Perimenopausal effects on bone turnover as measured by serum alkaline phosphatase and a new assay for intact osteocalcin and undercarboxylated osteocalcin.

Abstract

Although the suppression of cyclic estradiol production after menopause is associated with bone loss, the time course and rate of this decline in bone health has not been well defined. Markers of reproductive aging and bone activity could provide an indication of the bone turnover in the perimenopause (PM). Previous studies have suffered imprecise assessment and measurement of hormones and osteocalcin (OC). Subjects were recruited from local advertisement. Measures of morning OC were determined in plasma obtained from a larger study of 28 PM women (mean BMI = 24 +/- 0.6), studied on menstrual cycle day six, and related to previously determined mean concentrations of FSH (9 hourly), LH (49 q 10 minute), E2 and P (3 q 4 hour) plus E₁, total calcium (Ca), and fasting alkaline phosphatase (ALP). N-tact<super>RTM</super> Osteo SP Osteocalcin IRMA (Incstar, Stillwater, MN) measures intact OC, and was modified with serial dilution of standards to facilitate measurement of concentrations near the assay detection limit of 0.2 ng/mL, for assessment of undercarboxylated osteocalcin (ucOC). Linearity was demonstrated in assay standards and 2 unknown samples from 0.2 to 20 ng/mL. Detection of ucOC was optimal with a 50:1 mix ratio (heparinized plasma: calcium phosphate tribasic) using end-over-end mixing, then extraction of supernatant. In PM women with an FSH &ge; 20 (mean +/- SE = 41 +/- 7.1 IU/L), mean cycle length was far longer (81 +/- 16 <italic>vs</italic> 31 +/- 3 days, p < .001) and associated with higher ALP (79 +/- 8.8 <italic>vs</italic> 5 8 +/- 3.6 IU/L, p < .03), lower E₂ (26 +/- 7.8 <italic>vs</italic> 89 +/- 14.8 pg/mL, p < .001), lower E₁ (49 +/- 11 <italic>vs</italic> 81 +/- 13 pg/mL, p < .05) and lower Ca (9.6 +/- 0.1 <italic> vs</italic> 10.1, p < .001) compared to those with FSH < 20 (mean +/- SE = 8 +/- 1 IU/L). ALP was positively related to FSH (r = 0.41, p < .03), LH (r = 0.55, p = .002) and cycle length (r = 0.36, p = .052). In a subanalysis of 10 subjects, who had measurable OC and ucOC concentrations, there was an increase in percent ucOC with E₁ levels below 40 pg/mL compared to those above 40 pg/mL (46.3 +/- 13.2% <italic>vs</italic> 22.0 +/- 7.6%, p < .006). In conclusion, clinical markers of the PM transition (elevated FSH, LH, and prolonged and irregular menstrual cycles are associated with evidence of enhanced bone turnover (higher ALP). Low estrogen status may be linked to a decrease in the vitamin K-dependent, post-translational carboxylation of the bone protein OC.