Effects of Magnesium Deficiency on Duodenal and Ileal Magnesium Absorption and Secretion M. M I C H A E L PETITH, MD, and HAROLD P. SCHEDL, MD, PhD
Intestinal adaptation by the growing rat to a low-magnesium diet was studied by in situ perfusion of duodenum and ileum in vivo. Rats were f e d diets containing either 0.066 or 0.002% Mg for 3 weeks. Magnesium-restricted rats became hypomagnesemic and hypercalcemic. Net magnesium secretion was Studied by perfusing an initially magnesium-free saline solution; secretion was higher in duodenum than in ileum, and decreased significantly in the duodenum in response to magnesium restriction. Net magnesium absorption studied by intraluminal perfusion of 2.5 m M magnesium in saline was greater in duodenum than ileum in rats taking a low-magnesium diet, but duodenal and ileal absorption did not differ in animals taking the normal magnesium diet. Absorption did not adapt significantly to magnesium restriction in either segment. Adaptation of small-intestinal magnesium transport to a low magnesium diet is minimal, consisting mainly o f reduced duodenal magnesium secretion.
Magnesium transport by the rat intestine has been examined by a variety of techniques (1-4). However, the site of maximal magnesium absorption had not been clearly established, since in vitro studies comparing magnesium transport rates of duodenum and ileum in gut sacs are conflicting (1, 5, 6). While in situ perfusion studies show greater absorption in duodenum than ileum (7, 8), the reverse was found by in vivo isotopic studies (3). The role of magnesium intake in regulation of intestinal magnesium absorption has likewise not been clearly defined, since---in contrast to similar experiments in the rabbit (9)--/n vitro studies in the rat have failed to From the Gastroenterology Research Laboratories, Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, Iowa 52242. This study was supported in part by Research Grant AM02534 and Training Grant AM5390 from the National Institutes of Health. Dr. Petith is a recipient of a Clinical Investigator Award from the National Institute of Arthritis, Metabolism and Digestive Diseases (AM00242). Address for reprint requests: Dr. M. Michael Petith, Department of Internal Medicine, University Hospitals, Iowa City, Iowa 52242.
show adaptation o f small-intestinal magnesium absorption in response to dietary magnesium restriction (10), whereas an in vivo isotopic method in the same species did (11, 12). The current study was thus designed to redefine the site of maximal magnesium absorption in the small intestine by in situ perfusion and to determine the effect of dietary magnesium restriction on in situ net magnesium absorption and secretion. MATERIALS AND METHODS Male albino rats (CFE strain, Carworth Laboratories) weighing 140-185 g were randomized into two weightcomparable groups, which were separately caged in sets of up to 6 rats and housed in fully air-conditioned rooms. Diets (General Biochemical # 170490) were identical (0.63% calcium, 0.54% phosphorus) except for magnesium (NMD = normal magnesium diet, 0.066%; LMD = low-magnesium diet, 0.002%). Demineralized water was given ad libitum. After 19-23 days (mean for both groups 21.2 --- 0.4 sz) animals were fasted overnight (16-20 hr) with demineralized water ad libitum. An NMD and LMD rat were studied on the same day. Animals were anesthetized with intraperitoneal injections of Nero-
Digestive Diseases, Vol. 23, No. 1 (January 1978)
0002-9211/78/0100-0001505.00/19
DigestiveDisease Systems, Inc.
PETITH AND SCHEDL butal (50 mg/kg rat), supplemented by additional doses as needed. After a longitudinal incision of the abdomen, the bile duct was ligated, and transected, and segments of the duodenojejunum (proximal 7-12.5 cm, henceforth called "duodenum") and ileum (terminal 9-16.5 cm) were cannulated and then flushed with saline followed by air. The test solutions contained phenol red as nonabsorbed indicator for net water movements (50 mg/liter), and either 171 mM sodium chloride or 171 mM sodium chloride with 2.5 mM magnesium chloride. Through cannulas and polyvinyl tubing, each segment was connected with a pump (Bowman pump, model 2, Process and Instruments, Brooklyn, New York), and the contents of a reservoir with an initial volume of 10 ml of perfusate were recirculated for 2 hr at 2 ml/min through each segment. After perfusion, animals were killed by transecting both diaphragms. Perfused segments were cut at the inlet and outlet cannulas and gently stripped from the mesentery. Length was measured by placing the segments straight but without fixed tension on absorbent paper. Full thickness wet weights were read to 0.001 g on a Bethlehem Instruments torsion balance. Segments were then opened longitudinally, mucosal surface up, on a glass plate and scraped firmly with the edge of a microscope slide. The wet weights of the mucosa and underlying tissue were obtained to 0.0001 g on a Sartorius balance (Type 2258, Westbury, New York). The corresponding dry weights were obtained by drying in a vacuum oven (model 19, Precision Thelco) at 100~ C for 24 hr. The test solution and perfusates were processed for determination of magnesium by atomic absorption spectrometry (model 303, Perkin-Elmer) and of phenol red by spectrophotometry (13) (Spectronic 20, Bausch and Lomb). Net magnesium movements per hour were calculated from the difference between initial and final concentrations of magnesium and adjusted for net water movements, as published previously (7), and divided by g mucosal dry weight. Net water movements in percent of initial volume circulated are calculated as (1 - initial/final indicator concentration) • 100. Positive values indicate net absorption, negative values net secretion. All samples were analyzed in duplicate. In each animal, a blood sample was obtained from the tail after induction of anesthesia and from the inferior vena cava after termination of perfusion, and mean serum calcium and magnesium were calculated from the two samples. For statistical analysis, the unpaired Student's t test was used, except for comparing data from duodenum and ileum in the same animal, when the paired Student's t test was applied.
