth rate and p e r c e n t tibial ash. However, w h e n t h e Mg level was increased to .6%, growth and percent tibia ash were decreased, regardless of t h e level of Ca or P. Magnesium t o x i c i t y m a y also result from its interrelationship with flouride ( F ) (Gardiner et al, 1 9 6 1 ) . T h e addition of .08% F and . 2 5 % Mg t o chick diets caused chicks t o develop leg weakness and reduced bone ash. These lesions were n o t observed in chicks fed diets supple-
1 2
Department of Poultry Science. Department of Veterinary Pathology.
m e n t e d with Mg or F alone. Griffith et al. ( 1 9 6 3 ) f o u n d similar results, e x c e p t t h a t in diets containing . 9 % Mg, reduced b o n e ash occurred in t h e absence of high dietary flouride. Spierto et al. ( 1 9 6 9 ) investigated 4 5 c a and 3 2 p incorporation i n t o b o n e mineral of chicks fed . 4 5 % Mg and . 0 8 % F and f o u n d t h a t these chicks incorporated less 4 5 c a o r 32 p j n r , 0 femurs than control chicks. Lee and Britton ( 1 9 8 0 ) showed t h a t t h e effects of Mg toxicity varied with t h e a m o u n t s of P and chloride (CI) in t h e chick diets. In those studies, with chicks reared in b a t t e r y cages, excessive dietary Mg consistently induced characteristic b o n e lesions. In several trials with dietary t r e a t m e n t replicated and r a n d o m i z e d over b a t t e r y cages and replicated over time, p e r c e n t tibia ash and tibial Ca/P were significantly lowered. Half of t h e chicks fed rations with a . 3 % Mg addition exhibited shortened, twisted, and b o w e d tibiae with reduced p e r c e n t ash. With a . 5 % or . 9 % Mg addition t o chick rations, m o r e t h a n 80% of chicks fed such rations exhibited t h e same characteristic skeletal abnormalities. T h e r e d u c t i o n of p e r c e n t b o n e ash observed could be due to lack of mineralization, excessive cartilage proliferation, excessive osteoclastic resorption, or c o m b i n a t i o n s of these factors. Studies presented here utilized t h e same dietary scheme of Lee and B r i t t o n ( 1 9 8 0 ) t o confirm previous w o r k and characterize t h e t y p e of b o n e lesions induced by Mg toxicity. Also
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(Received for publication December 3, 1979)
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LEE ET AL.
parathyroid and ultimobranchial glands were inspected to arrive at some estimate of Ca/P homeostasis. MATERIALS AND METHODS
TABLE 1. Basal diet Ingredient Corn SBM (dehulled) Fat (corn oil or poultry fat) DL-methionine Vitamin premix' Trace mineral mix 2 Total Calculated composition ME (kcal/kg) % Protein Ca level for all treatments (%) Basal P level (%) Basal CI level (%)
(g/100 g) 49.00 39.00 5.00 .20 .25 .05 93.5 3 3084 22.3 1.00 .34 .015
1
Vitamin premix provides (per kg of diet): vitamin A, 4400 IU; vitamin D3, 880 ICU; vitamin E, 11 IU; riboflavin, 4.4 mg; Ca pantothenate, 9.6 mg; nicotinic acid, 44 mg; choline CI, 220 mg; vitamin B 12 , 6.6 Mg; vitamin B 6 , 2.2 mg; menadione sodium bisulfite, 3.49 mg; folic acid, .55 mg; d-biotin, .11 mg; thiamine mononitrate, 2.2 mg; ethoxyquin, 125 mg. 2 Trace mineral mix provides (mg/kg): Mn, 60; Zn, 50; Fe, 30; Cu, 5; I, 1.05; Ca, 75 (min) and 90 (max). 'Remaining 6.5 g/100 g are derived from MgO, NaH2PO„, KH 2 P0 4 , CaC03, CaHP04, Na 2 C0 3 , NaCl, KCI, K 2 C0 3 , and cellulose fiber inappropriate combinations.
RESULTS AND DISCUSSION As previously reported (Lee and Britton, 1980), chicks fed Mg-supplemented diets developed diarrhea, grew poorly (Table 2), and exhibited characteristic skeletal abnormalities. As a result of poor growth, tissues of these chicks are smaller and may tend to confound analyses or inspections of lesions found, especially in soft tissues. However, upon close inspection of data, several significant trends are observed. As dietary Mg increased, percent ash was decreased at all P levels (Table 3). As dietary P was increased bone ash was significantly increased at all levels of dietary magnesium, but never to the level of controls. Chloride had no significant main effect. Figure 1 shows a series of tibiae collected from chicks fed rations approximately equivalent to diets fed in industry with the exception that dietary magnesium is increased. The tibiae from birds receiving corn-soy rations with no additional Mg were straight and well formed. As dietary Mg increased, the tibiae were shortened, thickened and exhibited bowing and twisting. Inspection of bone Ca/P and Mg/P mass ratios (Tables 4 and 5, respectively) show significant decreases in Ca/P and increases in Mg/P, indicating that more Mg is being incorporated into bone. The higher Ca/P ratios at .12% P may indicate that at a rachitic level of phosphorus, more bone material is present in apatite form and less is in the form of secondary calcium phosphate or brushite (Posner, 1973), since this more soluble phosphate could be lost to provide P for needs other than bone. The decrease in bone Ca/P of chicks fed .48% P
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All possible combinations (24) of Mg at 0, .3, .5, or .9% levels; P at .12, .24, or .48% levels, and CI at .135 or .27% levels were added to a corn-soy basal ration (Table 1) containing 3084 kcal/kg ME and 22.3% protein, as reported earlier (Lee and Britton, 1980). The 24 experimental diets were fed to 10 chicks per diet. Chickens were housed in electrically heated battery brooders and provided feed and water ad libitum. All chicks were weighed at 4 weeks of age, and four chicks from each dietary treatment were selected at random, sacrificed, and both tibiae, parathyroid glands, ultimobranchial bodies, thyroid glands, and kidney tissue were removed. Both tibiae were stripped of flesh and one tibia and the soft tissues were placed in 10% phosphate buffered formalin solution. The
other tibia was prepared and ashed as described previously (Lee and Britton, 1980). Fixed tibiae were then decalcified in Kristen solution and embedded in paraplast. The thyroids, parathyroids, ultimobranchial glands, kidneys, and tibiae were embedded in paraplast, sectioned at 6 ix, stained with hematoxylin and eosin (H and E), and subsequently evaluated by light microscopy. All statistical analyses were carried out using the general linear method (GLM) procedure of a SAS computer program (Barr et al., 1976) with Duncan separation (P<.05) of main effect and interaction means where applicable. Trends were deemed significant at the (P<.05) probability level.
