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High P Diet Induces Acute Secretion of Parathyroidhormone without Alteration of Serum Calcium Levels in Rats

To find whether a high phosphorus (P) diet stimulate the secretion of PTH, a high-P diet was fed to rats and an increase in serum P levels has occurred. All rats were fed a control diet (0.5% calcium (Ca), 0.5% P) for 7 days, while they were being adapted, for 1 hour at 8:00 AM and again at 8:00 PM. Four groups were switched to the high-P diet (0.5% Ca, 1.5% P) at the time of their morning meal for 1 hour. The other 4 groups continued to receive the control diet. Blood samples were collected from the rats in the remaining group, which served as a pre-feeding control. Every 30 minutes after the start of feeding (30, 60, 90, 120 min), blood samples were collected from the rats in the groups fed the control and high-P diets. Serum P concentrations increased upon intake of the high P diet, within 30 minutes after the start of feeding. Serum PTH levels also increased upon intake of the high P diet, within 30 minutes after the start of feeding, and the levels were significantly higher in the high-P group than in the control group. However, no significant difference was observed in serum Ca levels between the two groups. From these results, our findings suggest that an increase in serum P concentration might be a trigger of PTH secretion without any changes of serum calcium levels.