It is well established that physical exercise can influence plasma concentrations of PTH and other parameters related to calcium metabolism. Serum levels of PTH have repeatedly been reported to increase during or after prolonged exercise(14,22,27,29,35). On the other hand, elevation in plasma calcium levels as induced by oral calcium load does not appear to influence PTH levels during acute exercise(14). This supports the hypothesis of possible exercise-related modifications in the homeostatic control of calcium. This topic of research is of considerable interest since exercise is proposed as a preventive measure against loss of bone mass (i.e., osteopenia), particularly for aging subjects. But few studies have been conducted to investigate changes in PTH and other systemic markers of bone and calcium metabolism in elderly people after a protocol of exercise (25), and bone and calcium metabolism in older men in response to physical exercise remains unclear. The purpose of the present study was to investigate the effects of a maximal exercise test before and at the end of a 6-wk endurance training program, on serum levels of PTH, osteocalcin, total calcium, phosphorus, ALP, albumin, and lactate in healthy elderly male subjects.
MATERIALS AND METHODS
This study was carried out on 24 healthy male subjects (age range, 55-73 yr). Their physical characteristics are given in Table 1. They were retired, and their health status had been assessed by means of a questionnaire concerning background and activity habits, an hematological examination, and an exercise electrocardiogram. None of these subjects showed metabolic or locomotor disorders, and they were free to eat what they wanted at home. Their physical activity consisted of regular walking, swimming, or cycling without specific training. All were fully informed about the risks and discomforts of the study before giving their informed written consent. They were free to withdraw from the study at any time. The protocol had been approved by the “Comité Consultatifde Protection des Personnes dans la Recherche Biomédicale de la Région Rhône-Alpes-Loire.”
Before the beginning, and at the end of the 6-wk period of endurance training, each subject performed a maximal exercise test (MET) on a Monark bicycle ergometer. During MET, workload was incremented by 20 W every minute until exhaustion. Heart rate was recorded continuously with EKG (Siemens Minograph 32) for maximal heart rate (HRmax) determination. Oxygen uptake was evaluated for each workload. Expired gases were collected in a mixing chamber(4 L) allowing continuous sampling (0.25 L·min-1) and connected to a Tissot Spirometer. FEO2 and FECO2 were measured with Ametek S-3A/I (Pittsburg, PA), and Normocap Datex (Helsinki, Finland) analyzers, respectively, with reference to gas mixtures determined by Schölander's technique. To ensure that maximal oxygen uptake (˙VO2max) had been reached, at least two of the following three criteria had to be met: an increase in ˙VO2 of less than 150 mL·min-1 despite an increase in power output, a respiratory exchange ratio higher than 1.10, and a heart rate within 10 beats·min-1 of the maximal heart rate predicted for the given age. The duration of MET (≈10 min) was kept identical before and after training by adjusting the first step of workload as close as possible to the subjects' capacities.
The endurance training program consisted of bicycle ergometer exercise, 1 h·d-1, 4 d·wk-1, for 6 wk. All subjects trained during the same season (October to November), to avoid differences owing to sun exposure or chronobiology. Exercise intensity was chosen to elicit 70% of HRmax during the first week of training, then 80% until the end of training. Subjects' heart rate was monitored continuously with a pulse watch (Cordless Pulse Monitor, model PU-801) during each training session.
Two blood samples (at rest and after MET) were collected from a forearm vein in each subject on each one of the two MET (before and at the end of endurance training). All investigations and blood samples were performed at the same time in the morning. The pre-MET blood sample was drawn between 0800 h and 0900 h after an overnight fast. The MET began at 1000 h, and the second blood sample was collected immediately (<1 min) after the end of the MET. Plasma volume (PV) was determined using Evan's blue dye dilution technique. Serum was aliquoted into separate specimen tubes, and routine blood biochemistry (total calcium, phosphorus, ALP, lactate, total proteins, albumin) was performed immediately in an autoanalyzer (Eppendorf-Merck, Germany). The total calcium values were adjusted for variations in serum albumin levels by the formula Ca adjusted = [calcium total + 0.02(40-alb)](7). The remaining serum aliquots were stored at-80°C. Parathyroid hormone, PTH-(1-84), was measured using a two-site immunoradiometric assay purchased from Incstar (Stillwater, MN). Osteocalcin was measured using a commercial human radioimmunoassay kit (reference OSTK PR, CIS bio international, Gif-sur-Yvette, France). Intra- and interassay variabilities of the kit assay were given by the supplier to be less than 3.7 and 6.6%, respectively, in the range of the values we have observed. As five assay batches were necessary to complete all assays, to avoid interassay drift, the four samples of each subject (i.e., before and after MET, before and after training) were assayed using the same assay batch. Moreover, as osteocalcin is a labile peptide, samples were stored at -20°C, all centrifugations and assays being conducted at +4°C. 1,25(OH)2D was assayed using a commercially available INCSTAR kit, which requires an extraction procedure and a radioreceptor assay specific for both 1,25(OH)2D2 and 1,25(OH)2D3. Extraction work has been improved by automation of the method on a Bench Mate (Zymark, Roissy, France) automaton, using Bond Elut C18OH extraction cartridges (Varian, Harbor City, CA).