RESULTS Data on body weights, serum calcium and magnesium as well as intestinal segments are shown in Table 1. Intestinal segment length, wet weight, and mucosal dry weight were similar in both diet groups for corresponding duodenal and ileal segments. Mucosal growth, indicated by mucosal dry weight per segment, was the same in both diet groups for corresponding segments. Total intestinal (duodenum, 2
TABLE 1. ANIMAL, SERUM, AND INTESTINAL SEGMENT DATA
NMD (normal M g diet) (N = 13)*
LMD (low Mg diet) (N = 13)
Body weight, u n f a s t e d
Initial, g 159.5 Final, g 275.5 Serum Magnesium, mM Before perfusion ( 13)0.97 After perfusion, 0 mM perfusate (5)1.06 2.5 mM perfusate (6) 1.16 Calcium, mM Before perfusion (13)2.08 After perfusion, 0 mM perfusate (5)2.29 2.5 mM perfusate (6)2.23 Duodenum Segment length, cm 9.3 FTWW**, g 0.44 0.045 MSDWtt, g Ileum Segment length, cm 12.8 FTWW, g MSDW, g Total small intestine L e n g t h , cm FTWW, g
• 3.0t • 4.3
158.9 230.8
• 2.7 • 4.15
• 0.03
(12)0.50
• 0.045
• 0.04
(6)0.53
• 0.065
• 0.04w
(6)0.71
__+0.055w182
• 0.05
(12)2.29
• 0.075
• 0.03w (6)2.41
• 0.10
• 0.09w (6)2.46
+ 0.12
- 0.4 • 0.02 -+ 0.002
10.0 + 0.3 0.47 • 0.02 0.044 • 0.003
-+ 0.5 0.48 --- 0.03 0.054 + 0.004
13.9 • 0.6 0.53 • 0.03 0.057 + 0.004
86.1 3.77
-
1.1
91.1
• 0.13
3.96
•
1.45
• 0.10
*Or as stated in p a r e n t h e s e s . ? M e a n + SEM. SP < 0.05 c o m p a r e d with N M D . w < 0.02 c o m p a r e d with value before perfusion (paired Stud e n t ' s t test).
82 < 0.05 compared with 0 mM Mg perfusate. **Full thickness wet weight. ?tMucosal scrapings dry weight.
midgut, ileum) length, but not wet weight, was significantly greater in L M D than in N M D rats. Serum magnesium was much lower in rats taking L M D than in those taking N M D , both before (tail blood sample) and after (sample from vena cava) perfusion of either perfusate. Values for L M D and N M D rats after perfusion were about 40% and 20% higher than those before, when perfused with 2.5 mM Mg perfusate, but there was no difference when magnesium-free solution was perfused. Serum calcium before perfusion was slightly but significantly higher in rats taking L M D than in controls, and was 5-10% higher after than before perfusion. H o w e v e r , this difference was not significant in L M D rats, and in N M D rats, it was significant for both rats perfused with 2.5 mM Mg and with magnesium-free solution. Digestive Diseases, Vol. 23, No. 1 (January 1978)
INTESTINAL MAGNESIUM TRANSPORT TABLE 2.
NET WATERMOVEMENTS
Percent per segment Perfusate 2.5 mM Mg Duodenum Ileum 0 mM Mg Duodermm Ileum
NMD (normal Mg diet)
LMD (low Mg diet)
(6) -21.5 -+- 3.6* (6) + 0.7 -+ 1.4?
