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438 SERUM OSTEOCALCIN: RESPONSE TO EXOGENOUS PARATHYROID HORMONE (PTH) IN X-LINKED HYPOPHOSPHATEMIA (XLH)

Osteocalcin (OC) is a bone-specific protein that can be quantitated in serum by radioitnmunoassay. Serum OC is a sensitive measure of bone formation, but its response to calcitropic hormones in vivo is still poorly understood. We observed the effects on serum OC of a 5 min i.v. infusion (mean 5.0 U/kg) of PTH in 7 XLH patients (3 males, 4 females; ages 8-35 yr) who had been treated with 1,25(OH)2D3 and phosphate (Pi). Pre-infuslon renal Pi reabsorption (TRPi) was determined as a mean of three values from sequential 30 min collection periods (0900 to 1030 hr). Post-infusion clearances in 3 subsequent periods (1030 to 1200 hr) were examined separately. The large variation (5-116 ng/ml) in pre-infusion serum OC among individuals was related to age differences and treatment status. Urinary cAMP response to PTH was maximal in the first post-infusion period (mean increase: 7.6-fold); the phosphaturic response (blunted in XLH) was greatest in the second (ΔTRPi=-14%). Serum OC decreased in all 7 patients. At 15 and 45 min, OC concentrations were 63 ± 57. and 69 ± 7%, respectively, of pre-infusion ones (x ± SE, p<.01); but by 75 min, mean OC values were no longer significantly different from pre-infusion (87 ± 14%). We suggest that serum OC responds rapidly to PTH and may therefore be useful in the study of skeletal sensitivity to the hormone.