and/or High Potassium Diet

Transcription

1 Calcium Mobilization in Hypomagnesemic Wethers Fed on a Low Magnesium and/or High Potassium Diet Yoshiaki TERASHIMA, Shizuka MATSUNOBU, Tetsuo YANAGISAWA and Hiroshi ITOH Faculty of Animal Science, Kitasato University, Towada-shi 034 (Received June 15,1987) Abstract Four wethers were subjected to four diet treatments for 7-10 days in a sheep fed on a low magnesium and/or high potassium diet. The diet treatments were a normal diet, low magnesium diet, high potassium diet and low magnesium/high potassium diet. On the final day of each diet feeding period, the intravenous infusion of EDTA solution at a rate of 0.263mmol/kg body weight was performed for 60 min to determine calcium mobilization during the infusion period. Mean plasma magnesium levels in animals receiving the low magnesium, high potassium and low magnesium/high potassium diets were 0.55,0.74 and 0.61mmol/l, respectively. They were significantly lower than those of the control diet fed-sheep. Diet treatments did not affect plasma calcium and potassium concentrations. Plasma-ionized calcium concentrations linearly decreased during the EDTA infusion in all groups. The rate of decrease in ionized calcium tended to be greater in hypomagnesemic sheep compared with those of the control sheep. The mean calcium mobilization rates in the control, low magnesium, high potassium and low magnesium/high potassium diet-fed sheep during the EDTA infusion period were 4.98mmol, 3.83mmol, 3.23mmol and 3.58mmol, respectively. These results suggest that hypomagnesemic sheep which are induced by the consumption of a low magnesium and/or high potassuim diet, could be susceptible to hypocalcemia due to depressed calcium mobilization. Jpn. J. Zootech. Sci., 59 (1): 75-81,1988 Key words: calcium mobilization, hypomagnesemia, grass tetany, sheep Hypomagnesemic tetany (grass tatany) is a metabolic disorder of ruminants mainly related to low blood serum magnesium levels. As the metabolism of magnesium (Mg) and calcium (Ca) are closely interrelated, hypomagnesemia would affect Ca metabolism. Experimentally induced Mg deficiency is associated with hypocalcemia in cattle and sheep1,2), although some reports have shown no effect of low Mg rations on Ca metabolism in sheep3). In ewes, hypocalcemia and hypomagnesemia are almost always combined in clinical cases of tetany4). BOHMAN et al. have shown that a metabolic deficiency of Ca was the cause of tetany in some cases5). Naturally, animals Jpn. J. Zootech. Sci., 59 (1):

2 TERASHIMA, MATSUNOBU, YANAGISAWA and ITOH on a Mg-deficient diet show changes in Mg metabolism, but they also show a striking disturbance in Ca metabolism. In such animals, several reports suggest that hypomagnesemia may be involved in an impaired ability to mobilize Ca from reserves (bone)6,7). But this is still obscure in hypomagnesemic sheep which are induced by being fed a low Mg and/or high K diet. The present experiment was designed to determine the Ca mobilization in hypomagnesemic sheep induced by a low Mg and/or high potassium (K) diet. Animals and diets Four crossbred wethers averaging Materials and Methods 45kg body weight were used to study calcium mobilization during the EDTA infusion period. An experimental semi-purified diet (Table 1) was prepared with primarily, corn starch, corn cobs, dextrose, wood cellulose and soy protein to provide a low magnesium diet. Experimental diets consisted of the control (0.23%Mg-0.76%K), low magnesium (0.04%Mg-0.75%K), high potassium (0.19%Mg-4.90%K) or low magnesium/high potassium (0.04%Mg-4.68%K) diet. Each diet was provided to meet the daily metabolizable energy requirements for sheep at 17:00 once daily8). Water was provided ad libitum. Wethers were kept in Table 1. Ingredient and chemical composition of basal semi-purified diet1) 1) The basal semi-purified diet was used as the low Mg diet. The control and low Mg/high K diets were prepared by adding 0.3% MgO and 6.5% KCl to the the basal semi-purified diet respectively, and the high K diet was prepared by adding 6.5% KCl to the control diet. 2) Contained per g feed additive: NaCl, 500mg; Vitamin A, 500 IU; Vitamin D, 75 IU. 3) By determination. Mineral composition figures appeared in this text are actually determined values. 4) Calculated metabolizable energy: 2.99Mcal/kg diet. 76