MAGNESIUM TOXICITY:BONE LESIONS
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TABLE 2. Twenty-eight day body weight means (g) of chicks fed control to toxic levels of Mg and varying levels of P and Cl1
Added to basal % Magnesium
a,b,c,d,e,f,g,h,i,j,k Means
.48% P
.24% P
.27% Cl
.135% Cl
.27% Cl
.135% Cl
.27% Cl
549def 378 h i 325' 160 k
552cdef 433g h 34 0 hi 294J
670«b 550def 544 e f 297'j
648 b 656 b 624bcde 394g hi
6 23bcde
768a 620 b c de 692ab 50lfg
wjth different
ietters
are
640bc 637 b cd 210J k
significantly different (P<.05).
'Analysis of variance significance: Mg (P<.001); P (P<.001); Cl (P<.001); Mg X P (P<.006); Mg X Cl (P<.014); Mg X P X Cl (P<.03).
diets could be explained by inhibition of the transformation from less dense brushite to apatite (Bachra et al., 1965). Upon microscopic examination, tibiae from chicks fed diets with toxic Mg additions appear rachitic when compared to controls. The proliferating and hypertrophied zones of the tibial growth plate in relation to sizes of bones were longer and wider (Fig. 2) than the same areas from chicks fed normal levels of magnesium. Vascular invasion from the metaphysis to the zone of cartilage transformation was decreased in birds fed higher levels of magnesium. In control chicks (Mg 0, P .48%, Cl .27%), blood vessels were numerous and invaded in a straight, regular manner from the metaphysis to the hypertrophied zone (Fig. 3). In contrast, chicks receiving rations with Mg additions of .3% to .9% had fewer invading blood vessels. The vessels that did invade did
not invade as far as controls and the overall pattern of invasion was disorganized. Magnesium intoxicated chicks had metaphyseal vessels that were capped with osteoid or bone and were surrounded with few or no osteoblasts. The osteoblasts that were present were smaller and flatter than those observed from controls. Trabecular bone in the area of endochondrial ossification was disorganized in chicks fed excessive dietary Mg. Wide seams of osteoid were observed (Fig. 4). Cortical bone was much thinner than that found in control chicks. Fractures were evident in many of the tibiae from Mg intoxicated chicks. The woven bone associated with these fractures also had wide seams of osteoid. Table 6 illustrates the occurrence of rickets (lack of mineralization) over the dietary scheme. Tibiae were scored 0, 1, or 2 (0 = no rickets, 1 = marginal, 2 = severe) for rickets, summed according to dietary treatment and the means
TABLE 3. Twenty-eight day tibial ash means (%) of chicks fed control to toxic levels of Mg and varying levels ofP and Cl1
% Magnesium added to basal
.3 .5 .9
PX
.12% P
.48% P
.24% P
.135% Cl .27% Cl
.135% Cl .27% Cl
.135% Cl .27% Cl
31.2 32.1 29.9 29.5
39.9 39.8 36.7 32.2
40.4 39.0 39.4 34.0
30.7 30.4 31.4 28.4 30.4 Z
38.8 36.7 37.9 34.7
37.iy
3. b c d x v z > ' ' > '•'• Means with different letters are statistically different (P<.05). 'Analysis of variance statistical difference: Mg (P<.0001); P (P<.001).
41.7 39.5 38.6 38.0 39.1 s
,. Mg X 37.1 a 36.2 b 35.6 C 32.7 d
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.3 .5 .9
.12% P .135% Cl
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______ . • JBfe
'__•__-_•
M_H:H ' IS * L
:
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DM' _P •H-_MH___I_-I %Mg
%P
0.0 0.24 SslMnHl
%CI
%Ca I S
0.27
1.0
j
* 4Pm
w
W '.._fli
•
I %Mg
%p
%CI
%Ca
0.3
0.24
0.27
1.0
FIG. 1. Gross b o n e m o r p h o l o g y . This figure shows the effect of increasing dietary magnesium on gross b o n e morphology. N o t e the twisting and bowing of tibiae at higher Mg levels.
TABLE 4. Twenty-eight
% Magnesium added to basal
.3 .5 .9
day tibial Ca/P mass ratio means of chicks fed control levels of Mg and varying levels of P and Cl1
.12% P .135% Cl
.27% Cl
1.82aDcdef 1.95ab I 9^abc i 9iabc 1.87aboie i.90a 1 gQabcdefg j g4abcde
to
toxic
.24% P
.48% P
.135% Cl
.27% Cl
.13 5% Cl
.27% Cl
I g3abcdef
j g5abcd
1.91abc 1.80abcdefg 171efgh
1.80abcdef 1.85abcd r .77cdefg
1 72efgh 1.79bcd 1.74defg 1.57b
1.79bc 1.78cdefg 1.70fgh 1.64gh
-ibcdcffifh -1 . 1 '6. Means with different letters are significantly different ( P < . 0 5 ) . ' A n a l y s i s of variance significance: Mg ( P < . 0 0 3 ) ; Mg X Cl ( P < . 0 4 ) ; P ( P < . 0 0 0 1 ) .
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' -__-_H
MAGNESIUM TOXICITY:BONE LESIONS
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TABLE 5. Twenty-eight day tibial Mg/P mass ratio means of chicks fed control to toxic levels of Mg and varying levels ofP and CI' .12% P
% Magnesium added to basal
.48% F
.135% CI
.27% CI
.135% CI
.27% CI
.135% CI
.27% CI
.045 k .054 hi J k .060g h i j k .115 a
.044 k ,07ldefghi .089ctlef .095abc
.047J k . 0 68 c f g h i J .069 e f g h i 0 9 3 abcd
.047J k .068efghij .069 e f S h i 093abcd
,050iJk 067 fghijk 07 ocdefg .112 a b
.055 h i J k .068 e f g h 'j 0 7 5 cdefgh .090bcde
a,b,c,d,e,f,g,h,i,j,k Means with different letters are significantly different (P<.05). 'Analysis of variance significance: Mg (P-C001); Mg X CI (P<.05).
calculated. The rachitic (R) score was significantly (P<.001) affected by dietary Mg and P. The Mg X P interaction reflects the fact that one level of phosphorus in the study was rachitic and, therefore, varied less with dietary Mg.
The effects of Mg toxicity on bone may be direct or indirect or both. Since excessive dietary magnesium is cathartic, there may
fisf /;•
V
'%,
• ; •
1 y fi FIG. 2. Growth plate anomalies. Control tibia (Mg 0%, P .48%, CI .27%). Note that entire growth plate is in field of view. H and E magnification = 12.8X vs. Mg toxic tibia (Mg .9%, P .48%, CI .27%). Note that the growth plate is lengthened and widened as in rickets. H and E magnification = 12.8X.