Results are expressed as means ± SEM. All comparisons (rest vs MET, before vs after training) were made using paired t-tests. Relationships between quantitative parameters were tested using linear correlation tests. Correlation tests and paired t-tests were performed using a commercial software (PCSM, Deltasoft éditeurs, Grenoble, France) requiring a PC type computer. Statistical significance was accepted at P < 0.05.
The effects of training on subjects' physical characteristics are shown inTable 1. No significant change in body weight and HRmax has been noted after training. Conversely, as a result of training, maximal aerobic power and maximal oxygen uptake have been found to be significantly enhanced at the end of the experiment (+14% and +8%, P < 0.001 and P < 0.05, respectively). As shown in Table 2, MET induced significant decreases in PV before and after training as compared with resting values (-7.7% and -8.4%, respectively), with highly significant concomitant increases in protein and albumin levels. But training did not affect resting or post-MET values of PV, albumin, and protein. Finally, as expected, MET induced significant increases in serum lactate levels before training and at the end of the training period.
Changes in serum PTH and osteocalcin concentrations are illustrated inFigure 1. MET induced a marked increase in PTH levels before training (19.0 ± 1.2 vs 17.1 ± 1.4 pg·mL-1, MET vs rest values respectively, P < 0.05). This increase in PTH after MET was positively correlated with the increase in lactate levels (r = 0.49, P < 0.05). After training, MET was found again to induce a significant increase in PTH levels (21.7 ± 1.6 vs 17.8 ± 1.6 pg·mL-1, MET vs rest values respectively, P < 0.01). Furthermore, the post-MET serum PTH levels were found to be significantly higher at the end of the 6-wk training protocol than in the pretraining situation (21.7 ± 1.6 vs 19.0 ± 1.2 pg·mL-1, after vs before training values respectively,P < 0.05). Consequently, the exercise-induced increase in PTH was found to be two-fold more pronounced at the end of the training period than in the pretraining period (+21.9% vs +11.1%, respectively, P < 0.05). Suprisingly, after training no relationship was evidenced between the MET-induced increases in PTH and lactate levels.
As shown in Figure 1, osteocalcin was enhanced by heavy exercise before training (7.01 ± 0.36 vs 6.18 ± 0.44 ng·mL-1, MET vs rest values respectively, P < 0.05), but no relationship was evidenced between osteocalcin and protein or albumin levels. On the other hand, MET did not induce any change in osteocalcin levels after training (5.12 ± 0.41 vs 4.90 ± 0.43 ng·mL-1, rest vs MET respectively, NS). Furthermore, osteocalcin levels were found to be significantly decreased after training compared with values observed before training (Fig. 1).
Biochemical data reported in Table 3 show that maximal exercise induced significant increases in total calcium, phosphorus, and ALP, that pattern being unaffected by training. However, total calcium was found unchanged when corrected for albumin serum levels, and resting values of 1,25(OH)2D remained unaffected by training (Table 3). Finally, no significant relationship was found between basal or posttraining VO2max and basal or postexercise levels of PTH, osteocalcin, and other biochemical parameters even though the variations in serum levels of PTH relative to MET tended to be positively correlated with basal ˙VO2max (P = 0.08). Moreover, no direct relationship was evidenced between training, as measured by improved˙VO2max, and the biochemical changes. That is, the responses to training in terms of relation to intense exercise were similar over the whole range of ˙VO2max.