3 Calcium Mobilization in Hypomagnesemic Sheep square design. They were fitted with bilateral polyethylene jugular catheters on the day before the EDTA infusion experiment. To allow stress-free blood sampling, an extension catheter was attached to the back of the wethers. EDTA infusion test tion was performed to induce temporary hypocalcemia. The solution was prepared to intravenous EDTA infusion was conducted on the last day of each diet treatment period. After a pre-infusion blood sample was withdrawn into heparinized tubes, the EDTA solution was given into the jugular vein for 60min at a constant rate with a peristaltic pump to provide a dose of 0.263mmol EDTA/kg body weight. After each EDTA infusion was completed, all animals were fed the control diet for 3-5 days, then switched to each experimental feeding regimen. Blood samples were taken at 15,30,45,60,75,90,120,150,180 the initiation and 240min after of the EDTA infusion and were kept in ice until plasma was removed by centrifugation. Plasma was analyzed for total-ca, Mg and K concentrations by atomic spectorophotometry. meter (Sera-250, Horiba Ltd., Japan). Ionized Ca levels in plasma were determined by an ionic To evaluate the rate of calcium mobilization from the EDTA infusion test, the data obtained from plasma total-ca, ionized Ca and the amounts of EDTA infused were analyzed via the method reported by CONTRERAS et al.6). An equation of form ionized Ca. An independent measurement of V can be obtained from the changes in the plasma concentration of calcium bound to EDTA during the infusion period, as described by CONTRERAS et al.6). Statistical analysis All parameters were subjected to an analysis of variance to determine significant The effect of hypomagnesemia on the rate of calcium mobilization was determined by a paired t-test. Results and Discussion plasma Mg concentrations compared with the control diet-fed animals (Table 2). The high K diet also induced hypomagnesemia (0.74mmol/l) after 7-10 day feeding period. The addition of excess K to a low Mg diet has been reported to increase the rate of decline of plasma Mg over that due to a low Mg diet alone10). But in the present experiment, the lowest plasma Mg level was observed in the low Mg diet-fed animals. Some wethers which were fed the low Mg diet slightly decreased feed intake after the 77

4 TERASHIMA, MATSUNOBU, YANAGISAWA and ITOH Table 2. Effect of diet treatment on plasma calcium, magnesium and potassium levels in wethers after 7-10 day feeding period diet change from the control diet. This may cause more reduced plasma Mg concentrations in the low Mg diet treatment. Many studies demonstrate that high K diets reduce Mg absorption from the digestive tract in the ruminants11-13). Therefore, hypomagnesemia observed in the high K diet-fed animals might result from the reduction in Mg absorption. On the other hand, Mg contents of some tissues in excess K diet-adapted rats have been shown to be greater than those in the control animalsl14). The contribution of high K to lower plasma Mg levels might be partly due to a mechanism other than reduced Mg absorption. Mean plasma total-ca concentrations were 2.05,2.18,2.02 and 2.14mmol/l, respectively, when animals were fed the control, low Mg, high K and low Mg/high K diets for a 7-10 day feeding period. Diet treatments did not affect plasma K levels in the present experiment, as shown in Table 2. It is possible that low dietary Mg is a contributing factor to hypocalcemia seen in grass tetany due to low Ca absorption. CLARK demonstrated that the transport and metabolism of Ca and Mg are interrelated, and that increasing dietary Mg from low to high levels increased Ca absorption and serum Ca levels15). However, the results of the present experiment indicated that animals fed the low Mg and/or high K diet developed hypomagnesemia but this did not affect plasma Ca concentrations. Changes in plasma-ionized Ca concentrations during and after the EDTA infusion are presented in Fig. 1. Each value at each time is expressed as a percentage of the initial values. They decreased linearly during the EDTA infusion in all groups. The rate of decrease in ionized Ca level tended to be greater in hypomagnesemic animals fed the low Mg and/or high K diet. during the EDTA infusion period was 0.006,0.010,0.008 The constant derived from the slope of regression line and in animals fed the control, low Mg, high K and low Mg/high K diet, respectively. But there was no significant difference among diet treatments as the values were considerably variable in this experiment. While plasma-ionized Ca in the control diet-fed sheep returned to almost the initial values 120min after the completion of the EDTA infusion, they still remained low in hypomagnesemic The mean amounts animals fed the low Mg/high K diet. of Ca mobilized during the EDTA infusion period (60min) in the control, low Mg, high K and low Mg/high K diet-fed animals were 4.98,3.83,