FIG. 3. Interruption of tibial vascular invasion. Control tibia (Mg 0%, P .48%, CI .27%). Arrows indicate round, plump, probably active osteoblasts. H and E magnification = 51.2X vs. Mg toxic tibia (Mg .9%, P .48%, CI .135%). 'Indicates a capping of vessel by osteoid and/or bone while arrow indicates reduced amount of osteoblasts which is indicative of problems of mineralization. H and E magnification = 51.2X.
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.3 .5 .9
.24% P
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LEE ET AL.
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FIG. 4. Osteoid seams on tibial endochondral bone. Endochondral bone from control tibia (Mg 0%, P .48%, CI .27%). Note the minimal amount of lighter staining osteoid. This tibia is well mineralized. H and E magnification = 51.2X vs. endochondral bone from Mg toxic tibia (Mg .9%, P .48%, CI .135%). Arrows point to the greatly increased width of the osteoid seam indicative of poor mineralization. H and E magnification = 51.2X.
be malabsorption or decreased retention of nutrients essential for b o n e growth, especially phosphorus. As for direct effect, R a m p et al.
Soft Tissues. Kidney tissue appeared to be unaffected by dietary t r e a t m e n t s . Thyroid glands were decreased in size p r o p o r t i o n a t e l y with t h e reduced b o d y weights induced by Mg toxicity. High dietary magnesium had a significant effect on the size and cellular arrangement of p a r a t h y r o i d glands (PTGs) and ultimobranchial glands (UBGs). Microscopically, the appearance of t h e PTGs and t h e UBGs varied greatly over the dietary scheme and with chick b o d y weight (Table 2). At the magnesium level of the control corn-soy basal diet, parathyroid glands increased in size and cellularity with increasing dietary phosp h o r u s . T h e appearance of UBGs varied in an opposite m a n n e r with dietary P than did the appearance of PTGs and varied little with dietary Cl. At low dietary Cl (.135%) and control Mg, the PTGs were large with a heterogeneous cell
TABLE 6. Twenty-eight day rickets scores (R)1 of tibiae from chicks fed control to toxic levels of Mg and varying levels of P and Cl % Magnesium added to basal 0
.3 .5 .9
.24% P
.13 5% Cl
.12% P .27% Cl
.13 5% Cl
.27% Cl
.48% P .13 5% Cl .27% Cl
2.00 a 2.00 a 2.00 a 2.00 a
2.00 a 2.00 a 1.75 ab 2.00 a
of l.OOcdc 1.25 b c d 1.75 ab
of 1.25 b c d .75def 2.00 a
of 0f .50ef 2.00 a
a,b,c,d,e,f Means with different letters are significantly different (P<.05). 1
0, normal tibiae; 1, slightly rachitic tibiae; 2, severely rachitic.
of 0f 1.00«ie 1.50ab<-'
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( 1 9 7 7 ) n o t e d an inverse relationship of Mg level and net mineral accumulation of chick e m b r y o tibiae killed by inhibitors in vitro. He concluded that t h e actions of Mg were probably physiochemical in nature and could involve any or all of three mechanisms (Nielsen, 1973): 1) inhibition of apatite precipitation (Bachra and Fischer, 1969), 2) increased solubility of amorphous calcium p h o s p h a t e (Kramer et al., 1927), or 3) decreased transformation of a m o r p h o u s calcium p h o s p h a t e to a more stable mineral phase (Bachra et al., 1965). In these mechanisms, Mg is probably acting at the level of bone fluid. This could explain the statistically significant interaction of Mg X Cl on tibia Mg/P and Ca/P mass ratios in this study and the plasma Mg lowering main effect of chloride in previous work (Lee and Britton, 1980). Magnesium clearing from b o n e fluid or plasma m a y represent the formation of " d o u b l e " salts of chloride with magnesium.
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MAGNESIUM TOXICITY:BONE LESIONS
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FIG. 6. Parathyroid gland chief cells. Control PTG chief cells (Mg 0%, P .48%, Cl .135%). Note that cells are in wide cords to nests, are plump, and are light staining. H and E magnification = 51.2X vs. Mg toxic PTG chief cells (Mg .5%, P .24%, Cl .135%). Note that cells are in tight, narrow cords, are small, and are dark staining. H. and E magnification = 51.2X
FIG. 5. Parathyroid gland size. Control PTG (Mg 0%, P .48%, Cl .27%). This gland is large and consists of a heterogeneous population of cells in loose cords. Light staining cells predominate. H and E magnification = 12.8X vs. Mg toxic PTG (Mg .9%, P .24%, Cl .135%). This gland is much smaller (atrophied) and consists of tight cords of small, very darkly stained cells. H and E magnification = 12.8X.
with a pattern similar to controls. However, there was a slight trend for more small dark chief cells as dietary Mg increased (Figs. 5 and 6). Control UBGs had more large perivascular spaces with few cellular nests. The UBGs from Mg intoxicated chicks were usually much more cellular with cells in nests or sheets. The individual cells were larger and lighter staining (Fig. 7). There were apparent reversals in these trends in some treatments. This variation is probably attributable to the observation that when PTGs were much larger and more cellular than other PTGs observed within the dietary treatment, the chick also had UBGs slightly more cellular than other UBGs observed within the treatment. The converse was observed as well. When extremely large and cellular UBGs were observed, the corresponding PTGs were
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population of predominantly plump, light staining (H and E) parathyroid chief cells aranged in loose cords 2 to 4 cells wide or in nests of 8 to 12 cells wide similar to what was reported by Itakura et al. (1978). As dietary Mg increased at low dietary Cl, the size of the PTGs markedly decreased. The glands became less heterogeneous with small dark staining cells predominating. At the highest Mg dietary level 1.9%) and the lowest dietary Cl level (.135%), the PTGs were atrophied with very small dark chief cells arranged in tight narrow cords only one to two cells wide. The effects of increased dietary Mg at high dietary Cl (.27%) were less severe than at the low dietary Cl. The PTGs decreased less in size with increasing Mg and were not atrophied at any dietary Mg level. The chief cell population remained heterogeneous
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FIG. 7. Ultimobranchial glands. Control UBG (Mg 0%, P .48%, CI .27%). Note the large perivascular spaces and few cells. H and E Magnification = 12.8X vs. Mg toxic UBG (Mg .9%, P .48%, CI .135%). Note the greatly increased number of cells. 11 and E magnification = 12.8X.
