Clinical Science and Molecular Medicine (1975) 49,481484.
The direct effect of calcium on the hyperparathyroidism of chronic renal failure M. KAYE Department of Medicine, Division of Nephrology and the McGilI University Medical Clinic, Montreal General Hospital, Montreal Quebec, Canada
(Received 25 June 1975)
s-ry 1. The direct effect of calcium on the hyperparathyroidism of chronic renal failure was studied in rats with induced chronic renal failure, who were fed on a diet low in phosphate and who received supplemental phosphate by injection. They were given a normal (0.8%), or low (0.1 %) or high (1.7%) calcium diet. 2. The animals on the low calcium diet had larger parathyroids and more severe bone disease at the end of 4 weeks, indicating the importance of calcium intake in directly influencing the degree of hyperparathyroidism. 3. Increasing the calcium content of the diet from 0.8% to 1.7% produced no additional benefits.
shown that dietary phosphate intake markedly affected the degree of hyperparathyroidism, it was possible that the effect of dietary calcium was mediated entirely through limitation of phosphate absorption. To clarify this the present study was initiated: dietary phosphate was maintained at a low value and oral calcium intake varied. Parathyroid suppression was found when the calcium intake was increased to a normal value but an additional increment above this had no further effect. Methods and materials The experimental protocol was identical with that used in previous studies (Kaye, 1974a). Male Holtzman rats (average body weight 95 g) were subjected to unilateral nephrectomy and segmental infarction of the opposite kidney on day 1. They were killed by aortic bleeding on day 28 in the post-absorptive state. Diet was similar to that employed previously: casein 12%, glucose 15%, corn starch 15%, corn oil 15%, vitamin mix 0.02%, salt mix 7% and water 36%. The diets contained 0.3% potassium, 0.5% sodium, 0.1% magnesium and 0.2% chromium sesquioxide as a non-absorbable internal marker. The diet was adequate for all known nutrients (Laboratory Animal Handbooks 2, 1969). Animals were individually caged and fed only when the previously given diet had been consumed, so that total food intake throughout the 28 days was measured. Deionized water containing 5 % glucose was provided ad libitum. The diets contained no added phosphate, with the only dietary source of phosphate coming from casein to give an analysed
Key words : calcium, hyperparathyroidism, phosphate, uraemia. Introduction In a previous study in rats with chronic renal failure it was shown that, as dietary calcium intake increased, there was a decrease in the degree of hyperparathyroidism as judged by gland size, the uptake of u-aminoisobutyric acid in the glands, bone composition and fractional phosphate excretion (Kaye, 1974a). Associated with the rising dietary calcium was an increase in faecal phosphate excretion so that net phosphate absorption decreased. As it was also Correspondence: Dr Michael Kaye, Montreal General Hospital, 1650 Cedar Avenue, Montreal, Quebec H3G 1A4, Canada.
M . Kaye
value of 0.1 % for phosphorus. Calcium as calcium carbonate was added at three levels to give a final analysed calcium content of 0.1% for the low calcium diet, 0.85% for the normal calcium diet, and 1.75% for the high calcium diet. These diets were evaluated for acid-producing qualities by feeding normal animals for 1 week and then measuring the 24 h urine excretion of ammonia and titratable acid. After addition of suitable quantities of ammonium chloride to the diets the total net acid excretion was 0.6 mmol/day for all of them. To provide phosphorus for growth the animals were injected at eight hourly intervals intraperitoneally with a neutral sohtion containing Na,HPO, (800 mmol/l) and KH2PO, (180 mmol/l), providing 960 mmol/l of phosphorus. The total daily dose of injected phosphorus was 0.48 mmol per animal for the first and second weeks, 0.77 mmol the third week and 0.96 mmol the fourth week. Calcium in serum and urine was measured by atomic absorption spectrophotometry and also in stool, diet and tissues after preliminary ashing at 450°C for 16 h. Serum ionized calcium was measured with a flow-through electrode (Orion
Research Inc., Cambridge, Mass., U.S.A.) and creatinine and phosphorus by standard AutoAnalyzer I1 methodology (Techicon Instrument Corp., N.Y ., U.S.A.). The shaft of the left femur was used for determination of ash weight, calcium and phosphorus content, all expressed as a percentage of fresh weight in g/lOO g. All animals received dimethylchlortetracycline (10 mg/kg) on days 13, 14, 21 and 22. Mineral appositional rate (pmlday) was measured between the two tetracycline labels on ground (20 pm-thick) transverse sections of the mid-shaft of the right femur. Resorption areas were measured in the last lumbar vertebra and expressed as a percentage of total trabecular area. Data are expressed as mean value 5 SEM with Student’s t-test applied for estimation of the significance of the differences between mean values on the 0.8% diet and on the low and high calcium diets.
Data obtained at the time of death are shown in
TABLE 1. Biochemical and other data from rats at the time of death Results (mean v a l u e s + s e ~ ) were obtained approx. 10 h after the last injection of phosphorus. Tubular reabsorption of phosphorus was measured from the 24 h urine sample obtained before the animals were killed. Significance of difference between experimental groups and 0.8% dietary calcium group: * P