Effects of low phosphorus and low calcium diets on the production and metabolic clearance rates of 1,25-dihydroxycholecalciferol in pigs

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Abstract
Severe dietary calcium (Ca) or phosphorus (P) deficiency leads to a similar degree of stimulation of (1) plasma and intestinal 1,25-dihydroxycholecalciferol (calcitriol) levels and (2) intestinal Ca absorption. In contrast, P depletion causes a much smaller increase in the renal 25-hydroxycholecalciferol-1-hydroxylase enzyme activity than does Ca depletion. To test the hypothesis that a reduced metabolic clearance rate (MCR) during P depletion might, in part, account for the observed increase in calcitriol levels, we have developed a simple primed-infusion technique to determine the MCR and production rate (PR) of calcitriol, in vivo, in pigs fed low Ca and low P diets. The MCR of calcitriol was unchanged (0·92 ml/min per kg metabolic body size (MBS)) when low P diets were fed, but showed a significant 15% increase (1·06 ml/min per kg MBS) during the feeding of low Ca diets. The PR of calcitriol increased by 2·3-fold when low P diets were fed and by fourfold during Ca depletion. Thus the increase in plasma calcitriol levels during P depletion of pigs is caused by an increase in production rather than a decrease in clearance. In contrast, Ca depletion is associated with a slightly higher PR than indicated by measurement of plasma calcitriol levels because of the increased MCR. J. Endocr. (1985) 105, 169–173