slightly larger. This implies a certain compensation in one gland t o t h e supposed overactivity of t h e other. A s t u d y with m o r e refined techniques is in order t o characterize this a p p a r e n t interaction. T h e appearance of Mg toxic PTGs suggests t h a t Mg toxicity and its resulting hypermagnesemia (Lee and Britton, 1980) may suppress PTG activity in chicks. O t h e r studies with o t h e r species (Gitelman et ai, 1 9 6 8 ; Massry et al, 1970) have s h o w n similar results. Also, if t h e increase in size and cellularity of UBGs observed by light microscopy suggests secretory activity, t h e n this s t u d y provides s o m e in vivo evidence tiiat supports t h e hypothesis (Care et ah, 1 9 7 1 ; P e n t o et ai, 1974) t h a t hypermagnesemia can stimulate t h e release of calcitonin. Monitoring of Mg in commercial p o u l t r y rations does n o t occur, since deficiency is unlikely u n d e r n o r m a l conditions. Magnesium levels of .4 to .7% of diets in practical rations
are quite possible with s o m e dietary additions. Calcium and p h o s p h o r u s supplements can be quite high in Mg, especially limestones and rock phosphates. Meat and b o n e meals and most other animal by-products such as fishmeals also can contain much Mg. Major dietary c o m p o n e n t s such as soybean meal, corn, wheat, or fillers like bran and hulls also are good sources of M g a n d the level is related t o the soils in which these p r o d u c t s were grown. Some of the unexplained incidences of field rickets (Jensen, 1 9 7 8 ; Wise, 1978) and other related leg and skeletal problems m a y relate t o elevated dietary magnesium.
REFERENCES Bachra, B. N., and H.R.A. Fischer, 1969. The effects of some inhibitors on the nucleation and crystal growth of apatite. Calcif. Tiss. Res. 3:348-357. Bachra, B. N., O. R. Trautz, and S. L. Simon, 1965. Precipitation of calcium carbonates and phosphates. III. The effect of magnesium and flouride ions on the spontaneous precipitation of calcium carbonates and phosphates. Arch. Oral Biol. 10:731-738. Barr, A. J., J. II. Goodnight, J. P. Sail, and J. T. Helwig, 1976. SAS, a user's guide to the statistical analysis system. North Carolina State Univ., Raleigh NC. Buckner, G. D., J. H. Martin, and W. M. Insko, Jr., 1932. The effect of MgC0 3 when added to diets of growing chicks. Poultry Sci. 11:58—61. Care, A. D., R.F.L. Bates, and H. J. Gitelman, 1971. Evidence for a role of cyclic AMP in the release of calcitonin. Ann. New York Acad. Sci. 185: 317-320. Chicco, C. F., C. B. Ammerman, P. A. Van Wallegham,
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FIG. 8. Ultimobranchial and parathyroid glands. This photomicrograph illustrates the phenomenon of interaction between ultimobranchial (upper left) and parathyroid glands (lower right) of a chick fed a Mgtoxic diet (Mg .5%, P .48%, CI .13 5%). The ultimobranchial glands show a great proliferation of cells, whereas the parathyroids are only slightly reduced in size, unlike other Mg toxic parathyroid glands that are atrophied when the ultimobranchials are not as large or cellular. H and E magnification = 12.8X.
MAGNESIUM TOXICITY:BONE LESIONS
Massry, S. R., J. W. Coburn, and C. R. Kleeman, 1970. Evidence for suppression of parathyroid gland activity by hypermagnesemia. J. Clin. Invest. 49:1619-1628. Milby, T. T., 1934. Factors influencing the malformation of the leg bones of growing chickens. Iowa Agr. Exp. Sta. Res. Bull. 172. Neilsen, S. P., 1973. Effects of magnesium on calcification of young bone in tissue culture. Calcif. Tiss. Res. 11:78-94. Nugara, D., and H. M. Edwards, Jr., 1963. Influence of dietary calcium and phosphorus levels on the magnesium requirement of the chick. J. Nutr. 80:181-184. Pento, J. T., S. M. Glick, A. Kagan, and P. C. Gorfein, 1974. The relative influence of calcium, strontium, and magnesium on calcitonin secretion in the pig. Endocrinology 94:1176-1180. Posner, A. S., 1973. Bone mineral on the molecular level. Fed. Proc. 32:1933-1937. Ramp, W. K., J. R. Thomas, and P. D. Nifong, 1977. Magnesium and the mineral metabolism of chick embryo tibiae in organ culture. Calcif. Tiss. Res. 24:93-98. Spierto, F., J. C. Rogler, and H. E. Parker, 1969. Effect of dietary magnesium and flouride on bone mineralization in young chicks. J. Nutr. 98:271-278. Wise, D. R., 1978. Nutrition-disease interactions of leg weakness in poultry. Pages 41—57 in Recent advances in animal nutrition. W. H. Aresign and D. Lewis, ed. Butterworths, London.
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P. G. Waldroup, and R. H. Harms, 1967. Effects of varying dietary ratios of magnesium, calcium, and phosphorus in growing chicks. Poultry Sci. 46:367-373. Gardiner, E. E., J. C. Rogler, and H. E. Parker, 1960. Magnesium requirement of the chick. Poultry Sci. 39:1111-1115. Gardiner, E. E., J. C. Rogler, and H. E. Parker, 1961. Interrelationships between magnesium and flouride in chicks J. Nutr. 75:270-274. Gitelman, H. J., S. Kukolj, and L. G. Welt, 1968. Inhibition of parathryoid gland activity by hypermagnesemia. Am. J. Physiol. 215:483—485. Griffith, F. D., H. E. Parker, and J. C. Rogler, 1963. Observations of a magnesium-flouride interrelationship in chicks. J. Nutr. 79:251-256. Itakura, C , Y. Yamasaki, and M. Goto, 1978. Pathology of vitamin D deficiency rickets in growing chickens. II. Parathyroid gland. Avian Pathol. 7:515-532. Jensen, L. S., 1978. Leg weakness in broilers and turkeys. Proc. Arkansas Nutr. Conf. 17—22. Keene, O. D., and G. F. Combs, 1962. Magnesium requirement of chicks and poults. Poultry Sci. 41:1654. Kramer, B., D. H. Shelling, and E. R. Orent, 1927. Studies upon calcification in vitro. II. On the inhibitory effect of the magnesium ion. Bull. Johns Hopkins Univ. 4 1 : 4 2 6 - 4 3 1 . Lee, S., and W. M. Britton, 1980. Magnesium toxicity: Effect on phosphorus utilization by broiler chicks. Poultry Sci. 59:1989-1994.