(6) -18.4 _+ 3.7 (6) + 2.7 -+ 1.8t
(7) -17.8 -+ 1.9 (7) + 4.7 -+ 1.1r
(7) -15.9 -+ 1.9 (7) + 4.0 -+ 1.5r
*Mean -+ SE, positive values indicate net absorption; negative values, net secretion. ?P < 0.002 compared with duodenum. SP < 0.05 compared with 2.5 mM Mg perfusate.
Magnesiium Movements Rates of magnesium movements in this paper are given in micromoles per hour for each segment based on mucosal scrapings dry weight. Figures 1 and 2 summarize data on net magnesium movements. Net magnesium absorption (Figure 1) from the 2.5 mM magnesium perfusate is greater in duodenum than ileum in both diet groups, but the difference is significant only in the LMD group. Duodenal magnesium absorption is slightly higher in the LMD than in the NMD group, but not significantly so, and ileal magnesium absorption is the same in both diet groups. Net magnesium secretion (Figure 2) into an initially magnesium-free solution is greater in duodenum than ileum for both diet groups and is significantly decreased in duodenum of the LMD group compared to that in the NMD group. Ileal net magnesium secretion is unaffected by LMD. There was net water secretion in duodenum and net water absorption in ileum in both groups (Table 2). Duodenal water secretion tended to be greater in rats taking NMD than those taking LMD and was also somewhat higher in rats perfused with 2.5 mM magnesium than with the magnesium-free solution; none of these differences was significant. Mean ileal water absorption was higher with the magnesiumfree peffusate than with 2.5 mM magnesium, and this difference was significant for the NMD group. DISCUSSION This paper reports on the response of small-intestinal magnesium transport to dietary magnesium deficiency in the rat, providing the first data on net magnesium secretion into the lumen and on magneDigestive Diseases, Vol. 23, No. 1 (January 1978)
sium absorption of individual segments by the in vivo perfusion technique. The LMD group showed the typical response to magnesium restriction in rats (14, 15), ie, development of hypomagnesemia and h y p e r c a l c e m i a (Table 1). Net m a g n e s i u m movements into an initially magnesium-free lumen (Figure 2) was significantly lower in the duodenum of LMD compared with NMD rats, whereas no difference was seen in ileum. The lower duodenal net secretion rates of LMD animals may be related to the lower chemical gradient due to hypomagnesemia (Table 1); this would imply lower ileal than duodenal permeability. Absorption down a concentration gradient from a perfusate containing 2.5 mM Mg failed to adapt to magnesium deficiency in either duodenum or ileum (Figure 1). Likewise, the mean absolute rise of serum magnesium during perfusion was the same in NMD and LMD rats (171 _+ 25 and 228 _+ 66/xM, for paired data); only the relative increase was greater in LMD rats because of lower initial values (Table 1). These findings agree with earlier in vitro studies showing no response of duodenal magnesium transport (10), but they do not confirm reports of a generalized increase of magnesium transport along the entire intestinal tract based on m e a s u r e m e n t s of comp a r t m e n t a l shift of t r a c e r 28Mg in intact rats (11, 12), at least with respect to duodenum and ileum. In agreement with earlier observations by in situ perfusion (7) and in vivo isotopic studies (3), move-
~" 70
I ~
I Duodenum Ileum
6O
5o
/
I
4o
# 2o
~I NMD
Fig 1. Net magnesium absorption (mean
LMD -+ SE) from 2.5 mM
magnesium in saline in duodenum (clear bars) and ileum (shaded bars) of growing rats taking either a normal magnesium diet (NMD, 0.066% Mg) or a low-magnesium diet (LMD, 0.002% Mg). *P < 0.005 compared with corresponding duodenum.
