AN ACUTE INTAKE OF PHOSPHATE INCREASES PARATHYROID-HORMONE SECRETION AND INHIBITS BONE-FORMATION IN YOUNG-WOMEN

We studied the effects of a single oral phosphate (Pi) dose as well as those of three consecutive oral phosphate doses on calcium and bone m etabolism. In the first part of the study (P1 study) 10 female volunte ers were given orally 1500 mg of Pi in water, as a single dose, or pla in water in randomized order at two different sessions. In the second part of the study (P3 study), 10 female volunteers were given orally 1 500 mg of Pi, as three separate 500 mg doses in water, or plain water in randomized order. Calcium and bone metabolism was monitored for 24 h by measuring the concentrations of serum ionized calcium (S-iCa), ur inary calcium, serum phosphate (S-P), urinary P, serum intact parathyr oid hormone (PTH), serum carboxy-terminal propeptide of type I collage n (PICP), serum osteocalcin (BGP), serum carboxy-terminal telopeptide of type I collagen (ICTP), urine deoxypyridinoline (DPD) and bone-spec ific alkaline phosphatase activity (B-ALP). The S-P increased (p = 0.0 0005 and p = 0.0005, in the PI and P3 studies, respectively), the S-iC a concentration declined significantly only in the PI study (p = 0.001 4), the urinary calcium excretion decreased (p = 0.02 and 0.013, in th e P1 and P3 studies, respectively), and the PTH concentration rose (p = 0.0083 and p = 0.014, in the P1 and P3 studies, respectively) during the phosphate experiment as compared with the control session. Of the three markers of bone formation studied, PICP declined in the PI stud y (p = 0.04), and B-ALP declined in both parts of the study (p = 0.027 , p = 0.026, in the P1 and P3 studies, respectively) after phosphate a dministration, whereas there was no significant change in BGP in eithe r of the studies. The markers of bone resorption, ICTP and DPD, were u naffected by the phosphate load in both studies. In conclusion, acute ingestion of phosphate leads to an increase in S-P, a decrease in S-iC a, and an increase in intact PTH secretion. Our results indicate that these events may lead to an acute inactivation of the early phases of bone formation. In this setting, there was no indication of enhanced b one resorption despite the increase in PTH secretion, which could be d ue to the combined effect of phosphate and PTH on bone resorption.