Comparative effects of calcitriol and parathyroid hormone on serum aluminum in vitamin D-depleted rabbits fed an aluminum-supplemented diet.

Under normal circumstances, the body barriers effectively limit the entry and retention of dietary aluminum. However, both parathyroid hormone (PTH) and calcitriol (physiologically active hormonal form of vitamin D3) have been reported to produce elevation of serum aluminum in animals fed an aluminum-supplemented ration. To compare the effects of calcitriol with those of PTH with reference to their putative effect to enhance aluminum absorption, an experiment was designed wherein the serum levels of both PTH and calcitriol would be changing markedly during a short time-frame. To condition the rabbits used for this comparison, they were fed a vitamin D-free diet, which caused the level of calcitriol and its precursors to decline rapidly. The calcitriol deficit together with the ensuing lack of calcium absorption resulted in a state of secondary hyperparathyroidism. Vitamin D-depletion was shown to be complete by the high level of serum PTH and a low (unmeasurable) level of serum calcitriol. To enable comparison of PTH with calcitriol, exogenous calcitriol infusion (60 IU/day) was started by osmotic pump simultaneously with the beginning of an aluminum (aluminum lactate) supplemented diet. Aliquots were collected for both serum PTH and serum calcitriol at intervals during the 7 day study. A rising serum aluminum level was highly correlated with the rising serum calcitriol level in the rabbits (r = 0.903, p = 0.036) during the first 4 days of the infusion. The mean serum aluminum levels rose nearly 13 parts per billion (ppb) in the 7 day period. Declining serum PTH (due to feedback mechanisms of calcitriol suppressing PTH synthesis) showed a negative correlation of serum aluminum and serum PTH (r = -0.959, p = < 0.01) during the first 4 days of infusion. Control rabbits (vitamin-D depleted) fed aluminum-supplemented rations have shown only a minimal transient rise in serum aluminum level which returned to the pre-test level by the end of the week. To test for any effect of PTH on serum aluminum in the absence of calcitriol, five rabbits were implanted with osmotic pumps infusing PTH (mean 6.0 U/hr) and started on an aluminum supplemented diet. These rabbits, having previously been depleted of vitamin D were already in a state of nutritional secondary hyperparathyroidism as shown by their elevated pretest PTH levels. During the 7 day infusion, the serum aluminum rose only a mean of approximately 1 part per billion (ppb).(ABSTRACT TRUNCATED AT 400 WORDS)