In healthy young subjects, parathyroid hormone (PTH) is secreted presumptively in a dual fashion, with low amplitude pulses apparently superimposed on tonic secretion. In contrast, PTH secretion has not been as well characterized in postmenopausal women, and relationships among bone density, estrogen status, and PTH release have not been explored. It is possible that a pulsatile pattern of PTH secretion is important for bone remodeling, since exogenous PTH administered in a pulsatile manner stimulates bone formation. To assess the importance of pulsatile PTH secretion as a determinant of bone mass, we measured PTH in blood sampled every 2 minutes for 6 h in four groups of older women: (1) high bone density receiving estrogen (n = 6), (2) high bone density not receiving estrogen (n = 5), (3) low bone density receiving estrogen (n = 6), and (4) low bone density not receiving estrogen (n = 8). The plasma PTH release profiles were subjected to deconvolution analysis, which resolves measured hormone concentrations into secretion and clearance components, and to an approximate entropy (ApEn) estimate, which provides an ensemble measure of the serial regularity or orderliness of the release process. In postmenopausal subjects, PTH was secreted in a fashion similar to that observed in young adults, with significant tonic secretion and PTH pulse occurrences averaging every 18-19 minutes. Pulsatile PTH secretion accounted for approximately 25% of the total secreted PTH. There were no differences in the amplitude or frequency of pulsatile PTH secretory parameters or in ApEn values among the four groups or compared with young controls. We conclude that in postmenopausal women, PTH secretory patterns and temporal organization are similar to those in healthy young subjects and are not altered in states of low bone density or estrogen deficiency. This suggests that abnormalities in orderly pulsatile PTH secretion are unlikely to play a major role in established postmenopausal osteoporosis.
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