Plasma parathyroid hormone levels and intestinal calcium binding protein in magnesium deficient rats

Rats were pair fed a magnesium deficient or control diet. Plasma parathyroid hormone (PTH) was estimated by radioimmunoassay using synthetic 1-34 PTH and intestinal calcium-binding protein (CaBP) was quantified directly by RIA in proximal duodenum, distal ileum and medium jejunum. In magnesium depleted rats, plasma magnesium levels were significantly decreased, a fall in plasma phosphate paralleled the decrease in plasma magnesium and plasma calcium levels were significantly increased after 14 days of magnesium deficiency. A significant rise in plasma PTH was observed on day 7 and 14 after magnesium deficiency. This increase disappeared on day 20. During the whole experimental period, no significant differences in CaBP levels were observed between the two groups of rats. Thus it is difficult to postulate an increase in vitamin D-dependent calcium absorption to explain the hypercalcemia found in magnesium deficient rats. Neither can the hypercalcemia be readily explained by an increased bone calcium mobilisation due to transient PTH increase since previous results have reported decreased bone resorption in magnesium deficient rats under similar experimental conditions.