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ABSTRACT Broiler chicks fed corn-soy rations supplemented with toxic levels of magnesium from one day of age grew poorly, developed diarrhea, and exhibited characteristic skeletal abnormalities. Tibiae from magnesium intoxicated chicks were shortened, thickened, and bowed. Percent tibial ash was greatly reduced. Upon microscopic examination, the bone lesion appeared rachitic as evidenced by widened and lengthened growth plates, excessive osteoid seams on endochrondral bone, and osteoid or bone capped metaphyseal blood vessels with very few associated osteoblasts. Tibial calcium to phosphorus mass ratios decreased while tibial magnesium to phosphorus mass ratios increased concomitantly with increased dietary magnesium. Calcium and phosphorus homeostasis was possibly affected as evidenced by a general decrease in size and cellularity of parathyroid glands and a general increase in size and cellularity of ultimobranchial glands. (Key words: magnesium, intoxication, calcium, phosphorus, chick, rickets) 1980 Poultry Science 59:2403-2411 INTRODUCTION Excessive dietary magnesium (Mg) usually results in skeletal abnormalities when fed t o chicks. Buckner et al. ( 1 9 3 2 ) and later Milby ( 1 9 3 4 ) observed deleterious effects, especially bowing of t h e metatarsus, when t h e M g C 0 3 c o n t e n t of t h e diet was high (1.4%). However, Gardiner et al. ( 1 9 6 0 ) and Keene and C o m b s ( 1 9 6 2 ) r e p o r t e d n o undesirable effects when Mg level in practical diets was increased. Nugara and Edwards ( 1 9 6 3 ) reported t h a t chicks did perform well with u p t o 3 2 0 0 p p m Mg in a diet containing . 3 % p h o s p h o r u s (P) and . 6 % calcium (Ca), b u t feeding of diets with Mg levels above 3 2 0 0 p p m decreased p e r c e n t tibia ash. Chicco et al. ( 1 9 6 7 ) r e p o r t e d t h a t Mg s u p p l e m e n t a t i o n of levels u p t o .4% in diets low t o n o r m a l in Ca and n o r m a l t o high in P improved growth rate and p e r c e n t tibial ash. However, w h e n t h e Mg level was increased to .6%, growth and percent tibia ash were decreased, regardless of t h e level of Ca or P. Magnesium t o x i c i t y m a y also result from its interrelationship with flouride ( F ) (Gardiner et al, 1 9 6 1 ) . T h e addition of .08% F and . 2 5 % Mg t o chick diets caused chicks t o develop leg weakness and reduced bone ash. These lesions were n o t observed in chicks fed diets supple-
1 2
Department of Poultry Science. Department of Veterinary Pathology.
m e n t e d with Mg or F alone. Griffith et al. ( 1 9 6 3 ) f o u n d similar results, e x c e p t t h a t in diets containing . 9 % Mg, reduced b o n e ash occurred in t h e absence of high dietary flouride. Spierto et al. ( 1 9 6 9 ) investigated 4 5 c a and 3 2 p incorporation i n t o b o n e mineral of chicks fed . 4 5 % Mg and . 0 8 % F and f o u n d t h a t these chicks incorporated less 4 5 c a o r 32 p j n r , 0 femurs than control chicks. Lee and Britton ( 1 9 8 0 ) showed t h a t t h e effects of Mg toxicity varied with t h e a m o u n t s of P and chloride (CI) in t h e chick diets. In those studies, with chicks reared in b a t t e r y cages, excessive dietary Mg consistently induced characteristic b o n e lesions. In several trials with dietary t r e a t m e n t replicated and r a n d o m i z e d over b a t t e r y cages and replicated over time, p e r c e n t tibia ash and tibial Ca/P were significantly lowered. Half of t h e chicks fed rations with a . 3 % Mg addition exhibited shortened, twisted, and b o w e d tibiae with reduced p e r c e n t ash. With a . 5 % or . 9 % Mg addition t o chick rations, m o r e t h a n 80% of chicks fed such rations exhibited t h e same characteristic skeletal abnormalities. T h e r e d u c t i o n of p e r c e n t b o n e ash observed could be due to lack of mineralization, excessive cartilage proliferation, excessive osteoclastic resorption, or c o m b i n a t i o n s of these factors. Studies presented here utilized t h e same dietary scheme of Lee and B r i t t o n ( 1 9 8 0 ) t o confirm previous w o r k and characterize t h e t y p e of b o n e lesions induced by Mg toxicity. Also
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(Received for publication December 3, 1979)
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LEE ET AL.
parathyroid and ultimobranchial glands were inspected to arrive at some estimate of Ca/P homeostasis. MATERIALS AND METHODS
TABLE 1. Basal diet Ingredient Corn SBM (dehulled) Fat (corn oil or poultry fat) DL-methionine Vitamin premix' Trace mineral mix 2 Total Calculated composition ME (kcal/kg) % Protein Ca level for all treatments (%) Basal P level (%) Basal CI level (%)
(g/100 g) 49.00 39.00 5.00 .20 .25 .05 93.5 3 3084 22.3 1.00 .34 .015
1
Vitamin premix provides (per kg of diet): vitamin A, 4400 IU; vitamin D3, 880 ICU; vitamin E, 11 IU; riboflavin, 4.4 mg; Ca pantothenate, 9.6 mg; nicotinic acid, 44 mg; choline CI, 220 mg; vitamin B 12 , 6.6 Mg; vitamin B 6 , 2.2 mg; menadione sodium bisulfite, 3.49 mg; folic acid, .55 mg; d-biotin, .11 mg; thiamine mononitrate, 2.2 mg; ethoxyquin, 125 mg. 2 Trace mineral mix provides (mg/kg): Mn, 60; Zn, 50; Fe, 30; Cu, 5; I, 1.05; Ca, 75 (min) and 90 (max). 'Remaining 6.5 g/100 g are derived from MgO, NaH2PO„, KH 2 P0 4 , CaC03, CaHP04, Na 2 C0 3 , NaCl, KCI, K 2 C0 3 , and cellulose fiber inappropriate combinations.
