Reduced parathyroid hormone response to peroral phosphate in osteoporotic patients

To determine whether PTH secretion can be stimulated in osteoporosis by peroral phosphate (1.5 g phosphorus), we have compared serum PTH and urinary cyclic AMP responses in 8 osteoporotic patients with vertebral compression fractures to those of 7 age-matched control subjects. While there was no statistically significant difference in the rise of serum phosphate and urinary phosphate excretion and in the fall of serumionized calcium concentrations, serum PTH and urinary cyclic AMP were increased in control subjects, but not in osteoporotic patients (osteoporotics vs. control group: PTH, p less than 0.01; cyclic AMP/glomerular filtrate, p less than 0.05). Moreover, plasma levels of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D were the same in the 2 groups of subjects. Serum levels of the carboxyl-terminal flanking peptide (PDN-21) encoded by the calcitonin gene and cosecreted with calcitonin were similar in normal and osteoporotic subjects before and 3 min after 1-min intravenous calcium infusions (2 mg/kg body weight), and of calcitonin after the calcium infusions. We conclude that calcitonin and PDN-21 responses to intravenous calcium are the same in normal and osteoporotic subjects, whereas stimulation of PTH secretion by peroral phosphate is impaired in some osteoporotic patients.