3
PETITH
[---] Duodenum Ileum t2 8 4
NMD
LMD
Fig 2. Net magnesium secretion (mean ~ sE) into an initially magnesium-free saline perfusate. Symbols same as in Figure I. *P < 0.02 compared with corresponding duodenum. **P < 0.05 compared with duodenum of NMD group,
ment of magnesium into the lumen (Figure 2) is higher in duodenum than ileum. In conjunction with our previous work in rats taking a magnesium-sufficient diet (1.2% Ca, 0.2% Mg) (16), mean rates of net magnesium secretion into an initially magnesium-free lumen can be c o m p a r e d and are 1.34 - 0.18, 0.44 - 0.04, 0.45 ___ 0.12, and 0.46 +- 0.08 for duodenum, ileum (current study), cecum and colon (16) respectively, expressed in micromoles magnesium per gram full thickness wet weight per hour (mean --- SEM). Corresponding rates of net magnesium absorption from a 2.5 mM Mg perfusate are 4.4 _-_ 0.7 (duodenum), 3.7 • 0.4 (ileum), 1.1 • 0.3 (cecum), and 1.3 • 0.3 (colon) expressed in micromoles magnesium per gram full thickness wet weight per hour. These data are in accord with in vitro experiments reporting lower absorption rates in colon than small intestine (6), but disagree with several in vitro (5, 6) and in vivo (7, 8) studies showing higher duodenal than ileal magnesium absorption in magnesium-sufficient rats. In the current study the difference betweenduodenal and ileal absorption rates was significant only for LMD but not NMD animals. We have no explanation for this apparent discrepancy, but the semisynthetic nature of our diets may have altered the proximal-to-distal gradient seen by other workers in rats taking regular chow. In order to assess the relative importance of individual intestinal sites for magnesium transport in the intact animal, the progressive increase in transit times along the small intestine (17) and probably more so along the large intestine must be considered. Thus, when intestinal continuity was pre-
4
AND SCHEDL
served, systemic uptake of intraluminal tracer 28Mg increased along the intestinal tract and was highest in the proximal large bowel (3), in apparent contrast to previous (5-8) studies involving disruption of intestinal transit. In contrast, the tracer studies also localize most of the loss of endogenous magnesium to the proximal small intestine (3), in agreement with our data. Thus, the distal alimentary tract may be the most important site of magnesium absorption in the intact animal, when intestinal motility determines transit and therefore contact times, whereas the interpretation of rates of intestinal magnesium secretion is much less affected by transit times. When magnesium transport is examined at a given intestinal site, eg, ileum or colon, magnesium movements correlate closely with net water movements (4). In the current study, the greater magnesium secretion in duodenum than in ileum in both NMD and LMD groups agrees with the pattern of water movement and is thus in accord with this concept. However, net water movement is not the primary determinant of absorption rates when two intestinal sites are compared. Factors related to magnesium t r a n s p o r t p e r se rather than net water movement appear to determine differences between duodenum and ileum.
REFERENCES 1. Ross DBL: In vitro studies on the transport of magnesium across t h e intestinal wall of the rat. J Physiol (London) 160:417--428, 1962 2. Alcock N, Maclntyre I: Inter-relation of calcium and magnesium absorption. Clin Sci 22:185-193, 1962 3. Chutkow JG: Sites of magnesium absorption and excretion in the intestinal tract of the rat. J Lab Clin Med 63:71-79, 1964 4. Behar J: Magnesium absorption by the rat ileum and colon. Am J Physiol 227:334--340, 1974 5. Hendrix JZ, Aicock NW, Archibald RM: Competition between calcium, stco qum, and magnesium for absorption in the isolated re~t :~,estine. Clin Chem 9:734-744, 1963 6. Aldor TAM, Moore EW: Magnesium absorption by everted sacs of rat intestine and colon. Gastroenterology 59:745-753, 1970 7. Urban E, Schedl HP: Net movements of magnesium and calcium in the rat small intestine in vivo. Proc Soc Exp Biol Med 132:1110-1113, 1969 8. Miller DL, Schedl HP: Effects of diabetes on intestinal magnesium absorption in the rat. Am J Physiol 231:1039-1042, 1976 9. Aikawa JK, David AP: 28Mg studies in magnesium-deficient animals. Ann NY Acad Sci 162:744-757, 1969 10. Kessner DM, Epstein FH: Effect of magnesium deficiency on gastrointestinal transfer of calcium. Proc Soc Exp Biol Med 122:721-725, 1966 Digestive Diseases, Vol. 23, No. 1 (January 1978)
INTESTINAL
MAGNESIUM
TRANSPORT
11. Chutkow JG: Studies on the metabolism of magnesium in the magnesium deficient rat. J Lab Clin Med 65:912-926, 1965 12. Chutkow JG: Effect of magnesium deficiency on location of the intestinal absorption of magnesium in rats. Proc Soc Exp Bio][ Med 123:836-840, 1966 13. Schedl HP, Clifton JA: Small intestinal absorption of steroids. Gastroenterology 41:491-499, 1961 14. Morehead RM, Kessner DM: Effects of magnesium deftciency and parathyroidectomy on gastrointestinal calcium transport in the rat. Am J Physiol 217:1608-1613, 1969
Digestive Diseases, Vol. 23, No. 1 (January 1978)
15. Krawitt EL: Effect of dietary magnesium deficiency on calcium transport by the rat small intestine. Proc Soc Exp Biol Med 141:569-572, 1972 16. Petith MM, Schedl HP: Divalent cation transport by rat cecum and colon in calcium and magnesium deficiency. Proc Soc Exp Biol Med 155:225-229, 1977 17. Summers RW, Kent TH, Osborne JE: Effects of drugs, ileal obstruction and irradiation on rat gastrointestinal propulsion. Gastroenterology 59:731-739, 1970
5
Intestinal adaptation by the growing rat to a low-magnesium diet was studied by in situ perfusion of duodenum and ileum in vivo. Rats were f e d diets containing either 0.066 or 0.002% Mg for 3 weeks. Magnesium-restricted rats became hypomagnesemic and hypercalcemic. Net magnesium secretion was Studied by perfusing an initially magnesium-free saline solution; secretion was higher in duodenum than in ileum, and decreased significantly in the duodenum in response to magnesium restriction. Net magnesium absorption studied by intraluminal perfusion of 2.5 m M magnesium in saline was greater in duodenum than ileum in rats taking a low-magnesium diet, but duodenal and ileal absorption did not differ in animals taking the normal magnesium diet. Absorption did not adapt significantly to magnesium restriction in either segment. Adaptation of small-intestinal magnesium transport to a low magnesium diet is minimal, consisting mainly o f reduced duodenal magnesium secretion.