RESULTS AND DISCUSSION As previously reported (Lee and Britton, 1980), chicks fed Mg-supplemented diets developed diarrhea, grew poorly (Table 2), and exhibited characteristic skeletal abnormalities. As a result of poor growth, tissues of these chicks are smaller and may tend to confound analyses or inspections of lesions found, especially in soft tissues. However, upon close inspection of data, several significant trends are observed. As dietary Mg increased, percent ash was decreased at all P levels (Table 3). As dietary P was increased bone ash was significantly increased at all levels of dietary magnesium, but never to the level of controls. Chloride had no significant main effect. Figure 1 shows a series of tibiae collected from chicks fed rations approximately equivalent to diets fed in industry with the exception that dietary magnesium is increased. The tibiae from birds receiving corn-soy rations with no additional Mg were straight and well formed. As dietary Mg increased, the tibiae were shortened, thickened and exhibited bowing and twisting. Inspection of bone Ca/P and Mg/P mass ratios (Tables 4 and 5, respectively) show significant decreases in Ca/P and increases in Mg/P, indicating that more Mg is being incorporated into bone. The higher Ca/P ratios at .12% P may indicate that at a rachitic level of phosphorus, more bone material is present in apatite form and less is in the form of secondary calcium phosphate or brushite (Posner, 1973), since this more soluble phosphate could be lost to provide P for needs other than bone. The decrease in bone Ca/P of chicks fed .48% P
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All possible combinations (24) of Mg at 0, .3, .5, or .9% levels; P at .12, .24, or .48% levels, and CI at .135 or .27% levels were added to a corn-soy basal ration (Table 1) containing 3084 kcal/kg ME and 22.3% protein, as reported earlier (Lee and Britton, 1980). The 24 experimental diets were fed to 10 chicks per diet. Chickens were housed in electrically heated battery brooders and provided feed and water ad libitum. All chicks were weighed at 4 weeks of age, and four chicks from each dietary treatment were selected at random, sacrificed, and both tibiae, parathyroid glands, ultimobranchial bodies, thyroid glands, and kidney tissue were removed. Both tibiae were stripped of flesh and one tibia and the soft tissues were placed in 10% phosphate buffered formalin solution. The
other tibia was prepared and ashed as described previously (Lee and Britton, 1980). Fixed tibiae were then decalcified in Kristen solution and embedded in paraplast. The thyroids, parathyroids, ultimobranchial glands, kidneys, and tibiae were embedded in paraplast, sectioned at 6 ix, stained with hematoxylin and eosin (H and E), and subsequently evaluated by light microscopy. All statistical analyses were carried out using the general linear method (GLM) procedure of a SAS computer program (Barr et al., 1976) with Duncan separation (P<.05) of main effect and interaction means where applicable. Trends were deemed significant at the (P<.05) probability level.
MAGNESIUM TOXICITY:BONE LESIONS
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TABLE 2. Twenty-eight day body weight means (g) of chicks fed control to toxic levels of Mg and varying levels of P and Cl1
Added to basal % Magnesium
a,b,c,d,e,f,g,h,i,j,k Means
.48% P
.24% P
.27% Cl
.135% Cl
.27% Cl
.135% Cl
.27% Cl
549def 378 h i 325' 160 k
552cdef 433g h 34 0 hi 294J
670«b 550def 544 e f 297'j
648 b 656 b 624bcde 394g hi
6 23bcde
768a 620 b c de 692ab 50lfg
wjth different
ietters
are
640bc 637 b cd 210J k
significantly different (P<.05).
'Analysis of variance significance: Mg (P<.001); P (P<.001); Cl (P<.001); Mg X P (P<.006); Mg X Cl (P<.014); Mg X P X Cl (P<.03).
diets could be explained by inhibition of the transformation from less dense brushite to apatite (Bachra et al., 1965). Upon microscopic examination, tibiae from chicks fed diets with toxic Mg additions appear rachitic when compared to controls. The proliferating and hypertrophied zones of the tibial growth plate in relation to sizes of bones were longer and wider (Fig. 2) than the same areas from chicks fed normal levels of magnesium. Vascular invasion from the metaphysis to the zone of cartilage transformation was decreased in birds fed higher levels of magnesium. In control chicks (Mg 0, P .48%, Cl .27%), blood vessels were numerous and invaded in a straight, regular manner from the metaphysis to the hypertrophied zone (Fig. 3). In contrast, chicks receiving rations with Mg additions of .3% to .9% had fewer invading blood vessels. The vessels that did invade did
not invade as far as controls and the overall pattern of invasion was disorganized. Magnesium intoxicated chicks had metaphyseal vessels that were capped with osteoid or bone and were surrounded with few or no osteoblasts. The osteoblasts that were present were smaller and flatter than those observed from controls. Trabecular bone in the area of endochondrial ossification was disorganized in chicks fed excessive dietary Mg. Wide seams of osteoid were observed (Fig. 4). Cortical bone was much thinner than that found in control chicks. Fractures were evident in many of the tibiae from Mg intoxicated chicks. The woven bone associated with these fractures also had wide seams of osteoid. Table 6 illustrates the occurrence of rickets (lack of mineralization) over the dietary scheme. Tibiae were scored 0, 1, or 2 (0 = no rickets, 1 = marginal, 2 = severe) for rickets, summed according to dietary treatment and the means
TABLE 3. Twenty-eight day tibial ash means (%) of chicks fed control to toxic levels of Mg and varying levels ofP and Cl1
% Magnesium added to basal
.3 .5 .9
PX
.12% P
.48% P
.24% P
.135% Cl .27% Cl
.135% Cl .27% Cl
.135% Cl .27% Cl
31.2 32.1 29.9 29.5
39.9 39.8 36.7 32.2
40.4 39.0 39.4 34.0
30.7 30.4 31.4 28.4 30.4 Z
38.8 36.7 37.9 34.7
37.iy
3. b c d x v z > ' ' > '•'• Means with different letters are statistically different (P<.05). 'Analysis of variance statistical difference: Mg (P<.0001); P (P<.001).
41.7 39.5 38.6 38.0 39.1 s
,. Mg X 37.1 a 36.2 b 35.6 C 32.7 d
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.3 .5 .9
.12% P .135% Cl
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LEE ET A L .
______ . • JBfe
'__•__-_•
M_H:H ' IS * L
:
__K
DM' _P •H-_MH___I_-I %Mg
%P
0.0 0.24 SslMnHl
%CI
%Ca I S
0.27
1.0
j
* 4Pm
w
W '.._fli
•
I %Mg
%p
%CI
%Ca
0.3
0.24
0.27
1.0
FIG. 1. Gross b o n e m o r p h o l o g y . This figure shows the effect of increasing dietary magnesium on gross b o n e morphology. N o t e the twisting and bowing of tibiae at higher Mg levels.
TABLE 4. Twenty-eight
% Magnesium added to basal
.3 .5 .9
day tibial Ca/P mass ratio means of chicks fed control levels of Mg and varying levels of P and Cl1
.12% P .135% Cl
.27% Cl
1.82aDcdef 1.95ab I 9^abc i 9iabc 1.87aboie i.90a 1 gQabcdefg j g4abcde
to
toxic
.24% P
.48% P
.135% Cl
.27% Cl
.13 5% Cl
.27% Cl
I g3abcdef
j g5abcd
1.91abc 1.80abcdefg 171efgh
1.80abcdef 1.85abcd r .77cdefg
1 72efgh 1.79bcd 1.74defg 1.57b
1.79bc 1.78cdefg 1.70fgh 1.64gh
-ibcdcffifh -1 . 1 '6. Means with different letters are significantly different ( P < . 0 5 ) . ' A n a l y s i s of variance significance: Mg ( P < . 0 0 3 ) ; Mg X Cl ( P < . 0 4 ) ; P ( P < . 0 0 0 1 ) .
