Effects of low phosphorus and low calcium diets on the production and metabolic clearance rates of 1,25-dihydroxycholecalciferol in pigs.
Severe dietary calcium (Ca) or phosphorus (P) deficiency leads to a similar degree of stimulation of plasma and intestinal 1,25-dihydroxycholecalciferol (calcitriol) levels and 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.
D000818:Animals; D002117:Calcitriol / Q000096:biosynthesis / Q000097:blood; D002118:Calcium / Q000172:deficiency* / Q000378:metabolism*; D004032:Diet; D005260:Female; D007408:Intestinal Absorption; D007413:Intestinal Mucosa / Q000378:metabolism; D008297:Male; D008657:Metabolic Clearance Rate; D010758:Phosphorus / Q000172:deficiency*; D013552:Swine
D010758:Phosphorus; D002117:Calcitriol; D002118:Calcium