Magnesium transport by the rat intestine has been examined by a variety of techniques (1-4). However, the site of maximal magnesium absorption had not been clearly established, since in vitro studies comparing magnesium transport rates of duodenum and ileum in gut sacs are conflicting (1, 5, 6). While in situ perfusion studies show greater absorption in duodenum than ileum (7, 8), the reverse was found by in vivo isotopic studies (3). The role of magnesium intake in regulation of intestinal magnesium absorption has likewise not been clearly defined, since---in contrast to similar experiments in the rabbit (9)--/n vitro studies in the rat have failed to From the Gastroenterology Research Laboratories, Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, Iowa 52242. This study was supported in part by Research Grant AM02534 and Training Grant AM5390 from the National Institutes of Health. Dr. Petith is a recipient of a Clinical Investigator Award from the National Institute of Arthritis, Metabolism and Digestive Diseases (AM00242). Address for reprint requests: Dr. M. Michael Petith, Department of Internal Medicine, University Hospitals, Iowa City, Iowa 52242.
show adaptation o f small-intestinal magnesium absorption in response to dietary magnesium restriction (10), whereas an in vivo isotopic method in the same species did (11, 12). The current study was thus designed to redefine the site of maximal magnesium absorption in the small intestine by in situ perfusion and to determine the effect of dietary magnesium restriction on in situ net magnesium absorption and secretion. MATERIALS AND METHODS Male albino rats (CFE strain, Carworth Laboratories) weighing 140-185 g were randomized into two weightcomparable groups, which were separately caged in sets of up to 6 rats and housed in fully air-conditioned rooms. Diets (General Biochemical # 170490) were identical (0.63% calcium, 0.54% phosphorus) except for magnesium (NMD = normal magnesium diet, 0.066%; LMD = low-magnesium diet, 0.002%). Demineralized water was given ad libitum. After 19-23 days (mean for both groups 21.2 --- 0.4 sz) animals were fasted overnight (16-20 hr) with demineralized water ad libitum. An NMD and LMD rat were studied on the same day. Animals were anesthetized with intraperitoneal injections of Nero-
Digestive Diseases, Vol. 23, No. 1 (January 1978)
0002-9211/78/0100-0001505.00/19
DigestiveDisease Systems, Inc.
PETITH AND SCHEDL butal (50 mg/kg rat), supplemented by additional doses as needed. After a longitudinal incision of the abdomen, the bile duct was ligated, and transected, and segments of the duodenojejunum (proximal 7-12.5 cm, henceforth called "duodenum") and ileum (terminal 9-16.5 cm) were cannulated and then flushed with saline followed by air. The test solutions contained phenol red as nonabsorbed indicator for net water movements (50 mg/liter), and either 171 mM sodium chloride or 171 mM sodium chloride with 2.5 mM magnesium chloride. Through cannulas and polyvinyl tubing, each segment was connected with a pump (Bowman pump, model 2, Process and Instruments, Brooklyn, New York), and the contents of a reservoir with an initial volume of 10 ml of perfusate were recirculated for 2 hr at 2 ml/min through each segment. After perfusion, animals were killed by transecting both diaphragms. Perfused segments were cut at the inlet and outlet cannulas and gently stripped from the mesentery. Length was measured by placing the segments straight but without fixed tension on absorbent paper. Full thickness wet weights were read to 0.001 g on a Bethlehem Instruments torsion balance. Segments were then opened longitudinally, mucosal surface up, on a glass plate and scraped firmly with the edge of a microscope slide. The wet weights of the mucosa and underlying tissue were obtained to 0.0001 g on a Sartorius balance (Type 2258, Westbury, New York). The corresponding dry weights were obtained by drying in a vacuum oven (model 19, Precision Thelco) at 100~ C for 24 hr. The test solution and perfusates were processed for determination of magnesium by atomic absorption spectrometry (model 303, Perkin-Elmer) and of phenol red by spectrophotometry (13) (Spectronic 20, Bausch and Lomb). Net magnesium movements per hour were calculated from the difference between initial and final concentrations of magnesium and adjusted for net water movements, as published previously (7), and divided by g mucosal dry weight. Net water movements in percent of initial volume circulated are calculated as (1 - initial/final indicator concentration) • 100. Positive values indicate net absorption, negative values net secretion. All samples were analyzed in duplicate. In each animal, a blood sample was obtained from the tail after induction of anesthesia and from the inferior vena cava after termination of perfusion, and mean serum calcium and magnesium were calculated from the two samples. For statistical analysis, the unpaired Student's t test was used, except for comparing data from duodenum and ileum in the same animal, when the paired Student's t test was applied.