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' -__-_H
MAGNESIUM TOXICITY:BONE LESIONS
2407
TABLE 5. Twenty-eight day tibial Mg/P mass ratio means of chicks fed control to toxic levels of Mg and varying levels ofP and CI' .12% P
% Magnesium added to basal
.48% F
.135% CI
.27% CI
.135% CI
.27% CI
.135% CI
.27% CI
.045 k .054 hi J k .060g h i j k .115 a
.044 k ,07ldefghi .089ctlef .095abc
.047J k . 0 68 c f g h i J .069 e f g h i 0 9 3 abcd
.047J k .068efghij .069 e f S h i 093abcd
,050iJk 067 fghijk 07 ocdefg .112 a b
.055 h i J k .068 e f g h 'j 0 7 5 cdefgh .090bcde
a,b,c,d,e,f,g,h,i,j,k Means with different letters are significantly different (P<.05). 'Analysis of variance significance: Mg (P-C001); Mg X CI (P<.05).
calculated. The rachitic (R) score was significantly (P<.001) affected by dietary Mg and P. The Mg X P interaction reflects the fact that one level of phosphorus in the study was rachitic and, therefore, varied less with dietary Mg.
The effects of Mg toxicity on bone may be direct or indirect or both. Since excessive dietary magnesium is cathartic, there may
fisf /;•
V
'%,
• ; •
1 y fi FIG. 2. Growth plate anomalies. Control tibia (Mg 0%, P .48%, CI .27%). Note that entire growth plate is in field of view. H and E magnification = 12.8X vs. Mg toxic tibia (Mg .9%, P .48%, CI .27%). Note that the growth plate is lengthened and widened as in rickets. H and E magnification = 12.8X.
FIG. 3. Interruption of tibial vascular invasion. Control tibia (Mg 0%, P .48%, CI .27%). Arrows indicate round, plump, probably active osteoblasts. H and E magnification = 51.2X vs. Mg toxic tibia (Mg .9%, P .48%, CI .135%). 'Indicates a capping of vessel by osteoid and/or bone while arrow indicates reduced amount of osteoblasts which is indicative of problems of mineralization. H and E magnification = 51.2X.
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.3 .5 .9
.24% P
2408
LEE ET AL.
«.;*:*»«
v «.
I:
*
FIG. 4. Osteoid seams on tibial endochondral bone. Endochondral bone from control tibia (Mg 0%, P .48%, CI .27%). Note the minimal amount of lighter staining osteoid. This tibia is well mineralized. H and E magnification = 51.2X vs. endochondral bone from Mg toxic tibia (Mg .9%, P .48%, CI .135%). Arrows point to the greatly increased width of the osteoid seam indicative of poor mineralization. H and E magnification = 51.2X.
be malabsorption or decreased retention of nutrients essential for b o n e growth, especially phosphorus. As for direct effect, R a m p et al.
Soft Tissues. Kidney tissue appeared to be unaffected by dietary t r e a t m e n t s . Thyroid glands were decreased in size p r o p o r t i o n a t e l y with t h e reduced b o d y weights induced by Mg toxicity. High dietary magnesium had a significant effect on the size and cellular arrangement of p a r a t h y r o i d glands (PTGs) and ultimobranchial glands (UBGs). Microscopically, the appearance of t h e PTGs and t h e UBGs varied greatly over the dietary scheme and with chick b o d y weight (Table 2). At the magnesium level of the control corn-soy basal diet, parathyroid glands increased in size and cellularity with increasing dietary phosp h o r u s . T h e appearance of UBGs varied in an opposite m a n n e r with dietary P than did the appearance of PTGs and varied little with dietary Cl. At low dietary Cl (.135%) and control Mg, the PTGs were large with a heterogeneous cell
TABLE 6. Twenty-eight day rickets scores (R)1 of tibiae from chicks fed control to toxic levels of Mg and varying levels of P and Cl % Magnesium added to basal 0
.3 .5 .9
.24% P
.13 5% Cl
.12% P .27% Cl
.13 5% Cl
.27% Cl
.48% P .13 5% Cl .27% Cl
2.00 a 2.00 a 2.00 a 2.00 a
2.00 a 2.00 a 1.75 ab 2.00 a
of l.OOcdc 1.25 b c d 1.75 ab
of 1.25 b c d .75def 2.00 a
of 0f .50ef 2.00 a
a,b,c,d,e,f Means with different letters are significantly different (P<.05). 1
0, normal tibiae; 1, slightly rachitic tibiae; 2, severely rachitic.
of 0f 1.00«ie 1.50ab<-'
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( 1 9 7 7 ) n o t e d an inverse relationship of Mg level and net mineral accumulation of chick e m b r y o tibiae killed by inhibitors in vitro. He concluded that t h e actions of Mg were probably physiochemical in nature and could involve any or all of three mechanisms (Nielsen, 1973): 1) inhibition of apatite precipitation (Bachra and Fischer, 1969), 2) increased solubility of amorphous calcium p h o s p h a t e (Kramer et al., 1927), or 3) decreased transformation of a m o r p h o u s calcium p h o s p h a t e to a more stable mineral phase (Bachra et al., 1965). In these mechanisms, Mg is probably acting at the level of bone fluid. This could explain the statistically significant interaction of Mg X Cl on tibia Mg/P and Ca/P mass ratios in this study and the plasma Mg lowering main effect of chloride in previous work (Lee and Britton, 1980). Magnesium clearing from b o n e fluid or plasma m a y represent the formation of " d o u b l e " salts of chloride with magnesium.
