PARATHYROID-HORMONE GENE-EXPRESSION IN HYPOPHOSPHATEMIC RATS

Phosphate is central to bone metabolism and we have therefore studied whether parathyroid hormone (PTH) is regulated by dietary phosphate in vivo. Weanling rats were fed diets with different phosphate contents for 3 wk: low phosphate (0.02%), normal calcium (0.6%), normal phosphate (0.3%),and calcium (0.6%); high phosphate( 1.2%), high calcium (1.2%), The low phosphate diet led to hypophosphatemia, hypercalcemia, and increased serum 1,25(OH)(2)D-3 together with decreased PTH mRNA levels (25+/-8% of controls, P < 0.01) and serum immunoreactive PTH (4.7+/-0.8: 22.1+/-3.7 pg/ml; low phosphate: control, P < 0.05). A high phosphate diet led to increased PTH mRNA levels. In situ hybridization showed that hypophosphatemia decreased PTH mRNA in all the parathyroid cells. To separate the effect of low phosphate from changes in calcium and vitamin D rats were fed diets to maintain them as vitamin D-deficient and normocalcemic despite the hypophosphatemia. Hypophosphatemic, normocalemic rats with normal serum 1,25(OH)(2)D-3 levels still had decreased PTH mRNAs. Nuclear transcript run-ons showed that the effect of low phosphate was posttranscriptional. Calcium and 1,25(OH)(2)D-3 regulate the parathyroid and we now show that dietary phosphate also regulates the parathyroid by a mechanism which remains to be defined.