Parathyroid hormone secretory response to EDTA-induced hypocalcemia in black and white premenopausal women

One consistent racial difference in mineral homeostasis is increased effici ency of renal calcium conservation in blacks which could account, in part, for differences in bone density and fracture risk. Since parathyroid hormon e (PTH) is the major regulator of calcium homeostasis, we investigated its secretion in black and white women in response to hypocalcemia. Two hour ED TA infusions (50 mg/kg) were performed in 34 premenopausal women (17 black, 17 white). Blood was sampled at 30-minute intervals during the infusion, a t 60-minute intervals for 3 more hours, and at 24 hours. Serum ionized calc ium decreased identically in both groups with a nadir at 2 hours and return ed to baseline within 24 hours. Serum 1-84 PTH levels rose similarly in bot h groups with a peak PTH level that was slightly higher in black women, and on average, slightly earlier than that in white women. Serum PTH levels re mained elevated in both groups at 24 hours with no overall group difference s in PTH response. In black, but not white women, serum 25OHD levels correl ated negatively with both basal PTH and peak PTH level, achieved with infus ion. Serum 1,25(OH)(2)D levels rose and osteocalcin levels decreased, with no group differences. We conclude that overall, premenopausal black women s how no clear differences in PTH secretory activity to an EDTA-induced hypoc alcemic stimulus. Basal vitamin D status appeared to be a determinant of th e degree of the PTH response in black women, with the peak PTH level being inversely correlated with levels of 25OHD. Since we have previously shown t hat the skeleton contributes less to acute calcium needs in blacks than in whites, the lack of a racial difference in PTH secretory responsivity sugge sts that calcium homeostasis is more likely maintained in blacks through gr eater PTH sensitivity at extraskeletal sites, such as the kidney.