•*-*:"
MAGNESIUM TOXICITY:BONE LESIONS
2409
i-- • .• ':."•*. *^*Jt^
'"•""' *$
•* *"*>*• t*;?-*-^ 3 *'
FIG. 6. Parathyroid gland chief cells. Control PTG chief cells (Mg 0%, P .48%, Cl .135%). Note that cells are in wide cords to nests, are plump, and are light staining. H and E magnification = 51.2X vs. Mg toxic PTG chief cells (Mg .5%, P .24%, Cl .135%). Note that cells are in tight, narrow cords, are small, and are dark staining. H. and E magnification = 51.2X
FIG. 5. Parathyroid gland size. Control PTG (Mg 0%, P .48%, Cl .27%). This gland is large and consists of a heterogeneous population of cells in loose cords. Light staining cells predominate. H and E magnification = 12.8X vs. Mg toxic PTG (Mg .9%, P .24%, Cl .135%). This gland is much smaller (atrophied) and consists of tight cords of small, very darkly stained cells. H and E magnification = 12.8X.
with a pattern similar to controls. However, there was a slight trend for more small dark chief cells as dietary Mg increased (Figs. 5 and 6). Control UBGs had more large perivascular spaces with few cellular nests. The UBGs from Mg intoxicated chicks were usually much more cellular with cells in nests or sheets. The individual cells were larger and lighter staining (Fig. 7). There were apparent reversals in these trends in some treatments. This variation is probably attributable to the observation that when PTGs were much larger and more cellular than other PTGs observed within the dietary treatment, the chick also had UBGs slightly more cellular than other UBGs observed within the treatment. The converse was observed as well. When extremely large and cellular UBGs were observed, the corresponding PTGs were
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population of predominantly plump, light staining (H and E) parathyroid chief cells aranged in loose cords 2 to 4 cells wide or in nests of 8 to 12 cells wide similar to what was reported by Itakura et al. (1978). As dietary Mg increased at low dietary Cl, the size of the PTGs markedly decreased. The glands became less heterogeneous with small dark staining cells predominating. At the highest Mg dietary level 1.9%) and the lowest dietary Cl level (.135%), the PTGs were atrophied with very small dark chief cells arranged in tight narrow cords only one to two cells wide. The effects of increased dietary Mg at high dietary Cl (.27%) were less severe than at the low dietary Cl. The PTGs decreased less in size with increasing Mg and were not atrophied at any dietary Mg level. The chief cell population remained heterogeneous
2410
LEE ETAL,
FIG. 7. Ultimobranchial glands. Control UBG (Mg 0%, P .48%, CI .27%). Note the large perivascular spaces and few cells. H and E Magnification = 12.8X vs. Mg toxic UBG (Mg .9%, P .48%, CI .135%). Note the greatly increased number of cells. 11 and E magnification = 12.8X.
slightly larger. This implies a certain compensation in one gland t o t h e supposed overactivity of t h e other. A s t u d y with m o r e refined techniques is in order t o characterize this a p p a r e n t interaction. T h e appearance of Mg toxic PTGs suggests t h a t Mg toxicity and its resulting hypermagnesemia (Lee and Britton, 1980) may suppress PTG activity in chicks. O t h e r studies with o t h e r species (Gitelman et ai, 1 9 6 8 ; Massry et al, 1970) have s h o w n similar results. Also, if t h e increase in size and cellularity of UBGs observed by light microscopy suggests secretory activity, t h e n this s t u d y provides s o m e in vivo evidence tiiat supports t h e hypothesis (Care et ah, 1 9 7 1 ; P e n t o et ai, 1974) t h a t hypermagnesemia can stimulate t h e release of calcitonin. Monitoring of Mg in commercial p o u l t r y rations does n o t occur, since deficiency is unlikely u n d e r n o r m a l conditions. Magnesium levels of .4 to .7% of diets in practical rations
are quite possible with s o m e dietary additions. Calcium and p h o s p h o r u s supplements can be quite high in Mg, especially limestones and rock phosphates. Meat and b o n e meals and most other animal by-products such as fishmeals also can contain much Mg. Major dietary c o m p o n e n t s such as soybean meal, corn, wheat, or fillers like bran and hulls also are good sources of M g a n d the level is related t o the soils in which these p r o d u c t s were grown. Some of the unexplained incidences of field rickets (Jensen, 1 9 7 8 ; Wise, 1978) and other related leg and skeletal problems m a y relate t o elevated dietary magnesium.
REFERENCES Bachra, B. N., and H.R.A. Fischer, 1969. The effects of some inhibitors on the nucleation and crystal growth of apatite. Calcif. Tiss. Res. 3:348-357. Bachra, B. N., O. R. Trautz, and S. L. Simon, 1965. Precipitation of calcium carbonates and phosphates. III. The effect of magnesium and flouride ions on the spontaneous precipitation of calcium carbonates and phosphates. Arch. Oral Biol. 10:731-738. Barr, A. J., J. II. Goodnight, J. P. Sail, and J. T. Helwig, 1976. SAS, a user's guide to the statistical analysis system. North Carolina State Univ., Raleigh NC. Buckner, G. D., J. H. Martin, and W. M. Insko, Jr., 1932. The effect of MgC0 3 when added to diets of growing chicks. Poultry Sci. 11:58—61. Care, A. D., R.F.L. Bates, and H. J. Gitelman, 1971. Evidence for a role of cyclic AMP in the release of calcitonin. Ann. New York Acad. Sci. 185: 317-320. Chicco, C. F., C. B. Ammerman, P. A. Van Wallegham,
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FIG. 8. Ultimobranchial and parathyroid glands. This photomicrograph illustrates the phenomenon of interaction between ultimobranchial (upper left) and parathyroid glands (lower right) of a chick fed a Mgtoxic diet (Mg .5%, P .48%, CI .13 5%). The ultimobranchial glands show a great proliferation of cells, whereas the parathyroids are only slightly reduced in size, unlike other Mg toxic parathyroid glands that are atrophied when the ultimobranchials are not as large or cellular. H and E magnification = 12.8X.
MAGNESIUM TOXICITY:BONE LESIONS
Massry, S. R., J. W. Coburn, and C. R. Kleeman, 1970. Evidence for suppression of parathyroid gland activity by hypermagnesemia. J. Clin. Invest. 49:1619-1628. Milby, T. T., 1934. Factors influencing the malformation of the leg bones of growing chickens. Iowa Agr. Exp. Sta. Res. Bull. 172. Neilsen, S. P., 1973. Effects of magnesium on calcification of young bone in tissue culture. Calcif. Tiss. Res. 11:78-94. Nugara, D., and H. M. Edwards, Jr., 1963. Influence of dietary calcium and phosphorus levels on the magnesium requirement of the chick. J. Nutr. 80:181-184. Pento, J. T., S. M. Glick, A. Kagan, and P. C. Gorfein, 1974. The relative influence of calcium, strontium, and magnesium on calcitonin secretion in the pig. Endocrinology 94:1176-1180. Posner, A. S., 1973. Bone mineral on the molecular level. Fed. Proc. 32:1933-1937. Ramp, W. K., J. R. Thomas, and P. D. Nifong, 1977. Magnesium and the mineral metabolism of chick embryo tibiae in organ culture. Calcif. Tiss. Res. 24:93-98. Spierto, F., J. C. Rogler, and H. E. Parker, 1969. Effect of dietary magnesium and flouride on bone mineralization in young chicks. J. Nutr. 98:271-278. Wise, D. R., 1978. Nutrition-disease interactions of leg weakness in poultry. Pages 41—57 in Recent advances in animal nutrition. W. H. Aresign and D. Lewis, ed. Butterworths, London.
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