RESISTANCE TO BONE-RESORBING EFFECTS OF PTH IN BLACK-WOMEN

Black women have a lower incidence of vertebral and hip fractures than white women, possibly due to differences in skeletal and mineral meta bolism, One suggested mechanism is that blacks hare decreased skeletal sensitivity to parathyroid hormone (PTH), To test this hypothesis, we infused h(1-34)PTH in healthy premenopausal black (n = 15) and white (n = 18) women over 24 h and measured serum and urine indices of bone turnover and calcium metabolism throughout the infusion, At baseline, the mean 25-hydroxyvitamin D (25(OH)D) concentration was significantly lo,ver in black women (46%), There were also nearly significant trend s toward higher PTH and lower urinary calcium and pyridinoline levels in black women, During infusion, there were no racial differences in t he mean (1-34)PTH levels achieved or in resultant elevations of serum calcium or 1,25-dihydroxyvitamin D (1,25(OH)(2)D) levels, Endogenous p arathyroid suppression (measured by (1-84)PTH levels) was also similar between blacks and whites, There was an initial decline in urinary ca lcium/creatinine in both groups with a greater reduction in black wome n early in the infusion period (p < 0.05 at 8 h), Furthermore, blacks had lower levels of urinary calcium/creatinine throughout the infusion (p < 0.05 group difference), Bone formation markers (car boxy-termina l propeptide of type I procollagen and osteocalcin) decreased within 8 h and continued to decline throughout the infusion with no distinguis hable racial differences (p < 0.05 time trend for both), The most dram atic difference between black and white women in response to PTH infus ion was represented by the bone resorption markers, Three separate met abolites of bone resorption (cross linked N-telopeptide of type I coll agen, cross-linked C-telopeptide of type I collagen, and free pyridino line) all showed substantially greater elevations in white (mean peak increments 399, 725, and 43%) compared with black women (mean peak inc rements 317, 369, and 17%) during the infusion (p < 0.05 group differe nces for all three variables), These data strongly suggest that blacks have decreased skeletal sensitivity to the acute resorptive effects o f increased PTH, This finding indicates that calcium homeostasis may b e accomplished in blacks (during times of relative calcium deficiency) by greater conservation of calcium from nonskeletal sources (most lik ely renal) with relative preservation of skeletal tissue, These differ ences in calcium economy could account, at least in part, for the incr eased bone mass and lower incidence of osteoporotic fractures in black women.