RESULTS Data on body weights, serum calcium and magnesium as well as intestinal segments are shown in Table 1. Intestinal segment length, wet weight, and mucosal dry weight were similar in both diet groups for corresponding duodenal and ileal segments. Mucosal growth, indicated by mucosal dry weight per segment, was the same in both diet groups for corresponding segments. Total intestinal (duodenum, 2
TABLE 1. ANIMAL, SERUM, AND INTESTINAL SEGMENT DATA
NMD (normal M g diet) (N = 13)*
LMD (low Mg diet) (N = 13)
Body weight, u n f a s t e d
Initial, g 159.5 Final, g 275.5 Serum Magnesium, mM Before perfusion ( 13)0.97 After perfusion, 0 mM perfusate (5)1.06 2.5 mM perfusate (6) 1.16 Calcium, mM Before perfusion (13)2.08 After perfusion, 0 mM perfusate (5)2.29 2.5 mM perfusate (6)2.23 Duodenum Segment length, cm 9.3 FTWW**, g 0.44 0.045 MSDWtt, g Ileum Segment length, cm 12.8 FTWW, g MSDW, g Total small intestine L e n g t h , cm FTWW, g
• 3.0t • 4.3
158.9 230.8
• 2.7 • 4.15
• 0.03
(12)0.50
• 0.045
• 0.04
(6)0.53
• 0.065
• 0.04w
(6)0.71
__+0.055w182
• 0.05
(12)2.29
• 0.075
• 0.03w (6)2.41
• 0.10
• 0.09w (6)2.46
+ 0.12
- 0.4 • 0.02 -+ 0.002
10.0 + 0.3 0.47 • 0.02 0.044 • 0.003
-+ 0.5 0.48 --- 0.03 0.054 + 0.004
13.9 • 0.6 0.53 • 0.03 0.057 + 0.004
86.1 3.77
-
1.1
91.1
• 0.13
3.96
•
1.45
• 0.10
*Or as stated in p a r e n t h e s e s . ? M e a n + SEM. SP < 0.05 c o m p a r e d with N M D . w < 0.02 c o m p a r e d with value before perfusion (paired Stud e n t ' s t test).
82 < 0.05 compared with 0 mM Mg perfusate. **Full thickness wet weight. ?tMucosal scrapings dry weight.
midgut, ileum) length, but not wet weight, was significantly greater in L M D than in N M D rats. Serum magnesium was much lower in rats taking L M D than in those taking N M D , both before (tail blood sample) and after (sample from vena cava) perfusion of either perfusate. Values for L M D and N M D rats after perfusion were about 40% and 20% higher than those before, when perfused with 2.5 mM Mg perfusate, but there was no difference when magnesium-free solution was perfused. Serum calcium before perfusion was slightly but significantly higher in rats taking L M D than in controls, and was 5-10% higher after than before perfusion. H o w e v e r , this difference was not significant in L M D rats, and in N M D rats, it was significant for both rats perfused with 2.5 mM Mg and with magnesium-free solution. Digestive Diseases, Vol. 23, No. 1 (January 1978)
INTESTINAL MAGNESIUM TRANSPORT TABLE 2.
NET WATERMOVEMENTS
Percent per segment Perfusate 2.5 mM Mg Duodenum Ileum 0 mM Mg Duodermm Ileum
NMD (normal Mg diet)
LMD (low Mg diet)
(6) -21.5 -+- 3.6* (6) + 0.7 -+ 1.4?