Although previous studies have repeatedly demonstrated alterations in PTH levels and other bone and calcium parameters under the effects of exercise in young and middle-aged men, the present study is the first to examine exercise-induced changes in these parameters in elderly men. The results of the present study clearly show that PTH and osteocalcin levels are immediately raised after exercise in 55- to 73-yr-old men and that this rise is affected by training.
The main determinant for PTH secretion is a low extracellular calcium concentration, and calcium was also shown to regulate PTH gene expression bothin vitro and in vivo(40). In our study, PTH levels have been found to be significantly enhanced after MET, although they remained within the normal range reported in subjects of similar age (1). This MET-induced rise in PTH levels occurred despite unchanged albumin-corrected serum calcium. This is in accordance with previous findings demonstrating that PTH dynamics during exercise is not influenced by total calcium levels (14). However, we did not measure levels of free ionized serum calcium, which could have decreased during exercise although total calcium remained constant, by binding to free fatty acids, known to increase during exercise (35). Moreover, the marked elevation in serum phosphorus concentrations during exercise may have been responsible for reduced ionized calcium levels through complex binding. The MET-related rise in phosphates reported here is consistent with previously reported data (28). Finally, we found a positive relationship between the exercise-induced rise in lactate and the increase in serum PTH, which is in accordance with previously reported data (39). On the other hand, adrenaline has been suggested as a possible factor of exercise-induced rise in PTH, since it is widely accepted that the adrenergic system, which is strongly activated by heavy exercise, affects both calcium and PTH(24,26). However, the association between exercise and PTH levels remains to be interpreted, and specific underlying mechanisms need to be clarified.
Training during 6 wk produced significant increases in ˙VO2max and maximal aerobic power in healthy elderly individuals. Pre- and postexercise plasma volume, and levels of albumin, phosphorus, and total calcium followed the same pattern of response to heavy exercise after training and pretraining. However, the MET-induced rise in PTH was found to be significantly higher after training than before training, but was no longer found to be correlated with increased lactate levels. The training-related difference observed in the rise in PTH levels in response to heavy exercise is an original finding of the present work. The reason for this change is not clear. As MET duration has been kept constant in this study, MET-related catecholamine levels were likely to be similar after training and in the pretraining situation (21), and do not seem to be the primary cause of changes in PTH response to exercise after training. Conversely, as stated by Salvesen et al. (39) in a study conducted in young subjects, it is possible that regular long-distance training induces a state of moderate hyperparathyroidism, with an enhanced response to a secretion stimulus. This response of PTH serum levels to exercise in the elderly does not automatically imply a skeletal response, which remains to be investigated. However, systemic factors play an important role in bone homeostasis, and our result might be of importance in these older subjects with regard to a potential risk for PTH-induced increased bone turnover. PTH has been shown to have paradoxal dual effects on the skeleton, both anabolic and catabolic, by means of stimulation of osteoblast and osteoclast differentiation, number, and activity (17). The net effect on bone mass appears to vary with PTH levels on one hand, and with the rhythm (continuous or intermittent) of its release (endogenous) or administration (exogenous), on the other hand. Continuous administration of PTH can induce bone loss, whereas intermittent administration has been found to significantly increase cancellous bone volume (11). The mechanism underlying the anabolic action of PTH is not fully known, but several studies have provided evidence that the PTH-induced stimulation of bone formation is mediated by local growth factors, with insulin-like growth factors (IGF) and transforming growth factor-β (TGFβ) being the primary candidates (4). However, if it has been assessed that PTH administration is able to prevent bone loss in various models of osteoporosis (11), a skeletal resistance to PTH has been evidenced in aging (13), which could be related to an age-related decrease in the skeletal content of IGF-1 and TGFβ(34). Moreover, although PTH is able to potentiate the stimulation of mechanically-induced signal transduction pathways in osteoblasts (5), it is known that the osteogenic response of bone to mechanical stimuli decreases with age (38). These data suggest potential for an imbalance between resorption and formation activities in bone, and further bone loss, as shown in overexercising elderly men (6,30,31).
Serum osteocalcin levels were monitored in this study in combination with PTH. This noncollagenous protein reflects osteoblastic activity and is considered as a sensitive index of bone formation (10). The present study clearly indicates that high-intensity exercise is able to induce a marked elevation of serum osteocalcin levels before but not after a 6-wk endurance training program in elderly subjects. On the other hand, ALP levels were similarly increased after MET, both before and after training. The increase in total ALP found after intense exercise (MET) both before and after training is questionable. Indeed, contrary to osteocalcin, total ALP is known not to be a specific biological marker of bone formation activity. This measure represents the totality of blood isoenzyme activity, both liver and bone derived alkaline phosphatase activity, and intense exercise is known to result in an increase in alkaline phosphatase activity in the liver of normal adults (32). This increase could be a result of increased energy demands (fatty acid mobilization and/or oxidation, for example) or liver dysfunction with cell injuries. However, there was no reason to suspect liver damage to have occurred in the short 6-wk training period owing to physical trauma or dramatic changes in diet or alcohol consumption, and specific markers of liver injury have not been assayed in the present study to test this hypothesis. On the other hand, the MET effects on osteocalcin levels in an aged population, both before and after training, are consistent with data previously reported in young athletes (23,35). The MET-induced increase in serum osteocalcin concentration before the beginning of training does not likely reflect a new synthesis of this protein by osteoblasts, since it has been reported that stimulation of osteoblastic cell cultures by 1,25(OH)2D3 is followed within 6 h by an increase in osteocalcin levels in the culture medium(16). The role played by exercise-induced fluid shifts should also be discussed. The marked reduction in plasma volume, caused by MET, is able to induce marked increases in serum protein levels, as shown in the results reported above. The MET-induced increase in osteocalcin observed before the beginning of training could then simply be a result of hemoconcentration. However, no significant correlation was found between osteocalcin changes and total protein or albumin changes either before or after training. Moreover, at the end of the training program, the similar MET-induced hemoconcentration occurred but was not accompanied by a decrease in osteocalcin levels. Finally, a decreased clearance of osteocalcin by kidneys might also be evoked. It is indeed well known that the half life of osteocalcin is short, being largely cleared by the kidneys, and serum osteocalcin levels have been reported to double a few minutes after nephrectomy (37). The dramatic decrease in kidney clearance known to be a result of intense exercise (36) could be the cause of the increase in MET related increase in osteocalcin levels before training. On the other hand, the fact that basal osteocalcin levels were significantly lower after training than before appears to be in contradiction with the generally accepted fact that physical training can induce an increase in osteocalcin levels(2,20,33). This decrease in post-training osteocalcin levels is in accordance with previous data reported in trained postmenopausal women (9), even though skeletal mechanical loading is known to be lower during cycling than during walking. This decrease could be related to the increased levels of PTH we have measured after heavy exercising, as it has been shown that osteocalcin levels can be decreased by acute increases in PTH both in rats and in humans(15,19). However, osteocalcin response to acute PTH treatment has been shown to differ from that of chronic treatment(15,19), and more studies are needed to draw any conclusion in terms of the relationship between PTH and osteocalcin levels after exercise training. Moreover, training may also involve some renal adaptation, which could have impaired the well known exercise-induced decrease in kidney clearance (36). The role played by kidneys in the differences between pre- and posttraining osteocalcin levels after heavy exercise needs to be investigated.
Finally, no change was evidenced between pre- and posttraining 1,25(OH)2D levels at rest, despite a probable stimulation of vitamin D metabolism by PTH (18). This result, which is contradictory to data obtained on young subjects (2), could be related to aging related inhibition of 1-hydroxylation, known to be influenced by testosterone and growth hormone levels(22). Overexercise-induced bone loss, usually linked with a deficiency in sexual hormones, is well documented in young female athletes(12) and in young male long distance runners(3). Further investigations in elderly men are needed to study the status of sexual hormones relative to training, knowing their facilitory effect on osteoblastic activity (8).
In summary, this study reports that, before training, the response to acute maximal exercise in the elderly is similar to that of young subjects, with marked increases in serum PTH, osteocalcin, and phosphorus levels. Conversely, at the end of a 6-wk daily endurance training program, subjects exhibited lower overall osteocalcin levels and more marked exercise-induced increase in PTH levels than in the pretraining period. The question remains as to how long the observed elevation in PTH is sustained and whether this intermittent rise in PTH with regular physical training is a stimulus to bone formation in the elderly. Further interpretation of these observations at the bone level would require histomorphometric assessment of bone tissue and long-term longitudinal studies of bone mineral density in the same subjects.
1. Arnaud, C. D. and K. K. Pun. Metabolism and assay of parathyroid hormone. In: Disorders in Bone and Mineral Metabolism
, F. L. Coe and M. J. Favus (Eds.). New York: Raven Press, 1992, pp. 107-122.
2. Bell, N. H., R. N. Godsen, D. P. Henry, J. Shary, and S. Epstein. The effects of muscle-building exercise
on vitamin D and mineral metabolism. J. Bone Miner. Res.
3. Bilanin, J. E., M. S. Blanchard, and E. Russek-Cohen. Lower vertebral bone density in male long distance runners. Med. Sci. Sports Exerc.
4. Canalis, E., J. M. Hock, and L. G. Raisz. Anabolic and catabolic effects of parathyroid hormone on bone and interactions with growth factors. In: The Parathyroids: Basic and Clinical Concepts
, J. P. Bilezikian, R. Marcus, and M. A. Levine (Eds.). New York: Raven Press, 1994, pp. 65-82.
5. Carvalho, R. S., J. E. Scott, D. M. Suga, and E. H. K. Yen. Stimulation of signal transduction pathways in osteoblasts by mechanical strain potentiated by parathyroid hormone. J. Bone Miner. Res.
6. Chappard, D., L. Vico, J. C. Chatard, et al. Relations entre l'activité physique, la masse osseuse et les activités cellulaires osseuses dans une population d'hommes sains âgés.Sci. Sports
7. Cunningham, J., G. V. Segre, E. Slatopolsky, and L. V. Avioli. Effect of heavy exercise
on mineral metabolism and calcium
regulating hormones in humans. Calcif. Tissue Int.
8. Dalsky, G. P. Effect of exercise
on bone: permissive influence of estrogen and calcium
. Med. Sci. Sports Exerc.
9. Dalsky, G. P., K. S. Stocke, A. A. Ehansi, E. Slatopolsky, W. C. Lee, and S. J. Birge. Weight-bearing exercise
training and lumbar bone mineral content in postmenopausal women. Ann. Int. Med.
10. Delmas, P. D. Clinical use of biochemical markers of bone remodeling in osteoporosis
11. Dempster, D. W., F. Cosman, M. Parisien, V. Shen, and R. Lindsay. Anabolic actions of parathyroid hormone on bone. Endocr. Rev.
12. Drinkwater, B. L., K. Nilson, C. H. Chesnut III, W. J. Bremner, S. Shainholtz, and M. B. Southworth. Bone mineral content of amenorrheic and eumenorrheic athletes. N. Engl. J. Med.
13. Fox, J. and M. B. Mathew. Heterogeneous response to PTH
rats: evidence for skeletal PTH
resistance. Am. J. Physiol.
14. Grimston, S. K., K. E. Tanguay, C. M. Gundberg, and D. A. Hanley. The calciotropic hormone response to changes in serum calcium
in female long distance runners. J. Clin. Endocrinol. Metab.
15. Gundberg, C. M., M. I. Fawzi, M. E. Clough, and M. S. Calvo. A comparison of the effects of parathyroid hormone and parathyroid hormone-related protein on osteocalcin
in the rat. J. Bone Miner. Res.
16. Hauschka, P. V., J. B. Lian, D. E. C. Cole, and C. M. Gundberg. Osteocalcin
and matrix Gla protein: vitamin K-dependent proteins in bone. Physiol. Rev.
17. Heersche, J. N. M., C. G. Bellows, and J. E. Aubin. Cellular actions of parathyroid hormone on osteoblast and osteoclast differentiation. In: The Parathyroids: Basic and Clinical Concepts.
, J. P. Bilezikian, R. Marcus, and M. A. Levine (Eds.). New York: Raven Press, 1994, pp. 83-91.
18. Henry, H. L., C. Dutta, N. Cunningham, et al. The cellular and molecular regulation of 1.25(OH)2
production.J. Steroid Biochem. Mol. Biol.
19. Hodsman, A. B., L. J. Fraher, T. Otsbye T., J. D. Adachi, and B. M. Steer. An evaluation of several biochemical markers for bone formation and resorption in a protocol utilizing cyclical parathyroid hormone and calcitonin therapy for osteoporosis
. J. Clin. Invest.
20. Karlsson, M. K., P. Vergnaud, P. D. Delmas, and K. J. Obrant. Indicators of bone formation in weight lifters. Calcif. Tissue Int.
21. Kindermann, W., A. Schnabel, W. M. Schmitt, G. Biro, J. Cassens, and F. Weber. Catecholamines, growth hormone, cortisol, insulin and sex hormones in anaerobic and aerobic exercise
. Eur. J. Appl. Physiol.
22. Klausen, T., L. Breum, H. A. Sorensen, S. Schifter, and B. Sonne. Plasma levels of parathyroid hormone, vitamin D, calcitonin, and calcium
in association with endurance exercise
. Calcif. Tissue Int.
23. Kristoffersson, A., J. Hultdin, I. Holmlund, K. Thorsen, and R. Lorentzon. Effects of short-term maximal work on plasma calcium
, parathyroid hormone, osteocalcin
and biochemical markers of collagen metabolism. Int. J. Sports Med.
24. Kukreja, S. C., G. K. Hargis, E. N. Bowser, W. J. Henderson, E. W. Fisherman, and G. A. Williams. Role of adrenergic stimuli in parathyroid hormone secretion in man. J. Clin. Endocrinol. Metab.
25. Lips, P., F. C. Van Ginkel, J. C. Netelenbos, A. Wiersinga, and W. J. F. Van der Vijgh. Lower mobility and makers of bone resorption in the elderly. Bone Miner.
26. Ljunghall, S., G. Akerström, L. Benson, L. Hetta, C. Rudberg, and L. Wide. Effects of epinephrine and norepinephrine on serum parathyroid hormone and calcium
in normal subjects. Exp. Clin. Endocrinol.
27. Ljunghall, S., H. Joborn, L. E. Roxin, J. Rastad, L. Wide, and G. Akerstrom. Prolonged low-intensity exercise
raises the serum parathyroid hormone levels. Clin. Endocrinol.
28. Ljunghall, S., H. Joborn, J. Rastad, and G. Akerstrom. Plasma potassium and phosphate concentrations-influence by adrenaline infusion, β-blockade and physical exercise
. Acta Med. Scand.
29. Ljunghall, S., H. Joborn, L. E. Roxin, E. T. Skarfors, L. E. Wide, and H. O. Lithell. Increase in serum parathyroid hormone levels after prolonged physical exercise
. Med. Sci. Sports Exerc.
30. Michel, B. A., D. A. Bloch, and J. F. Fries. Weight-bearing exercise
, overexercise and lumbar bone density over age 50 years. Arch. Intern. Med.
31. Michel, B. A., E. L. Lane, D. A. Bloch, H. H. Jones, and J. F. Fries. Effect of changes in weight-bearing exercise
on lumbar bone mass after age fifty. Ann. Med.
32. Nagel, D., D. Seiler, H. Franz, and K. Jung. Ultra-long distance running and the liver. Int. J. Sports Med.
33. Nichols, D. L., C. F. Sanborn, S. L. Bonnick, et al. The effects of gymnastics training on bone mineral density. Med. Sci. Sports Exerc.
34. Nicolas, V., A. Prewett, P. Bettica, et al. Age-related decreases in insulin-like growth factor-I and transforming growth factor-β in femoral cortical bone from both men
and women: implications for bone loss with aging
. J. Clin. Endocrinol. Metab.
35. Nishiyama, S., S. Tomoeda, T. Ohta, A. Higuchi, and I. Matsuda. Differences in basal and postexercise osteocalcin
levels in athletic and nonathletic humans. Calcif. Tissue Int.
36. Poortmans, J. R. Exercise
and renal function.Sports Med.
37. Price, P. A., M. K. Williamson, and J. W. Lothringer. Origin of the vitamin K-dependent bone protein found in plasma and its clearance by kidney and bone. J. Biol. Chem.
38. Rubin, C. T., S. D. Bain, and K. J. McLeod. Suppression of the osteogenic response in the aging
skeleton. Calcif. Tissue Int.
39. Salvesen, H., A. G. Johansson, P. Foxdal, L. Wide, K. Piehl-Aulin, and S. Ljunghall. Intact serum parathyroid hormone levels increase during running exercise
in well-training men
. Calcif. Tissue Int.
40. Silver, J. Regulation of parathyroid hormone synthesis and secretion. In: Disorders in Bone and Mineral Metabolism
. F. L. Coe and M. J. Favus (Eds.). New York: Raven Press, 1992, pp. 83-106.