Resistance to Bone Resorbing Effects of PTH in Black Women

Abstract

Black women have a lower incidence of vertebral and hip fractures than white women, possibly due to differences in skeletal and mineral metabolism. One suggested mechanism is that blacks have 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 lower in black women (46%). There were also nearly significant trends toward higher PTH and lower urinary calcium and pyridinoline levels in black women. During infusion, there were no racial differences in the 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 parathyroid suppression (measured by (1-84)PTH levels) was also similar between blacks and whites. There was an initial decline in urinary calcium/creatinine in both groups with a greater reduction in black women 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 (carboxy-terminal propeptide of type I procollagen and osteocalcin) decreased within 8 h and continued to decline throughout the infusion with no distinguishable racial differences ( p < 0.05 time trend for both). The most dramatic difference between black and white women in response to PTH infusion was represented by the bone resorption markers. Three separate metabolites of bone resorption (cross-linked N-telopeptide of type I collagen, cross-linked C-telopeptide of type I collagen, and free pyridinoline) all showed substantially greater elevations in white (mean peak increments 399, 725, and 43%) compared with black women (mean peak increments 317, 369, and 17%) during the infusion ( p < 0.05 group differences for all three variables). These data strongly suggest that blacks have decreased skeletal sensitivity to the acute resorptive effects of increased PTH. This finding indicates that calcium homeostasis may be accomplished in blacks (during times of relative calcium deficiency) by greater conservation of calcium from nonskeletal sources (most likely renal) with relative preservation of skeletal tissue. These differences in calcium economy could account, at least in part, for the increased bone mass and lower incidence of osteoporotic fractures in black women. (J Bone Miner Res 1997;12:958-966)

Urinary calcium excretion, sodium intake and blood pressure in a multi-ethnic population: results of the Wandsworth Heart and Stroke Study

“…34 However, another study found no association between 25-(OH)-D 3 levels and UCa when comparing blacks with whites. 35 The present study found that the ethnic differences were still significant when fasting UCa was measured, suggesting that they were unlikely to be due to differences in intestinal calcium absorption and therefore unlikely to be related to vitamin D levels which mainly acts by increasing intestinal calcium absorption.…”

Prevalence of CKD-MBD in pre-dialysis patients using biochemical markers in Enugu, South-East Nigeria

“…Some studies have suggested that this high prevalence might be due to blacks having reduced calcium sensing capacity, 13 or due to greater skeletal resistance to PTH among blacks. 14 The patients and controls had normal mean serum calcium levels which were not significantly different between them. Again, in the CHOICE study, 15 serum calcium level at the commencement of dialysis was 2.34 mmol/L, similar to values in our study.…”

(1-34) Parathyroid Hormone Infusion Acutely Lowers Fibroblast Growth Factor 23 Concentrations in Adult Volunteers

“…One hour after the intravenous catheter was placed, baseline blood and urine samples were collected and the (1-34) PTH infusion was initiated at a rate of 0.055 mg/kg per hour. This rate was based on previous studies examining the effect of (1-34) PTH infusion on renal mineral ion handling in healthy volunteers (21)(22)(23). The time of the initiation of the infusion (8:30 AM) was standardized for all participants.…”