(6) -18.4 _+ 3.7 (6) + 2.7 -+ 1.8t
(7) -17.8 -+ 1.9 (7) + 4.7 -+ 1.1r
(7) -15.9 -+ 1.9 (7) + 4.0 -+ 1.5r
*Mean -+ SE, positive values indicate net absorption; negative values, net secretion. ?P < 0.002 compared with duodenum. SP < 0.05 compared with 2.5 mM Mg perfusate.
Magnesiium Movements Rates of magnesium movements in this paper are given in micromoles per hour for each segment based on mucosal scrapings dry weight. Figures 1 and 2 summarize data on net magnesium movements. Net magnesium absorption (Figure 1) from the 2.5 mM magnesium perfusate is greater in duodenum than ileum in both diet groups, but the difference is significant only in the LMD group. Duodenal magnesium absorption is slightly higher in the LMD than in the NMD group, but not significantly so, and ileal magnesium absorption is the same in both diet groups. Net magnesium secretion (Figure 2) into an initially magnesium-free solution is greater in duodenum than ileum for both diet groups and is significantly decreased in duodenum of the LMD group compared to that in the NMD group. Ileal net magnesium secretion is unaffected by LMD. There was net water secretion in duodenum and net water absorption in ileum in both groups (Table 2). Duodenal water secretion tended to be greater in rats taking NMD than those taking LMD and was also somewhat higher in rats perfused with 2.5 mM magnesium than with the magnesium-free solution; none of these differences was significant. Mean ileal water absorption was higher with the magnesiumfree peffusate than with 2.5 mM magnesium, and this difference was significant for the NMD group. DISCUSSION This paper reports on the response of small-intestinal magnesium transport to dietary magnesium deficiency in the rat, providing the first data on net magnesium secretion into the lumen and on magneDigestive Diseases, Vol. 23, No. 1 (January 1978)
sium absorption of individual segments by the in vivo perfusion technique. The LMD group showed the typical response to magnesium restriction in rats (14, 15), ie, development of hypomagnesemia and h y p e r c a l c e m i a (Table 1). Net m a g n e s i u m movements into an initially magnesium-free lumen (Figure 2) was significantly lower in the duodenum of LMD compared with NMD rats, whereas no difference was seen in ileum. The lower duodenal net secretion rates of LMD animals may be related to the lower chemical gradient due to hypomagnesemia (Table 1); this would imply lower ileal than duodenal permeability. Absorption down a concentration gradient from a perfusate containing 2.5 mM Mg failed to adapt to magnesium deficiency in either duodenum or ileum (Figure 1). Likewise, the mean absolute rise of serum magnesium during perfusion was the same in NMD and LMD rats (171 _+ 25 and 228 _+ 66/xM, for paired data); only the relative increase was greater in LMD rats because of lower initial values (Table 1). These findings agree with earlier in vitro studies showing no response of duodenal magnesium transport (10), but they do not confirm reports of a generalized increase of magnesium transport along the entire intestinal tract based on m e a s u r e m e n t s of comp a r t m e n t a l shift of t r a c e r 28Mg in intact rats (11, 12), at least with respect to duodenum and ileum. In agreement with earlier observations by in situ perfusion (7) and in vivo isotopic studies (3), move-
~" 70
I ~
I Duodenum Ileum
6O
5o
/
I
4o
# 2o
~I NMD
Fig 1. Net magnesium absorption (mean
LMD -+ SE) from 2.5 mM
magnesium in saline in duodenum (clear bars) and ileum (shaded bars) of growing rats taking either a normal magnesium diet (NMD, 0.066% Mg) or a low-magnesium diet (LMD, 0.002% Mg). *P < 0.005 compared with corresponding duodenum.