5 Calcium Mobilization in Hypomagnesemic Sheep Fig. 1. Changes in ionized calcium concentration in wethers during and after the EDTA infusion. Each value expressed as a percentage of the initial values represents a mean of four animals. and 3.58mmol, respectively (Table 3). The values of the low Mg/high K diet-fed Although there was no significant difference in the rate of Ca mobilization among the control, low Mg and high K diet treatments, all hypomagnesemic animals tended to reduce the Ca mobilization compared to those of the control group. When the values are expressed as a percentage of the control values, the Ca mobilization rates were 77, 65 and 72% in the low Mg, high K and low Mg/high K diet-fed sheep, respectively. It has been reported in some clinical cases of grass tetany that hypomagnesemia and hypocalcemia are associated4). However, although the metabolism of Ca and Mg are closely interrelated as mentioned above, these relationships have not been precisely defined and the mechanisms involved are not understood. The present results indicate that hypomagnesemic animals which are fed the low Mg and/or high K diet had a normal range of plasma Ca concentrations. However, hypomagnesemic animals showed reduced Ca mobilization when acute hyhypocalcemia was induced by the EDTA infusion. These results suggest that hypomagnesemic animals on a low Mg/high K diet may have difficulty in obtaining adequate Ca from the reserves (bone) under certain conditions, such as heavy lactation. The reason for the inability to mobilize Ca from their reserves could not be explained via this experiment. There may be several factors contributing to reduced Ca mobilization in the hypomagnesemic animals observed in the present experiment. Many experiments have shown the ability of Mg administration to correct the hypocalcemia associated with numerous clinical hypomagnesemic conditions1,16,17). RAIsz and NIEMAN demonstrated 79

6 TERASHIMA, MATSUNOBU, YANAGISAWA and ITOH Table 3. Effect of diet treatment on calcium mobilization rate in wethers after 7-10 day feeding period from in vitro bone-culture studies that increasing the Mg concentration had no effect on in-vitro bone resorption, but that decreasing Mg inhibited calcium mobilization mediated by the parathyroid hormone18). Some experiments suggest that magnesium deficiency has caused the impaired release of the parathyroid hormone and decreased bone response to the parathyroid hormone19,20). From these observations, it is possible that hypomagnesemic animals which were fed the low Mg and/or high K diet had an impaired parathyroid hormone secretion or a decreased bone response to this hormone. More experiments should be conducted to clarify the relationship between hypomagnesemia induced by the feeding of a low Mg and/or high K diet and parathyroid activity in the ruminant. Acknowledgement Research support was provided by the Kitasato University, School of Veterinary Medicine and Animal Science research grant (No. 5911) and a grant-in-aid (No ) for scientific research from the Ministry of Education, Science and Culture in Japan. References 1) SMITH, R. H., Nature (London) 191: ) L'ESTRANGE, J. L. and R. F. E. AXFORD, J. Agric. Sci., 62: ) HJERPE, C. A., Am. J. Vet. Res., 29: ) HEMINGWAY, R. G. and N. S. RITCHIE, Proc. Nutr. Soc., 24: ) BOHMAN, V. R., F. P. HORN, E. T. LITTLEDIKE, J. G. HURST and D. GRIFFIN, J. Anim. Sci., 57: ) CONTRERAS, P. A., R. MANSTON and B. F. SANSOM, Res. Vet. Sci., 33: ) FORBES, R. M. and H. M. PARKER, J. Nutr., 110: ) National Research Council, Nutrient requirements of sheep. 5 th rev. ed., National Academy of Sciences, Washington, D. C., ) DUNCAN, D. B., Biometrics, 11: ) FIELD, A. C. and N. F. SUTTLE, J. Comp. Pathol., 89: ) GREENE, L. W., J. P. FONTENOT and K. E. WEBS, JR., J. Anim. Sci., 56:

7 Calcium Mobilization in Hypomagnesemic Sheep 12) WYLIE, M. J., J. P. FONTENOT and L. W. GREENE, J. Anim. Sci., 61: ) TOMAS, F. M. and B. J. POTTER, Aust. J. Agric. Res., 27: ) QUARTE, C. G., in Magnesium in Health and Disease (CANTIN, M. and M. S. SEELIG, eds.) Spectrum Publication Inc., New York ) CLARK, I., Endocrinology, 85: ) HEATON, F. W. and P. FOURMAN, Lancet 2 : ) SHILS, M. E., Am. J. Clin. Nutr., 15: ) RAISZ, L. G. and I. NIEMANN, Endocrinology, 85: ) ANAST, C. S., J. L. WINNACKER, L. R. FORTE and T. W. BURNS, J. Clin. Endocrinol. Metab., 42: ) MACMANUS, J. F., F. W. HEATON and P. W. LUCAS, J. Endocrinol., 49: