Increased 1,25-dihydroxyvitamin D3 synthesis in rats fed a high-phosphorus diet.

Rats fed vitamin D-deficient diets containing 0.6% Ca and 0.3%, 0.6%, 1.2%, or 1.8% P exhibited progressive increments of hypocalcemia and hyperphosphatemia. In vitro assay of 25-hydroxyvitamin D3-1-alpha-hydroxylase (1-alpha-hydroxylase) activity in isolated kidney cortical mitochondria showed that hyperphosphatemia in the presence of hypocalcemia was associated with an increase in enzyme activity. The results indicate that the stimulation of 1-alpha-hydroxylase associated with depressed plasma Ca in rats fed a high-P diet overrides any inhibition of the enzyme that may be caused by excess plasma phosphate.