3
PETITH
[---] Duodenum Ileum t2 8 4
NMD
LMD
Fig 2. Net magnesium secretion (mean ~ sE) into an initially magnesium-free saline perfusate. Symbols same as in Figure I. *P < 0.02 compared with corresponding duodenum. **P < 0.05 compared with duodenum of NMD group,
ment of magnesium into the lumen (Figure 2) is higher in duodenum than ileum. In conjunction with our previous work in rats taking a magnesium-sufficient diet (1.2% Ca, 0.2% Mg) (16), mean rates of net magnesium secretion into an initially magnesium-free lumen can be c o m p a r e d and are 1.34 - 0.18, 0.44 - 0.04, 0.45 ___ 0.12, and 0.46 +- 0.08 for duodenum, ileum (current study), cecum and colon (16) respectively, expressed in micromoles magnesium per gram full thickness wet weight per hour (mean --- SEM). Corresponding rates of net magnesium absorption from a 2.5 mM Mg perfusate are 4.4 _-_ 0.7 (duodenum), 3.7 • 0.4 (ileum), 1.1 • 0.3 (cecum), and 1.3 • 0.3 (colon) expressed in micromoles magnesium per gram full thickness wet weight per hour. These data are in accord with in vitro experiments reporting lower absorption rates in colon than small intestine (6), but disagree with several in vitro (5, 6) and in vivo (7, 8) studies showing higher duodenal than ileal magnesium absorption in magnesium-sufficient rats. In the current study the difference betweenduodenal and ileal absorption rates was significant only for LMD but not NMD animals. We have no explanation for this apparent discrepancy, but the semisynthetic nature of our diets may have altered the proximal-to-distal gradient seen by other workers in rats taking regular chow. In order to assess the relative importance of individual intestinal sites for magnesium transport in the intact animal, the progressive increase in transit times along the small intestine (17) and probably more so along the large intestine must be considered. Thus, when intestinal continuity was pre-
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served, systemic uptake of intraluminal tracer 28Mg increased along the intestinal tract and was highest in the proximal large bowel (3), in apparent contrast to previous (5-8) studies involving disruption of intestinal transit. In contrast, the tracer studies also localize most of the loss of endogenous magnesium to the proximal small intestine (3), in agreement with our data. Thus, the distal alimentary tract may be the most important site of magnesium absorption in the intact animal, when intestinal motility determines transit and therefore contact times, whereas the interpretation of rates of intestinal magnesium secretion is much less affected by transit times. When magnesium transport is examined at a given intestinal site, eg, ileum or colon, magnesium movements correlate closely with net water movements (4). In the current study, the greater magnesium secretion in duodenum than in ileum in both NMD and LMD groups agrees with the pattern of water movement and is thus in accord with this concept. However, net water movement is not the primary determinant of absorption rates when two intestinal sites are compared. Factors related to magnesium t r a n s p o r t p e r se rather than net water movement appear to determine differences between duodenum and ileum.
REFERENCES 1. Ross DBL: In vitro studies on the transport of magnesium across t h e intestinal wall of the rat. J Physiol (London) 160:417--428, 1962 2. Alcock N, Maclntyre I: Inter-relation of calcium and magnesium absorption. Clin Sci 22:185-193, 1962 3. Chutkow JG: Sites of magnesium absorption and excretion in the intestinal tract of the rat. J Lab Clin Med 63:71-79, 1964 4. Behar J: Magnesium absorption by the rat ileum and colon. Am J Physiol 227:334--340, 1974 5. Hendrix JZ, Aicock NW, Archibald RM: Competition between calcium, stco qum, and magnesium for absorption in the isolated re~t :~,estine. Clin Chem 9:734-744, 1963 6. Aldor TAM, Moore EW: Magnesium absorption by everted sacs of rat intestine and colon. Gastroenterology 59:745-753, 1970 7. Urban E, Schedl HP: Net movements of magnesium and calcium in the rat small intestine in vivo. Proc Soc Exp Biol Med 132:1110-1113, 1969 8. Miller DL, Schedl HP: Effects of diabetes on intestinal magnesium absorption in the rat. Am J Physiol 231:1039-1042, 1976 9. Aikawa JK, David AP: 28Mg studies in magnesium-deficient animals. Ann NY Acad Sci 162:744-757, 1969 10. Kessner DM, Epstein FH: Effect of magnesium deficiency on gastrointestinal transfer of calcium. Proc Soc Exp Biol Med 122:721-725, 1966 Digestive Diseases, Vol. 23, No. 1 (January 1978)
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TRANSPORT
11. Chutkow JG: Studies on the metabolism of magnesium in the magnesium deficient rat. J Lab Clin Med 65:912-926, 1965 12. Chutkow JG: Effect of magnesium deficiency on location of the intestinal absorption of magnesium in rats. Proc Soc Exp Bio][ Med 123:836-840, 1966 13. Schedl HP, Clifton JA: Small intestinal absorption of steroids. Gastroenterology 41:491-499, 1961 14. Morehead RM, Kessner DM: Effects of magnesium deftciency and parathyroidectomy on gastrointestinal calcium transport in the rat. Am J Physiol 217:1608-1613, 1969
Digestive Diseases, Vol. 23, No. 1 (January 1978)
15. Krawitt EL: Effect of dietary magnesium deficiency on calcium transport by the rat small intestine. Proc Soc Exp Biol Med 141:569-572, 1972 16. Petith MM, Schedl HP: Divalent cation transport by rat cecum and colon in calcium and magnesium deficiency. Proc Soc Exp Biol Med 155:225-229, 1977 17. Summers RW, Kent TH, Osborne JE: Effects of drugs, ileal obstruction and irradiation on rat gastrointestinal propulsion. Gastroenterology 59:731-739, 1970
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